Despite substantial progress in numerical modelling in recent decades, predictability for weather and extreme events is often limited and the assessments of future changes remain uncertain. This underscores the need to improve our understanding of the complex, nonlinear interactions of dynamical and physical processes that influence predictability at different lead times and determine the location, timing, and magnitude of extreme events.
This session will discuss our current understanding of how physical and dynamical processes connect atmospheric motions across temporal and spatial scales and how this relates to intrinsic and practical predictability of various weather phenomena. We particularly welcome contributions advancing our understanding, prediction, and future projections of weather and climate extremes, from both an applied and theoretical viewpoint, and with socio-economic impacts, e.g. on power systems.
Topics of interest include but are not limited to:
(1) Synoptic-scale atmospheric dynamics affecting the timing, positioning, and amplitude of weather events (e.g., the stationarity and amplitude of Rossby waves).
(2) Large-scale atmospheric and oceanic influences (e.g., the stratosphere, the Artic, or tropical oceans) on atmospheric variability and predictability in the midlatitudes.
(3) Intrinsic limits of predictability for various atmospheric phenomena and their link to the multi-scale, non-linear nature of atmospheric dynamics.
(4) Practical limits of predictability and the representation of atmospheric phenomena in numerical weather prediction and climate models including sensitivities to the model physics.
(5) Weather and climate extremes, including compound extreme events, their dynamics, predictability, and representation in weather and climate models.
(6) Statistical and mathematical approaches for the study of extreme events.
(7) Impact and risk assessment analyses of extreme events, in particular with a focus on renewable power systems and Europe.
(8) Extreme event attribution and changes in extreme event occurrences under climate change.
Recently, weather prediction models based on artificial intelligence (AI) have become equally to slightly more accurate than the established operational weather prediction models in terms of deterministic scores. The much lower computational cost of the AI-models facilitates the generation of large ensembles, hence it is important to assess if their error growth properties are realistic. Here, we investigate error growth from initial condition perturbations with varying amplitudes, simulated with the AI-weather prediction models (PANGU, GraphCast, FourCastNet) and with a “classic” fluid equation-based weather model (ICON). From past research and the global convection-permitting ICON simulations, it is expected that small-amplitude initial condition perturbations would grow very fast initially in areas with latent heat release, then spreading out to larger and larger scales, eventually setting a fixed and fundamental limit to the predictability of weather. This phenomenon is known as the butterfly effect. We find however, that in contrast to ICON, the AI-based models completely fail to reproduce the rapid initial growth rates. Instead their growth rates remain similar to those typically found on synoptic-scales, which incorrectly suggests an unlimited predictability of the atmosphere. In contrast, if the initial perturbations are large in amplitude and comparable to current uncertainties in the estimation of the initial state, the AI-based models basically agree with results from the “physically-based” ICON simulations, although some deficits are still present, mostly related to their particularly low effective resolution. This provides an example of how machine learning models can fail to reproduce a fundamental physical principle, even though they can accurately mimic many observed behaviors.
How to cite: Selz, T. and Craig, G.: Can AI-based weather prediction models simulate the butterfly effect?, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-556, https://doi.org/10.5194/ems2024-556, 2024.
We use a moist adjoint model to compute the initial-condition perturbations that minimize the significant 48-72-h synoptic-scale forecast errors associated with the 15 November 2018 cyclone over the East Coast of the United States. These adjoint-optimal perturbations, which have a maximum amplitude of about 2 K in potential temperature, are widespread, extending throughout the troposphere and across much of North America. We investigate the most impactful subsets of the perturbations by truncating them in physical and spectral space and rescaling them to be equal in an energy norm to the full, unmodified perturbations. When the perturbations are confined to localized target regions, including those with the highest adjoint sensitivity or the strongest convective instability, they have weaker impacts on the forecast than when the perturbations within the target regions are removed and the rest of the perturbations are retained. Additionally, when the perturbations are filtered to retain only wavelengths longer than 1000 km, they have greater impacts on the forecast than when the perturbations are filtered to retain only wavelengths shorter than 1000 km. These results suggest that the 15 November 2018 forecast bust was strongly sensitive to widespread, large-scale uncertainties in the initial conditions, rather than those in localized, small-scale regions that could more feasibly be reduced by targeted observations.
Left: 600-hPa potential-temperature adjoint perturbations (color fill every 0.5 K) for the (b) box, (d) full, and (e) hole experiments. GFS analysis for 600-hPa geopotential height (black contours, contoured every 15 dam). Green box in (b), (e) bounds the box and hole regions.
Right: 72-h SLP errors (color fill, contoured at 3.5 hPa) for (a) control (unperturbed) simulation and for the adjoint perturbations (b) box, (d) full, and (e) hole. SLP contours from the GFS analysis (black) and the forecast (magenta) contoured every 6 hPa.
How to cite: Durran, D., Lloveras, D., and Doyle, J.: Can Observation Targeting Be a Wild Goose Chase? An Adjoint-Sensitivity Study of a U.S. East Coast Cyclone Forecast Bust, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-205, https://doi.org/10.5194/ems2024-205, 2024.
Despite rapid improvement in global-scale numerical weather prediction systems, forecast centers repeatedly experience notable declines in their systems’ forecast accuracy. These occasional very poor forecasts are often referred to as “forecast busts” and lead to perceptible differences in the anticipated weather. It is therefore important to understand the causes and the origins of "forecast busts” through a systematic analysis. Previous studies made use of reforecasts (or often referred to as hindcasts) to investigate hundreds of “forecast busts” from a fixed forecast model. The mean characteristics but also the different flavors of “forecast busts” have been investigated based on ERA-Interim reforecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF). However, a deeper analysis of these events, particularly by using reforecast of ECMWF's most recent reanalysis (ERA5), is still missing.
In this study, we identify “forecast busts" over Europe based on the day 6 anomaly correlation coefficient forecast skill of geopotential height at 500 hPa using ERA5 reforecasts of the period 1979-2023. Characteristics of these events are compared to the characteristics of “forecast busts” in an earlier model version (ERA-Interim), but also – for the first time – compared to exceptionally good forecasts in order to gain a better understanding of the strengths and weaknesses of the CY41R2 of the ECMWF Integrated Forecast System in predicting the large-scale pattern over Europe. To further extend the range of analyses on “forecast busts”, we systematically investigate various flavors of error patterns over Europe on forecast day 6 and discuss their distinct error pattern evolution and weather impacts over Europe.
How to cite: Hauser, S., Martin, J. E., Cavallo, S., and Parsons, D. B.: Characteristics of large-scale circulation "forecast busts” over Europe in ERA5 medium-range reforecasts, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-995, https://doi.org/10.5194/ems2024-995, 2024.
Over the past years, Europe was stricken by unprecedented heat and drought extremes. Heatwaves (HWs) pose an increasingly major threat to society, which requires to further our understanding of their predictability some days or weeks ahead. In the present study, we focus on medium-range lead times of 5-12 days, for which the successful prediction of HW onsets is strongly dependent on the adequate prediction of large-scale Rossby wave patterns and their dynamics.
Using ECMWF and GEFSv12 ensemble reforecasts, we statistically assess the medium-range predictability of HW onsets over multiple European regions for the period 2001-2018. Heatwaves are objectively diagnosed as time periods where the 90th percentile in 2m maximum temperatures is exceeded both grid point-wise and integrated over the region for at least 3 days, giving about 50 HWs for each region. Predictive skill is evaluated mainly for large-scale flow features but also in terms of the ensemble’s capability to predict the extremeness of near-surface temperatures. By adopting the concept of Euro-Atlantic weather regimes, we are further able to stratify HW onsets and their predictability by the concurrent large-scale atmospheric flow pattern.
For both the British and Central European region, we find that the medium-range predictive skill is significantly better for HW onsets associated with either Scandinavian or European blocking compared to cases with no pronounced regime. This skill advantage mostly concerns the large-scale flow (500hPa geopotential patterns) and to some extent 850hPa temperatures, particularly in times of a Scandinavian blocking. In contrast, near-surface maximum temperature forecasts do not exhibit increased skill for regimes associated with above-average large-scale predictability, which may point to the inherent difficulty of predicting smaller scales and boundary layer processes.
Finally it is explored how exceptionally good or poor forecasts at 10 days lead time are related to the atmospheric state during or shortly after forecast initialisation. For Central European HW onsets, poor large-scale flow predictive skill is associated with significantly increased baroclinicity further upstream and a more intense North Atlantic jet stream whereas particularly good forecasts are on average characterized by an initial atmospheric state close to climatology. Forecast skill for near-surface temperatures is not affected by such dynamical precursors, but rather by pre-existing soil moisture anomalies, with drier than normal soils being linked to better predictions and vice versa. For the British region, exceptionally good forecasts of both large-scale flow and near-surface temperatures are associated with an already established blocking over the continent. In comparison to Central Europe, antecedent soil moisture anomalies do have a same-sign, but less prominent effect on near-surface temperature forecasts.
How to cite: Lemburg, A. and Fink, A. H.: Investigating the medium-range predictability of European heatwave onsets in relation to weather regimes using ensemble reforecasts, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-740, https://doi.org/10.5194/ems2024-740, 2024.
The 2023/24 El Niño ranks as the 2nd strongest El Niño in the 21st century thus far. The intensity of the event was successfully predicted as early as March 2023, based on the buildup of upper ocean heat content in the western equatorial Pacific. Nevertheless, the unusual evolution pattern of this event, including the two-step warming tendency and two warming centers during the 2023/24 El Niño, and the influence from outside of the equatorial Pacific, remain to be explained. Here we show that the 2023/24 strong El Niño was mainly contributed to by three factors. Firstly, the buildup of heat content in the western equatorial Pacific in the early months of 2023 fostered a strong canonical El Niño, characterized by a steady increase in SST anomalies across the central-eastern equatorial Pacific, peaking in late 2023. Secondly, the intense sea surface temperature warming outside of the equatorial Pacific suppressed the El Niño and confined the warming mainly to the eastern basin. Lastly, the westerly wind bursts that occurred in the western-central equatorial Pacific in autumn induced another relatively weak warm center in the central equatorial Pacific toward the end of 2023. While the latter two factors coincidentally offset each other, leaving the buildup of heat content appearing as the primary cause of the strong 2023/24 El Niño, they explained the detailed structure of this El Niño. Our results not only confirm the essential role of equatorial ocean heat recharge for El Niño development, but also demonstrate the necessity of accounting for multi-scale interactions from a global perspective to understand and predict El Niño.
How to cite: Lian, T., Liu, T., Hu, R., Wang, J., Song, X., and Chen, D.: Predicting the 2023/24 El Niño from a multi-scale and global perspective, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-55, https://doi.org/10.5194/ems2024-55, 2024.
Variability in the jet stream can have a significant influence on the distribution of extreme weather, with winter-long anomalies leading to extreme seasons. During winter of 2009/2010, the Atlantic and African jets were anomalously merged for most of the winter, resulting in a persistent zonally oriented single jet. At the same time, intense and prolonged negative phase of the North Atlantic Oscillation (NAO) and unusually cold and extreme weather conditions were reported over the Northern Hemisphere. Such a merging was only observed to occur for a whole winter during the winters of 1968-69 and 1969-70. Preliminary results indicate that such persistent winter merged jets could be more frequent in a future global warming scenario and thus it is important to understand this rare dynamical jet state and its effects on the weather patterns. In this study, we explore the associated distributions of extreme weather during merged-jet winter months, in comparison to the distribution of extremes during negative NAO months. We show that merged jet winter months have a signature weather pattern distribution that is different from the negative NAO phase, affecting Northern Africa, Europe and south-west Greenland, which is associated with synoptic storms tending to either propagate zonally towards the Mediterranean, with a secondary northward then westward branch propagating to Greenland. We further find a complex relation to Arctic-Eurasian temperature anomalies. Once we remove the NAO-related high-latitude cold surface temperature anomalies, we find that Atlantic-African jet merging, especially the winter-long persistent ones, occur during winters with unusually warm Arctic and cold Eurasia. This is consistent with the theory that winter-long jet merging represents a dynamical regime transition to a merged jet, which is more likely to occur when mid-latitude baroclinicity is weaker.
How to cite: Suresan, S., Harnik, N., and Caballero, R.: Winter-long Atlantic-African jet merging and the effect on extreme weather, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-931, https://doi.org/10.5194/ems2024-931, 2024.
Recent work has provided robust evidence for the systematic co-occurrence of wintertime cold spells in North America and wet and windy extremes in Europe, which we term compound pan-Atlantic extremes. Both cold spells and wet and windy extremes are individually highly impactful, and their concurrence further amplifies their effects for actors with international exposure who are vulnerable to correlated losses. This study aims to investigate further the atmospheric processes associated with compound pan-Atlantic cold, wet and windy extremes and how these processes are represented in CMIP6 models.
On aggregate, cold spells in different parts of North America statistically co-occur with wind and precipitation extremes in specific European regions. However, North American cold spells can arise from multiple dynamical pathways, altering the location and timing of the associated European extremes for individual cold spells. Here, we use ERA5 reanalysis data (1940-2014) to identify North American wintertime cold spells in three different regions as well as the co-occurring wet and windy extremes in northern and southern Europe. We relate these compound pan-Atlantic extreme occurrences to dynamical pathways based on Pacific and Atlantic weather regimes. We then compare the CMIP6 historical simulation model data (1940-2014) with the ERA5 results. First, we discuss the performance of the models in capturing the weather regime frequencies and dynamical pathways to cold spells. Second, we review the ability of the models to replicate the spatial and temporal pattern of pan-Atlantic extremes for the three cold spell regions.
The results of this study contribute to the evaluation of the model fidelity in reproducing pan-Atlantic compound extremes and the associated circulation, with direct implications for the assessment of climate projections.
How to cite: Leeding, R. and Messori, G.: Dynamics of pan-Atlantic winter compound extremes in ERA5 and CMIP6 models., EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-265, https://doi.org/10.5194/ems2024-265, 2024.
Our work focuses on the dynamic drivers of cold spells in North America and wet and windy extremes in Europe, which we refer to as pan-Atlantic compound surface extremes. In particular, we consider the role of different atmospheric dynamical pathways in leading to an intensification of the Alaskan Ridge, which previous work has highlighted as a key precursor to the pan-Atlantic extremes. We specifically emphasise the role of Rossby wave activity acting on various spatial and temporal scales. The separation is performed on the ERA5 data projected into the normal-mode functions, as one of the available wave decomposition techniques. We select planetary (zonal wavenumbers 1-3) and synoptic (zonal wavenumbers 4-8) components to understand what processes are related (or dominant) at different Rossby wave scales. One potentially important mechanism for Alaskan Ridge amplification is related to stratospheric wave reflection events, which are associated with the vertical propagation of planetary-scale waves, and occur approximately two weeks before the peak of North American cold spells. On a shorter timescale (within 10 days), Rossby wave trains with smaller spatial scales are enhanced 5 days prior to the peak. Preliminary analysis suggests that the enhancement is likely to result from the merging of meridionally and zonally propagating waves within the modulation of the planetary waves. Our results highlight the intricate connection between different dynamical processes in the atmosphere and amplified Alaskan Ridges, which in turn are a precursor to pan-Atlantic compound extremes. The dynamical and statistical link to surface extremes could aid in predicting such events.
How to cite: Strigunova, I. and Messori, G.: Scale-Dependent Rossby Wave Dynamics and Alaskan Ridge Amplification: Impacts on Pan-Atlantic Compound Extreme Events, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-805, https://doi.org/10.5194/ems2024-805, 2024.
Mediterranean weather extremes are often driven by regional low-pressure systems, here under the name of Mediterranean cyclones. In this work we take a Lagrangian (cyclone-centred) and a Eulerian (geographical) perspectives on the association between bi-variate compounds of rain-wind and wave-wind extremes and Mediterranean cyclones. Our aim is to provide new insight on the processes driving the compounding of extreme weather conditions in the extended Mediterranean region. Three main research questions are adressed.
(i.) What is the average spatial distribution of compound extremes around cyclone centres?
(ii.) How is the regional compound frequency modulated by the presence of cyclones?
(iii.) Is the presence of cyclonic air-streams (warm conveyor belts and dry intrusions) and fronts relevant for triggering compound extremes?
The results are strongly dependent on the compound type, on the cyclone class and on the season and region of occurrence. We find that in transition seasons compound extremes are far rarer than in the winter season, and that they are mostly associated with a nearby low-pressure centre. The fraction of cyclone-related compounds is weaker during the winter season, even though winter baroclinic cyclones are associated with the highest frequency and spatial extension of compound extremes. Finally, the presence of warm conveyor belts (inflow and ascent), dry intrusion outflow and cold fronts around the cyclone centre is shown to be important for the occurrence of regional compound extremes across all seasons. Warm conveyor belt ascent regions show high occurrence of both types of compound extremes; wave-wind compounds are also favoured in regions of dry intrusion outflow.
How to cite: Portal, A., Rousseau-Rizzi, R., Raveh-Rubin, S., Catto, J., Givon, Y., and Martius, O.: Mediterranean compound extremes and their link to Mediterranean cyclones, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-812, https://doi.org/10.5194/ems2024-812, 2024.
In the beginning of September 2023, Europe experienced a pronounced atmospheric omega-blocking, marked by a persistent anticyclone in the center flanked by two low pressure systems to the southwest and southeast. By definition, an atmospheric block interrupts the mean westerly flow and leads to prolonged persistent conditions lasting for at least five days. The core of the omega-blocking in September 2023 was located over Central Europe and Southern Scandinavia, which experienced a heatwave in the first week of September 2023. On the other hand, areas situated on the southwestern (Spain) and southeastern flanks (Greece, Bulgaria, and later Libya) were hit by extreme precipitation leading to major flood events.
We employ the method of ensemble boosting to explicitly simulate omega blocking situations with spatially compounding extremes (heatwave and extreme precipitation) with the Community Earth System Model 2 (CESM2). Using this model re-initialization approach, slight perturbations are introduced to initial conditions 10 to 25 days prior to the event, leading to the generation of hundreds of coherent physical event trajectories. This allows to study two sets of research questions: the first focuses on assessing the capability of the climate model to reproduce the 2023 event in its severity, while the second deals with identifying the primary factors of the omega block that result in the most severe impacts on the ground.
In our presentation, we introduce the research concept and address the following research questions: Is the CESM2 model capable of reproducing an omega blocking event with spatially compounding extremes in the magnitude of the September 2023 event? Could the September 2023 event have been even more devastating by chance? What characteristics of the omega block precondition the occurrence of the most extreme impacts in terms of heatwaves and extreme precipitation within the boosted ensemble?
How to cite: Mittermeier, M., Guo, Y., Beyerle, U., Suarez-Gutierrez, L., Bevacqua, E., and Fischer, E.: Spatially compounding extremes under omega blocking, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-964, https://doi.org/10.5194/ems2024-964, 2024.
We investigate the representation of hot-dry compound extremes in decadal predictions and their correlations with the corresponding univariate extremes. We use a multi-model ensemble (MME) of 125 members from the CMIP6 Decadal Climate Prediction Project (DCPP) hindcast simulations and compare it with different observational references. Our analysis focuses on the average of forecast years 2 to 5, with the different forecasts initialised every year from 1960 to 2014. We analyse the skill of predicting hot, dry and compound hot-dry events in the MME. Specifically, we select the days above the 90th percentile of the daily maximum temperature for hot events (tx90p). For dry events, we use two indicators, based on the Standardised Precipitation Index (SPI) and the Standardised Precipitation Evapotranspiration Index (SPEI), with accumulation periods of 3, 6 and 12 months, and we consider a dry event a month when the SPI or SPEI value ≤ -1. Finally, we identify days that present both hot and dry conditions according to these criteria as compound hot-dry days (HDSPEI3,-6,-12 and HDSPI3,-6,-12).
For the univariate extremes, results show strong skill for hot extremes, to a large extent driven by the warming trend. On the contrary, dry extremes show less uniform skill, with isolated areas of significant correlations. We find regional skill in some areas of the globe for hot-dry compounds. Through a comparison with historical simulations, we find that initialisation of the predictions leads to additional skill in localised regions, with most areas showing non-significant differences. However, the models still appear to underestimate the connections between compound and univariate extremes, especially between hot-dry compounds and dry conditions. These results show the potential and the limitations of predicting compound events in decadal forecasts, highlighting the pivotal role of dry conditions in assuring a skilful prediction.
How to cite: Aranyossy, A., Donat, M., De Luca, P., Delgado-Torres, C., and Solaraju-Murali, B.: Hot, dry and compound hot-dry extremes in decadal predictions, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-507, https://doi.org/10.5194/ems2024-507, 2024.
The western Mediterranean region experienced an exceptional and unprecedented early heatwave in April 2023, shattering historical temperature records, particularly in the Iberian Peninsula and northwestern Africa. This event serves as a stark illustration of a drought–heatwave compound event. In this study, we explore the soil moisture–temperature interactions that underpinned this event, using the most up-to-date observations and robust statistical analysis. Our findings reveal that soil moisture deficit preconditions, concurring with a strong subtropical ridge as a synoptic driver, significantly contributed to the amplification of this record-breaking heatwave. Specifically, we estimate that the most extreme temperature records would have been 4.53 times less likely and 2.19°C lower had the soils been wet. The dynamics of the land-atmosphere interactions during the heatwave showed a pronounced soil moisture-temperature coupling, with reduced soil moisture leading to increased sensible heat flux, which significantly increased air temperatures. Employing the flow analogues technique, we further demonstrated that similar atmospheric conditions in the past, under dry soil moisture conditions, consistently resulted in higher temperature extremes compared to those under wet conditions.
Importantly, our results indicate that in terms of land–atmosphere coupling during extreme events, semiarid regions in spring can behave like temperate regions during summer when soil moisture content is more variable. Therefore, our findings suggest that in these semiarid regions, including other Mediterranean climate regions where hot–dry compound events are aggravating (e.g., California, central Chile), soil moisture may be a good diagnostic of spring heatwave risk and hold potential for subseasonal heatwave forecasting.
Reference:
Lemus-Canovas, M., Insua-Costa, D., Trigo, R.M. et al. Record-shattering 2023 Spring heatwave in western Mediterranean amplified by long-term drought. npj Clim Atmos Sci 7, 25 (2024). https://doi.org/10.1038/s41612-024-00569-6
How to cite: Lemus-Canovas, M., Insua-Costa, D., Trigo, R. M., and Miralles, D. G.: Tracing the impact of drought on the Western Mediterranean's record-shattering Spring 2023 heatwave, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-1010, https://doi.org/10.5194/ems2024-1010, 2024.
Eastern China experienced persistent regional extreme heatwaves in the summer of 2022, with disparate spatial features and formation mechanisms in different months. We quantitatively assessed the relative contributions of three oceans, i.e., tropical Indian Ocean and Pacific and North Atlantic, and the local soil moisture–temperature feedback using linear regression. The results showed that the monthly mean atmospheric circulation anomalies failed to explain the extreme heatwave in June 2022. The combined contribution of the tropical Indo-Pacific and North Atlantic sea surface temperature anomalies (SSTAs), together with the local soil moisture–temperature feedback, explaining approximately 10% of the temperature anomalies. In July, the tropical Indo-Pacific SSTAs promoted anomalous atmospheric circulation and extreme heat via meridional circulation originating in the Maritime Continent, accounting for approximately 10% of the temperature anomalies, with North Atlantic SSTAs contributing the same percentage by a mid-latitude steady Rossby wave. Local soil moisture–temperature feedback accounted for 42% of the anomalies. The tropical Indo-Pacific SSTAs produced a strong western North Pacific anticyclone in August, but their direct contribution to the temperature anomalies was negligible. The North Atlantic SSTAs contributed 9% of the total via the mid-latitude steady Rossby wave. Local soil moisture–temperature feedback contributed 66%, suggesting that the July heatwave and drought exerted a significant impact on the subsequent August extreme heatwave. Global warming has greatly facilitated extreme heatwaves, accounting for about 30%–40% of these events in summer 2022. These results also suggest that the climatic effects of tropical Indo-Pacific and North Atlantic SSTAs on Eastern China are evident in the month-to-month variation in summer. Our results thus contribute to the understanding and prediction of extreme heatwaves in Eastern China.
How to cite: Jiang, J., Liu, Y., Mao, J., and Wu, G.: Extreme heatwave over Eastern China in summer 2022: the role of three oceans and local soil moisture feedback, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-220, https://doi.org/10.5194/ems2024-220, 2024.
Storm Ciarán was an impactful weather system that affected western Europe in the first days of November 2023. The main threat from Ciarán were extreme winds, that particularly hit the areas around the English Channel: the highest wind gust recorded was 207 km/h at Pointe du Raz, in Bretagne (France), just as the storm was rapidly deepening. In this work, we discuss some dynamical characteristics of storm Ciarán related to the occurrence of those extreme winds: its origin as a diabatic Rossby wave, and its evolution as a warm-seclusion cyclone with the development of a sting jet.
Diabatic Rossby waves (or vortices) are a particular type of cyclones driven by latent heat release along a baroclinic zone. These shallow systems have a smaller scale than usual extratropical cyclones, and are driven by the low-level vorticity generation that results from intense, localized condensational heating. Ciarán’s evolution resembled the infamous storm Lothar (1999), because both underwent explosive intensification starting from the diabatic Rossby wave stage, and both resulted in extreme winds.
Rapidly deepening cyclones that develop a warm seclusion are also associated with strong surface winds, as they can provide a favorable environment to the development of sting jets. A sting jet is an airstream that accelerates as it descends out of the tip of the hooked cloud head, generating strong low-level winds into the frontal-fracture region of the cyclone. Storm Ciarán indeed originated as a diabatic Rossby wave and then developed into a rapidly deepening warm-seclusion cyclone containing a sting jet. We thus illustrate its evolution, highlighting the occurrence of a “diabatic Rossby wave-sting jet” dynamical pathway leading to the generation of high-impact winds.
How to cite: Riboldi, J. and Volonté, A.: The dynamics of Storm Ciarán (2023): from diabatic Rossby wave to warm-seclusion cyclone with a sting jet, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-572, https://doi.org/10.5194/ems2024-572, 2024.
The growing increase in frequency and intensity of extreme weather events (EWEs) has a wide impact on energy systems and consumers, such as energy transmission infrastructures - namely overhead power lines (OPL). The main objective of this work is to assess recent high-impact storms and to present the methodology of risk analysis of the extreme weather events on power outages in Portugal. The considered events are windstorms, snowstorms and heavy precipitation associated with extratropical cyclones. In this study, a comprehensive assessment of these events, synoptic conditions and large-scale dynamics are investigated using the ERA5 reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). Furthermore, the level of risk associated with each of the identified events is classified according to the probability of occurrence and consequences, in a risk matrix, and through the cause-and-effect analysis. It is shown that, in Portugal, the extreme wind – corresponding to level 11 of the Beaufort Wind Force Scale, that is, values equal to or higher than 105 km h-1 – is the main factor that provoked the OPL disruption, between 28% and 40% of analyzed events associated with windstorms. Considering the occurrence of compound events - wind and rain - the probability of damage to OPL is between 21% and 30%; for wind and ice, it is 3%–5%. EWEs represent increasing serious risk for electrical systems, and thus this study highlights the need to develop effective solutions to minimize the associated impacts, such as the modification and upgrade of the current design and engineering standards, and electrical network monitoring.
References:
Gonçalves, Ana, Margarida Correia Marques, Sílvia Loureiro, Raquel Nieto, and Margarida L.R. Liberato (2023) "Disruption Risk Analysis of the Overhead Power Lines in Portugal", Energy, 263, 125583. https://doi.org/10.1016/j.energy.2022.125583
Gonçalves, A.C.R., Nieto R., Liberato M.L.R. (2023) "Synoptic and Dynamical Characteristics of High-Impact Storms Affecting the Iberian Peninsula during the 2018–2021 Extended Winters", Atmosphere, 14, 1353. https://doi.org/10.3390/atmos14091353
Acknowledgements:
This work is supported by the Portuguese Science and Technology Foundation through the projects WEx-Atlantic (PTDC/CTAMET/29233/2017, LISBOA-01-0145-FEDER-029233, NORTE-01-0145-FEDER-029233) and UID/GEO/50019/2019. FCT is providing for Ana Gonçalves doctoral grant (2021.04927.BD). This work is supported by national funds by FCT - Portuguese Foundation for Science and Technology, under the project UIDB/04033/2020
(https://doi.org/10.54499/UIDB/04033/2020). The EPhysLab group is also funded by Xunta de Galicia, Consellería de Cultura, Educación e Universidade, under project ED431C 2021/44 “Programa de Consolidación e Estructuración de Unidades de Investigación Competitivas.
How to cite: Liberato, M. L. R., Gonçalves, A., Loureiro, S., Nieto, R., and Correia Marques, M.: Power disruption risk analysis in Portugal associated with extratropical cyclones, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-636, https://doi.org/10.5194/ems2024-636, 2024.
Downslope windstorm (DW) is a mesoscale phenomenon with considerable impact on nature and human activities in mountainous regions. It is characterised not only by the descent of air down the lee slopes but also by wind acceleration on the windward side of a mountain. Climate studies of such complex and local events on larger scale are rare. In this work, a high-resolution climate model is used to study the DW characteristics in the present and future climate in the Scandinavian mountains.
The HARMONIE-Climate (HCLIM) regional climate model is used on two nested grids with horizontal grid spacing of 12 and 3 km. The DW’s are identified using a two-step algorithm. First the favorable terrain for DW’s is marked, followed by the identification of DW events in these areas based on stability, horizontal and vertical wind speed. Synoptic circulation types (CT) are used to distinguish DW’s according to different large-scale environments. The Scandinavian mountains are divided into four subregions, and the analysis is focused on winter, which exhibits the highest frequency of DW’s.
The simulation at 3-km grid spacing agrees considerably better with observations in DW-prone regions, indicating that such high resolution is needed to reproduce strong DW’s. The CT’s with highest frequency of DW occurrence are those with strong cross-mountain pressure gradient. For every CT, DW’s were additionally grouped as either cold or warm winds in the lee of a mountain, distinguishing between bora-like (cold) and foehn-like (warm) DW’s. Due to the considerable terrain complexity of the Scandinavian Mountains, a straightforward link between CT’s and the DWs’ effect on local warming or cooling is not evident. Results show that even in a single grid point DW’s can cause warming and cooling regardless of a CT or wind direction. We focus on one such exemplary grid point to highlight how different synoptic settings can lead to very similar local conditions but with opposite DW character. Additionally, insights into future climate projections of DW’s are presented.
How to cite: Jureša, P., Belušić, D., Hansen, F., and Marimbordes, S.: Downslope windstorms in the Scandinavian mountains from a kilometer-scale regional climate model, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-719, https://doi.org/10.5194/ems2024-719, 2024.
This study investigates convective wind gusts measured by National Meteorological Institute (INMET, in Portuguese) operational automated surface weather stations network in southern Brazil from 2005 to 2015. Only events unequivocally associated with deep convective storms were selected, as evaluated with a set of criteria that include the employment of geostationary satellite data. Wind gusts exceeding 25 ms -1 are categorized as severe, while non-severe gusts with moderate intensity range from 15 ms -1 to 24.9 ms -1, and weaker gusts from 10 ms -1 to 14.9 ms -1.
Convective parameters computed from the Fifth Generation of the European Centre for Medium-Range Weather Forecasts Reanalysis (ERA5) are used to investigate the atmospheric conditions conducive to severe wind gusts. Emphasis is placed on assessing the ability of convective parameters to discriminate between severe and sub-severe wind gust environments. The results indicate that the mean wind computed from the surface to the 6 km layer was the only parameter that showed some discriminatory ability. Overall, convective parameters alone are unable to distinguish environments conducive to the formation of storms capable of generating intense winds from those generating weak wind gusts. This result aligns with ingredient-based analysis, as individual parameters alone do not encompass all the necessary ingredients required for the development of these storms.
Therefore, we evaluate the combined product of ERA5-based convective available potential energy (CAPE) and bulk wind difference in the 0-6 km layer (BWD). The results indicate that as the gust intensity increase, the high CAPE high shear space gradually becomes more preferable. In other words, there is a tendency for more severe storms to cluster in the sector of the diagram where both CAPE and BWD values are moderate to high.
How to cite: Ferreira, V., de Lima Nascimento, E. D. L., and de Oliveira dos Santos, L.: Atmospheric environmets of convectively generated wind gusts in Southern Brazil, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-833, https://doi.org/10.5194/ems2024-833, 2024.
Extreme storm events have a profound impact on aviation operations, often leading to significant damages and hazardous flying conditions. This study focuses on four prominent storms that affected both Europe and the USA between February 2022 and June 2023, resulting in widespread disruptions to aviation services.
By leveraging historical and contemporary data on sea level pressure (SLP), this research applies the attribution methodology outlined by Yiou \cite{Yiou} to compute weather analogues. By means of 6-hourly ERA5 data-sets spanning from 1950 to 2023, split into two distinct 35-year periods—[1950–1984], denoting a factual period, and [1988–2023], representing a counterfactual period — we identify the cyclone time for each event, designated as the time when cyclones associated with the storms reach their lowest SLP. Thirty best analogue cyclones are selected within defined spatial domains for each period, encompassing all months of the year in one case and only the months of the involved season in the other.
Composite maps of SLP and geopotential height anomalies are computed for the time-steps corresponding to the found analogues, assessing differences between the two periods. The analysis extends to precipitation and eddy dissipation rate (EDR), a crucial indicator to assess the level of air turbulence during flights.
These comprehensive analyses aim to provide a comprehensive assessment of atmospheric conditions during the studied extreme events, to enhance understanding of the atmospheric dynamics of extreme storm events and their implications for aviation safety, flight operations, and air traffic management. The findings are crucial for developing strategies to mitigate risks and improve adaptation for future storm events in the aviation industry.
How to cite: Rapella, L., Alberti, T., and Faranda, D.: Quantifying the Impact of Extreme Storm Events on Aviation Operations over Europe and the USA, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-303, https://doi.org/10.5194/ems2024-303, 2024.
Event identification and tracking tools have become essential in weather and climate research due to the rapid growth of weather and climate model data. These algorithms automate the detection of extreme weather-related events and hazardous phenomena and their corresponding features, thereby enhancing predictive capabilities. Our study introduces an open and adaptable framework designed for the automated identification and tracking of various hazard types from a Lagrangian perspective, spanning multiple spatial and temporal scales and providing insights into the dynamics and behavior of hazardous events. This framework is versatile and applicable to a range of scenarios from historical hazard analyses to future climate scenarios evaluation. Additionally, this work facilitates the investigation of weather patterns that give rise to the hazards and enables the assessment of their impacts. The algorithm relies on connected components and distance thresholds to identify and track events in space and time, permitting event merging and splitting dynamics over time. Furthermore, it characterizes events based on diverse spatio-temporal features such as duration, volume, intensity, and trajectory. The framework includes visualization tools for displaying event trajectories. It enables statistical analyses of detected time series derived from historical records and climate model outputs (e.g., identifying trends, exploring correlations between feature pairs, and analyzing feature distributions). To prove the framework's efficacy and versatility, we applied it to detect major hazards including heatwaves, cold spells, heavy precipitation events, hydrological droughts, and even combinations of these events identified as compound events. Leveraging ERA5 reanalysis, satellite data, and radar data we constructed an extended historical catalog of these hazards in France and conducted statistical analyses to characterize event properties and structures. Subsequently, applying the framework to Cordex data allowed us to assess potential future hazard scenarios.
How to cite: Cazzaniga, G., Bonhomme, L., Burq, A., Vrac, M., and Faranda, D.: Automated identification and tracking framework for hazard cataloging, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-697, https://doi.org/10.5194/ems2024-697, 2024.
We examine the complex meteorological processes involved in the development of multiple dust storms in the middle of March 2022 over North Africa by means of multi-level high-resolution reanalysis datasets, and satellite and surface observations. These dust storms ultimately transported a remarkable amount of dust into Europe, and in particular to the Iberian Peninsula.
The analyses show that baroclinic Rossby Wave Breaking (RWB) events over two regions: the eastern North Atlantic and the western Eurasian border, resulted in the creation of a downscale confluence of low-level jets that ablated dust in the Sahara Desert. One of these active baroclinic regions involved multiple wave-breaks over western Russia and the Middle East that transported unseasonably cold air southwestward into Egypt and Sudan. A second active baroclinic region occurred along the North African Atlantic Coast where rapid downstream dispersion of energy was focused in response to very intense cyclogenesis over Labrador. The cold pool which propagated into northwestern Africa and cold air over northeastern Africa effectively sandwiched hot Saharan air in between the Hoggar and Atlas Mountain Ranges. The fronts separating these air masses became juxtaposed with the two mountain ranges creating ideal conditions for blocking and multiple barrier jets’ genesis. These barrier jets each combined with larger scale semi-geostrophic low-level jet formation, resulted in the ablation of dust into two separate large plumes both of which were transported poleward out of North Africa into Europe around a upper-level cutoff-low near Iberia slowly displacing downstream.
The blocked RWB forcing the penetration of cold air over eastern Africa constitutes a substantial difference with respect to other more frequent events where RW trains intensify, penetrate equatorward into North Africa and propagate eastward.
Work funded by MCIN/AEI/ 10.13039/501100011033 under Grant PID2020-115153RB-I00 (rROSSETA Project).
How to cite: G. Orza, J. A. and Kaplan, M. L.: Multi-scale analysis of the meteorological processes forcing African dust ablation and transport in the extreme mid-march 2022 case, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-986, https://doi.org/10.5194/ems2024-986, 2024.
High-concentration Asian dust storm (ADS) events significantly increase atmospheric particulate matter concentrations, adversely affecting respiratory health and causing substantial economic losses in environmental and agricultural productivity. Understanding the mechanisms, pathways, and behavior patterns of ADS is essential for providing early warnings for periods and regions where severe dust events are anticipated, enabling appropriate preventive measures. This research aims to identify key meteorological variables and classify synoptic patterns of high-concentration sand dust events (dust warnings) during the beginning stages at the source regions and the peak concentration stages observed over Korea, using unsupervised learning methods such as Principal Component Analysis (PCA) and K-means clustering. The study analyzed the ADS cases observed from 2002 to 2022, utilizing the ECMWF reanalysis data (ERA5). The results indicate that during the origin stage, the primary meteorological variables were the geopotential height and temperature at lower layers (900-1000hPa), while temperature and humidity were key variables during the peak concentration observation stage. Each stage was classified into five cluster patterns. Temporal analysis of these patterns revealed that most ADSs occurred more frequently at night, except for pattern 4 that was predominantly observed during daytime, and pattern 5 that was exclusively observed at night. During the peak concentration stage, three out of five patterns showed more frequent occurrences at night, while patterns 1 and 4 had more frequent daytime occurrences, and pattern 5 was predominantly observed at night. This research contributes to the improvement of high-concentration ADS prediction by classifying and analyzing the synoptic patterns related to their occurrences.
How to cite: Lee, S. and Park, S. K.: Identification of Key Meteorological Variables and Synoptic Patterns of High-Concentration Asian Dust Storms, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-771, https://doi.org/10.5194/ems2024-771, 2024.
The region of Emilia Romagna in northeastern Italy, with its capital in Bologna, experienced a sequence of flooding events on May 2nd, 10th, and 16th, 2023. These three events were linked to the passage of three extratropical cyclones, identified as Mediterranean cyclones, over the same region within a short period. We investigate the influence of climate change on each of these cyclones using an Extreme Event Attribution framework.
We use the analogues approach to identify similar cyclones in two periods characterized by weak and strong climate change. Cyclones are defined using a multivariate method based on sea level pressure, surface wind speed, and precipitation rate. We first normalize the fields via quantile normalization. We then calculate the spatially averaged Euclidean distance between the cyclones and other time steps within the phase space defined by the normalized sea level pressure, wind speed, and precipitation rate. Analogues are identified as those with the minimum Euclidean distance.
We assess detected changes using daily ERA5 reanalysis data, comparing the meteorological hazards of the analogues of the past period (1950–1985) with the present (1987–2022). To evaluate future changes, we conduct a multi-model study with high-resolution regional models within the EURO-CORDEX program, comparing historical (1970–2000) with future projections (2070–2100). We found an increase in precipitation associated with two of the cyclones during the present period compared to the past. Future projections also indicate an increase in precipitation compared to the present climate. Hence, our findings suggest a possible role of climate change in increasing the severity of such cyclones.
How to cite: Ginesta, M., Lu, C., Coppola, E., Yiou, P., and Faranda, D.: Climate change fingerprint on the 2023 Emilia Romagna floods, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-448, https://doi.org/10.5194/ems2024-448, 2024.
This study investigates the dynamics and thermodynamics driving sub-hourly heavy precipitation events (SHHPs) associated with Regional Low-pressure (RegL) systems in mainland Portugal. SHHPs are significant natural hazards, impacting various sectors such as agriculture, particularly viticulture. Using observations from automated surface weather stations from the Portuguese Weather Service (IPMA) from 2000 to 2022, with a 10-minute temporal resolution, this research focuses on three representative stations across different climatic regions. Viana do Castelo station represents the northernmost region, known for high precipitation and strong Atlantic influence. Castelo Branco station, in the central region, is less exposed to Atlantic systems and more influenced by convective systems from western Spain. Faro station, in the south, experiences less influence from maritime polar air masses and more from maritime tropical air masses or Mediterranean/North African systems. The southern region of Portugal demonstrates higher precipitation variability, characterized by intense events occurring on fewer rainy days. The study establishes a link between SHHPs and low-pressure systems west of the Iberian Peninsula (IP). These systems exhibit a cold core, particularly evident at mid-levels, and feature a positive vorticity anomaly extending from the upper troposphere to lower levels. This configuration induces differential positive vorticity advection, especially increasing with height, developing upward motion east of the low-pressure systems over western Iberia. Moreover, these systems facilitate moisture advection over western Iberia at lower levels, evident in positive anomalies of 2-meter dew point temperatures, and promote instability conditions, as diagnosed by various instability indices. Additionally, higher values of total column cloud ice water during heavier precipitation events suggest its potential as a predictor for such occurrences. The study also suggests a potential association of some SHHPs with cut-off lows, though this hypothesis requires further validation. Overall, this research provides a systematic assessment of atmospheric conditions driving SHHPs, contributing to a better understanding of these hazardous events in mainland Portugal.
Acknowledgments: Research funded by National Funds by FCT – Portuguese Foundation for Science and Technology, under the project UIDB/04033/2020 and LA/P/0126/2020 (https://doi.org/10.54499/UIDB/04033/2020). Vine & Wine Portugal—Driving Sustainable Growth Through Smart Innovation, PRR & NextGeneration EU, Agendas Mobilizadoras para a Reindustrialização, Contract Nb. C644866286-011.
How to cite: Cruz, J., Belo-Pereira, M., Fonseca, A., and Santos, J.: Exploring dynamic and thermodynamic drivers associated with heavy sub-hourly precipitation events in Portugal, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-140, https://doi.org/10.5194/ems2024-140, 2024.
The impact of El Niño on the Hadley Circulation (HC) has been a topic of previous studies, but the results have been inconclusive. We study how El Niño affects the HC during different stages of its cycle. In development years, the HC anomaly shows an equatorial quasi-symmetric pattern, while in decay years, it shows an asymmetric pattern. And it is shown a transition in the variability of the first two modes of the HC during these stages, with the first mode exhibiting a larger explained variance in the decaying stage. The regime change in HC variability corresponds to underlying anomalous SST distributions, as confirmed by sensitive experiments. The differences in tropical SST during different stages of El Niño cause differences in SST meridional gradients, which determine the location of convergence. This explains why the HC anomalies have different spatial structures during El Niño development and decay years. Quantitative assessment reveals stronger HC-SST response amplitudes during the decaying stage compared to the development stage. Employing the Kuo-Eliassen (KE) equation, diabatic heating anomalies during the decaying stage explain the difference in air-sea response intensity between the two stages. Diabatic heating variations are identified as the primary contributor to amplification or reduction of air-sea response intensity during the respective El Niño stages, providing insights into the different air-sea processes throughout the El Niño lifespan. Our results show that the meridional distribution of SST during different El Niño stages has significantly distinct impacts on meridional circulation and clarify the differences in El Niño's effects on climate.
How to cite: Feng, J., Ji, X., Li, J., Chen, X., and Wang, C.: A quantitative explanation for the different Impacts of El Niño Development and Decay Stages on the Hadley Circulation, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-402, https://doi.org/10.5194/ems2024-402, 2024.
The tropical Atlantic, as a vital component of the tropical oceans, exerts significant influence on both local and global climates through its sea surface temperature variability and interactions among tropical basins. We found a significant influence of the sea surface temperature anomaly over tropical south Atlantic (TSA-SSTA) on ENSO Events and the Meiyu onset date. A significant negative correlation between the preceding TSA-SSTA and subsequent summer ENSO events was revealed. The warm (cold) TSA-SSTA corresponds to the following summer La Niña (El Niño)-like SSTA in the tropical Pacific. Warm TSA-SSTA can trigger local anomalous upward motion, resulting in precipitation and positive Walker circulation anomalies in the equatorial Pacific. This configuration facilitates the strengthening of easterly anomalies in the lower level of the equatorial Pacific, further promoting the development of La Niña-like SSTA in the central to eastern equatorial Pacific. These mechanisms and conclusions remain valid even after linearly removing the signal of the preceding winter ENSO. At the same time, when TSA-SSTA is warm the Meiyu onset date tends to be early. This is because that warm TSA-SSTA triggers significant positive Gill responses in the equatorial Pacific, further generating local anticyclonic circulation anomalies over the Maritime Continent of the northwest Pacific. Under the combined effects of large-scale forcing and the "precipitation-circulation Sverdrup positive feedback" mechanism in the northwest Pacific, anomalous anticyclones can develop and be sustained, resulting in an earlier Meiyu onset date. Furthermore, the interannual relationship between the TSA-SSTA and ENSO also exhibits decadal variability, closely related to the Pacific Decadal Oscillation (PDO).
How to cite: Liu, Y., Zhang, S., Ma, T., Sheng, C., and Dong, B.: The connection between sea surface temperature anomaly over tropical south Atlantic and ENSO and the associated impact on East Asian climate, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-403, https://doi.org/10.5194/ems2024-403, 2024.
Most tropical cyclones (TCs) generated over the eastern North Pacific (ENP) do not make landfall. Consequently, TCs in this basin have received less attention, especially those that occur away from the mainland. Furthermore, there have been few studies of the climatic effects of ENP TCs. This study explores the feedback relationship between ENP TCs and the intensity of the El Niño–Southern Oscillation (ENSO), including El Niño and La Niña events, from the perspective of accumulated cyclone energy (ACE). Observational and modeling results indicate that the ENP ACE 3 months earlier can still affect the intensity of El Niño and La Niña events, although the SST persistence is main contributor. Thereinto, the impact of ENP TCs on El Niño appears to be approximately equal to that on La Niña. Moreover, this impact is independent of the persistence of the sea surface temperature (SST) in the Niño 3.4 region and the Madden–Julian Oscillation. Generally, the greater the ENP ACE, the stronger the El Niño, and the smaller the ENP ACE, the stronger the La Niña, this is especially the case for those TCs that develop over the July‒September period. In addition, results show that the ENP TCs modulate ENSO intensity by changing anomalous zonal wind at the low-level atmospheric layer. And the joint impacts of the low-level zonal wind anomalies on the Walker circulation and the east-west thermocline gradient lead to the time characteristics that ENP TCs lead ENSO intensity by about 3 months. This study proposes a cross-time-scale role for tropical cyclones in the development of ENSO, further expanding the theoretical framework for ENSO's development, and offering significant scientific relevance and practical value for its prediction.
How to cite: Wang, Q. and Tan, Z.-M.: Impact of tropical cyclones over the eastern North Pacific on El Niño–Southern Oscillation intensity, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-513, https://doi.org/10.5194/ems2024-513, 2024.
Tibetan Plateau (TP) has the highest elevation worldwide, and functions as an important modulator of both regional and global climates. Known as Asia’s water tower, TP is the headwater area of some major rivers, including the Yangtze River, Yellow River, Mekong River, Brahmaputra River, Ganges River, and Indus River. Precipitation over the TP exerts a great impact not only on local water resources but also on those downstream. Thus, variations in TP precipitation, especially their spatial pattern, are worth further study. In recent decades, the dipole pattern trends in precipitation over the TP has raised concerns, i.e., opposite precipitation trends in the northern TP (NTP) and in southern TP (STP). However, the physical processes of the variations in TP precipitation, especially the roles of local mesoscale systems, Tibetan Plateau vortices (TPVs), are not yet clear. In this work, dipole pattern variations are found in TPVs-associated precipitation, which experienced an interdecadal shift in the last two decades, that is, increases in NTP and decreases in STP first and then varies opposite trends. The interdecadal shift in the TPVs-associated precipitation trends greatly contributes to the interdecadal shift in the TP precipitation trends. Furthermore, the causes of variations in TPV frequency are explored, and find that the changes in zonal winds over the TP and the meridional winds across the northwestern TP boundary are closely related to the regionally different variations in TPV frequency. Subsequently, the changes in the sea surface temperature of India Ocean are demonstrated to be responsible for the wind variations.
How to cite: Li, L. and Zhang, R.: Interdecadal shift in dipole pattern precipitation trends over the Tibetan Plateau: Roles of local vortices, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-99, https://doi.org/10.5194/ems2024-99, 2024.
The source of potential vorticity (PV) substance (PVS) of the atmosphere is at the earth’s surface, whereas the PVS variation within a limited region depends on the sum of the effective PV flux crossing the boundary surface surrounding the region. Because the PV circulation (PVC) is the time- integral of effective PV flux, the monthly mean gross PVS in the northern troposphere is determined by the sum of PVC penetrating the enveloped three boundaries, e. i., the PVC cross- the equatorial vertical section CEPVC, the PVC cross- the tropopause CUPVC, and the PVC cross- the earth’s surface CBPVC. Because the CBPVC is associated with surface potential temperature and circulation, for an equilibrium gross PVS in the hemisphere, the change in CEPVC or/and CUPVC can induce the climate change near the surface through the adjustment of the atmospheric internal PVC changes.
Based on reanalysis data and by employing statistics and diagnostics, the characteristics of the July mean PVS and PVC in the northern troposphere, and the relationship between the Asian monsoon and the PVC crossing its three boundaries in July are investigated. Results demonstrate that, corresponding to a CBPVC pattern with negative (positive) PVC over the north (south) on the Tibetan Plateau, strong positive PV anomalies and westerly flows develop in the troposphere over the plateau， leading to increased precipitation in the downstream area along the Meiyu front. The combination effects of the CEPVC and CUPVC can trigger the adjustment of the troposphere internal PVC, resulting in PVS and circulation redistribution within NH and forming a meridional seesaw of precipitation over the East Asian summer monsoon area. Attribution diagnoses reveal that the tropical sea surface temperature anomaly contributes significantly to the CEPVC, whereas the meridional gradient of PVS over the northwestern Tibetan Plateau on the tropopause contributes more to the CUPVC.
How to cite: Wu, G., Liu, Y., He, B., Sheng, C., and Li, Y.: Potential Vorticity Circulation Crossing the Boundaries of Northern Troposphere and the East Asian Monsoon in July, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-224, https://doi.org/10.5194/ems2024-224, 2024.
Arctic sea ice has undergone a significant decline in the Barents–Kara Seas since the late 1990s. Previous studies have shown that the decrease in sea ice caused by increased poleward moisture transport is modulated by tropical sea temperature changes (mainly referring to La Niña events). The occurrence of multi-year La Niña events has increased significantly in recent decades, and their impact on Arctic sea ice needs to be further explored. In this study, we investigate the relationship between sea ice variation and different atmospheric diagnostics during multi-year La Niña and other La Niña years. The decline in BKS sea ice during multi-year La Niña winters is significantly stronger than that during other La Niña years. It is because the multi-year La Niña tends to accompany the warm Arctic-cold continent pattern with a barotropic high-pressure blocked over the Ural region. Consequently, more frequent northward atmospheric rivers intrude into the BKS, intensifying long-wave radiation downward to the underlying surface and melting sea ice in the Barents–Kara Seas. However, in the early mature winter of other La Niña cases, negative North Atlantic Oscillation presents in the North Hemisphere high latitudes, which obstructs the atmospheric rivers to the south of Iceland. We infer that such a different response of sea ice decline in Barents–Kara Seas to different La Niña is related to stratospheric processes. Considering likely future climate change, more frequent multi-year La Niña events may account for substantial Arctic sea ice loss in recent decades. In particular, it was proposed that the multi-year La Niña probably occur more frequently in a warming climate.
How to cite: Zhong, W.: Wintertime Arctic Sea Ice Decline Related to Multi-Year La Niña Events, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-89, https://doi.org/10.5194/ems2024-89, 2024.
During winter months, the high-latitude stratospheric circulation is characterized by a prominent eastward flow, also known as the stratospheric polar vortex. However, this flow pattern is regularly disturbed by planetary waves (PW) originating from the troposphere, that occasionally cause either a temporary weakening or a complete reversal to a summer-like westward flow regime. Such events are called minor and major stratospheric sudden warmings (SSWs), correspondingly. Major SSW events tend to take place approximately every two years on average.
In winter 2023/2024, the stratospheric polar vortex was weak and highly variable. Three major SSWs were detected based on the standard definition at 10 hPa in the period between January and March 2024. Two short duration events (under 9 days) and one long duration event (over 20 days) occurred in mid-January, mid-February and early March, respectively. This number of SSW events within a single winter is notably uncommon. In this study, we investigate the dynamical evolution of the polar stratosphere, the tropospheric forcing and precursors, and the surface impacts throughout the winter, with a focus on the first two short-duration events. The study is mostly based on the fifth generation ECMWF reanalysis (ERA5) data.
It has been shown in previous studies that synoptic evolution of blocking highs and anomalous lows in the upper troposphere conditions the duration of SSW events. For the short SSW event in January, we show that blocking highs developed over the Euro-Atlantic region were indeed responsible for enhanced planetary waves activity prior to the SSW central date. Later an anomalous, westward-propagating High over the North Pacific suppressed the zonal-mean PW activity flux into the stratosphere and caused a quick termination of the event. Despite a similar development timeline, the second short-duration SSW event in February had a different tropospheric forcing. In this case, we investigate the role of snow accumulation in creating an increased meridional heat flux over Ural Mountains. Finally, a canonical long-duration Major SSW developed in early March.
How to cite: Vorobeva, E. and Orsolini, Y.: A series of major sudden stratospheric warming events in winter 2023/2024, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-163, https://doi.org/10.5194/ems2024-163, 2024.
Societies across the Mediterranean and wider European region can be significantly affected by weather extremes such as heatwaves, cold spells, windstorms and intense rainfall events. Temperature extremes are connected to atmospheric blocking, while heavy precipitation and windstorms have been associated with Mediterranean Cyclones (MCs). However, the connection between atmospheric blocking and MCs is still understudied, despite evidence suggesting their mutual importance for cyclone development and for exacerbating and synchronizing surface extremes. The aim of the present study is the systematic investigation of the frequency of MC development downstream of atmospheric blocks over the Euro-Atlantic region, and to examine how different cyclone track characteristics along with cyclone-attributed-precipitation might be modulated under such conditions. To this end we employ the combined ‘best tracks’ MCs dataset with objectively identified (using potential vorticity anomalies) blocking features in ERA5 for the 1970-2020 period. We find that in the presence of atmospheric blocks, MCs that develop downstream tend to be more intense and to be associated with increased precipitation compared to other MCs. Moreover, under blocking conditions, the distribution of precipitation varies geographically between the north-west and south-east Euro-Mediterranean region, with moisture transport contributing to this difference. MCs developing under this scenario form particular subsets of MCs, with preferred seasonality and geographical distribution, compared to all MCs. Lastly, certain MCs tend to be more static, while their mobility exhibits variability under blocking conditions. These results have important implications for the predictability of MCs and their various impacts in the region on both weather and climate time scales.
How to cite: Loizou, P. and Raveh-Rubin, S.: Atmospheric blockings and downstream cyclones in the Euro-Mediterranean sector, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-102, https://doi.org/10.5194/ems2024-102, 2024.
Rossby Wave Breaking (RWB) events describe the last stage in the life cycle of baroclinic atmospheric disturbances. These breaking events can strongly influence the large-scale circulation and are tightly related to low-frequency weather regimes. In addition, RWBs are often associated with weather extremes such as heat waves, blockings, and extreme precipitation events. Here we examine the three-way interaction between RWB, weather regimes, and surface weather in the North Atlantic. This is done by combining a RWB detection algorithm, a storm-tracking routine, and a clustering technique to identify low-frequency circulation regimes in the North-Atlantic. We find that regardless of weather regime, most cyclones and anticyclones are associated with an Anticyclonic Wave Breaking (AWB) and/or a Cyclonic Wave Breaking (CWB) at some point during their lifetime, while very few storms do not involve any upper-level wave breaking (~11%). Moreover, storm characteristics (e.g., actual and relative positions, intensities, and displacements) differ depending on the associated breaking type. In “same-pairing” cases (i.e., cyclones with CWB and anticyclones with AWB) the surface system is positioned so that its associated upper-level winds would enhance the breaking (the anomalous circulation is in the same direction as the background shear). In “opposite-pairing” cases (i.e., cyclones with AWB and anticyclones with CWB), the upper-level winds associated with the surface system do not act to enhance the breaking which occurs in the direction of the background shear. In addition, we find that the surface storm characteristics are significantly altered with the weather regime, with distinct and clearly preferred storm paths in each cluster. We suggest a picture in which the resulting RWB frequencies and positions in each cluster are modified by the corresponding tracks of cyclones and anticyclones, with the maximum breaking found where there is a constructive interaction with the low-frequency flow. The positions of RWBs, in turn, shape the overall cluster structure and contributes to the persistence of the weather regime. An improved understanding of the relation between weather systems, RWB events, and weather regimes can also help us improve our understanding of and confidence in projected future circulation changes (e.g., by relating changes in the frequency and positions of RWB events, storm-tracks, and the North-Atlantic jet).
How to cite: Tamarin Brodsky, T. and Harnik, N.: The three-way feedback between North-Atlantic circulation regimes, Rossby wave breaking, and surface weather., EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-917, https://doi.org/10.5194/ems2024-917, 2024.
After establishing the link between Gulf Stream heat transport and subsequent atmospheric blocking in Mathews and Czaja (2024), revealing nearly double the amount of blocking over Greenland during DJF following increased heat transport through the Florida Straits in late summer, and confirming this connection using the ECMWF IFS in Mathews et al. (2024)—where surface latent heat fluxes over the Gulf Stream were suppressed, resulting in a reduction of blocking by up to 30% across the entire northern hemisphere, attributed to the diminished transport of boundary layer air into the upper troposphere along warm conveyor belts—we now adopt a playful exploration of this mechanism through a heuristic model.
This simple three-box model connects a basic slab ocean to an atmospheric boundary layer via heat fluxes, which then connects to the upper troposphere through slantwise convection following Emanuel (1983). Mimicking real-world dynamics, the model shows increased (decreased) ocean heat content before (after) an atmospheric block. Additionally, convection is triggered only when negative potential vorticity is present in the atmospheric boundary layer as theorised by Bennetts and Hoskins (1973).
By adjusting various feedback parameters the system bifurcates, leading to a series of period doublings before entering a chaotic regime. By utilising the instantaneous dynamical systems properties of the attractor as described by Faranda et al. (2017), we infer the predictability of the system with different arrangements of control parameters.
This simple model underscores the importance of accurately representing the oceanic pathway to atmospheric blocking in more sophisticated climate models for precise predictions of how atmospheric blocking changes in a warmer climate.
How to cite: Mathews, J., Czaja, A., Vitart, F., Roberts, C., and Deshmukh, V.: Chaos, Coupling and Climate: The Oceanic Pathway to Atmospheric Blocking, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-1053, https://doi.org/10.5194/ems2024-1053, 2024.
Rossby Wave Packets (RWPs) are atmospheric perturbations located at the upper levels in mid‐latitudes which, under certain circumstances, terminate in Rossby Wave Breaking (RWB) events. When RWB events are sufficiently persistent and spatially extended, they are synoptically identical to atmospheric blockings, which are linked to heatwaves and droughts. Therefore, studying RWB events that occur after RWPs propagation and their link with blocking is key to enhance extreme weather events detection between 10–30 days in advance. In this study we assessed the occurrence of RWB events after the propagation of transient RWPs, whether long‐lived RWPs (RWPs with a lifespan > 8 days, or LLRWPs) are linked to large‐scale RWB events that could form a blocking event, and the proportion of blocking situations that occur near large-scale RWB events. To do so, we applied a tracking algorithm to detect and follow transient RWPs in the southern hemisphere during summertime between 1979-2021, developed a wave breaking algorithm to identify RWB events, and searched for blocking events with different intensities. Results show that LLRWPs and the other RWPs displayed large‐scale RWB events around 40% of the time, and most of the RWB events in both distributions last around 1–2 days, which is not long enough to identify them as blocking situations. Nearly 17% of blockings showed a RWB event nearby, but barely 5% of these blockings were linked to RWPs, thus suggesting that transient RWPs are not strongly linked to blocking development. Lastly, large‐scale RWB events associated with RWPs that lasted less than 8 days are influenced by El Niño‐Southern Oscillation
How to cite: Perez, I., Barreiro, M., Ehstand, N., Hernández‐García2, E., and Lopez, C.: Wave breaking events and their Link to Rossby wave packets and atmospheric blockings during Southern Hemisphere Summer, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-466, https://doi.org/10.5194/ems2024-466, 2024.
The Rossby Wave Sources (RWS), RWS trigger wave-like patterns arising from the upper troposphere of the north-eastern Pacific region, causing a response around the Northern Hemisphere with alternating regions of positive and negative correlation values between RWS and geopotential height at 500 hPa (Z500). Increased RWS intensity during summer is related to negative temperature anomalies over western North America, and positive
temperature anomalies over eastern North America, concurrently with increased precipitation over the western subtropical Atlantic and Northern Europe during summer. Colder than normal conditions in the North Pacific Ocean intensify the RWS and its impact on the global atmospheric circulation. We use 7 CMIP6 large ensembles for the period 1950-2014 to analyse how they reproduce the response in the global atmosphere to RWS over the North-eastern Pacific Ocean. The waveguides that RWS produce from the North-eastern Pacific around the globe show that CMIP6 large ensembles have displaced crests and troughs from the North-eastern Pacific compared to the locations of ERA5, although all of them show the spread of the signal worldwide.
This location displacement of crests and troughs relies on how intense compared to ERA5 is the bias of the modelled westerly subpolar jet stream intensity reaching Europe from the North American continent. Models with a stronger bias in intensity show that the negative RWS-Z500 correlations over Scandinavia are weakened. Correspondingly, the same applies to the south-to-north component of the bias at 200 hPa. As in ERA5, the CMIP6 large ensembles show that warmer than normal SST over the North Atlantic, constraint the teleconnection only to the North American continent.
How to cite: Fuentes Franco, R., Scaife, A., Lockwood, J., Dunstone, N., and Koenigk, T.: Rossby waves sources over the Eastern Northern Pacific and their impact on the global atmosphere from a set of CMIP6 large ensembles, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-583, https://doi.org/10.5194/ems2024-583, 2024.
Model simulations show a robust increase in El Niño-Southern Oscillation (ENSO)-related precipitation variability in a warmer climate, but there remains uncertainty in whether the characteristics of ENSO events themselves may change in the future. Our study aims to disentangle these effects by addressing how the global impacts of observed large El Niño events would change in under present and future background climate conditions.
Pacemaker simulations with the EC-Earth3-CC model were performed replaying the 3 strongest observed El Niño events from the historical record (1982/83, 1997/98, 2015/16). Model tropical Pacific sea surface temperature (SST) anomalies were restored towards observations, while imposing different background states, mimicking present and future climate conditions (following the SSP2-4.5). All variables outside the restoring region evolve freely in a coupled-atmosphere ocean transient simulation. For each start date, 30 ensemble members with different initial conditions were run for 2 years. Using this approach we ask ‘what impacts would arise if the observed El Niño occurred in the past or future’?
In response to the same imposed El Niño SST anomalies, precipitation anomalies are shifted towards the Eastern equatorial Pacific in the future compared to the present day, leading to changes in the extratropical response to El Niño. Some examples are an amplification of the surface temperature response over north-eastern North America, northern South America and Australia in boreal winter. We link the changes of El Niño related tropical Pacific precipitation to a decrease in the climatological zonal SST gradient in the equatorial Pacific, as we move from past to future climatologies, which potentially leads to a higher convection sensitivity to SST anomalies over the Central and Eastern equatorial Pacific in the future. Changes in the future climate response to extreme El Niño events are not homogeneous among regions. For example, cold and hot anomalies driven by extreme El Niño over North America and Australia in the future are amplified, whereas the response over southern Africa gets muted due to shifts in atmospheric circulation.
Our study aims to attribute the absolute future climate response to extreme El Niño events to either changes in El Niño teleconnections or climate change.
How to cite: Trascasa-Castro, P., Ruprich-Robert, Y., and Maycock, A.: Future changes in the global teleconnections of extreme El Niño events, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-239, https://doi.org/10.5194/ems2024-239, 2024.
Climate change is altering the frequency and intensity of extreme events globally, such as heat waves, droughts, flooding, and tropical storms. It is, therefore, of primary importance to quantify the influence of climate change on the properties of specific extreme events. The scientific community has developed different methodologies to answer that question. One extended approach, commonly known as the pseudo-global-warming (PWG) approach, consists of removing the anthropogenic climate change signal from event-constrained initial conditions of a physics-based weather model. However, performing state-of-the-art atmospheric model simulations of every single extreme event requires significant computational resources. Artificial Intelligence (AI)-based weather models offer a chance to speed up this process due to its much lower computational cost.
In this study, we apply the PGW approach to the AI-based FourCastNet model forecasts of a selection of extreme events. To account for the uncertainty associated with the influence of climate change, we remove the signal in thermodynamic variables from several CMIP6 models independently from the AI model's initial conditions. At the same time, we also test the sensitivity to the initialisation date. Using this approach, we quantify the impact of climate change on a heatwave event and tropical storm. Our results indicate that climate change significantly contributed to the intensity and characteristics of these two extreme events. We also discuss the strengths as well as limitations of the AI-based model in the simulation and attribution of extreme events by comparing our results to reanalysis and to more classical attribution methods. This research contributes to our understanding of the impacts of climate change on extreme events and highlights the potential of AI-based models for climate attribution studies.
How to cite: Jiménez-Esteve, B., Barriopedro, D., Johnson, J. E., and García-Herrera, R.: Climate Change Attribution of Extreme Events Using AI-based Weather Models, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-280, https://doi.org/10.5194/ems2024-280, 2024.
Extra-tropical cyclones (ETCs) are an important part of the climate system and are responsible for much of the weather in the mid-latitudes. The most extreme of these weather systems can cause damaging winds and heavy precipitation and can have adverse impacts on society. While the current generation of climate models are starting to agree on how the number, intensity and location of ETCs will change in the future, considerable uncertainty remains in how impact-relevant parameters such as wind gusts and precipitation will change. Here we use a novel combination of CMIP6 model projections and idealised model simulations of baroclinic waves to identify how precipitation and 10-m winds and wind gusts associated with ETCs may change in the future. We have performed a large ensemble (~6500 members) of baroclinic life cycle simulations with OpenIFS, which is a version of ECMWF’s Integrated Forecast System. The ensemble was created by varying 7 parameters with Latin hypercube sampling: average surface temperature; surface relative humidity; jet width, height and strength; lapse rate; and surface roughness. CMIP6 projections indicate an increase in mean temperature, lapse rate, jet width, and upper-level jet wind strength, and little change in relative humidity, jet height or surface roughness. Using CMIP6 projections from different scenarios, we map future climate projections onto the same 7-dimensional parameter space and thus identify the baroclinic wave simulations which are closest to the mean background states predicted in CMIP6 in the future. As our baroclinic wave simulations produce considerably more output fields at much higher temporal resolution compared to CMIP6, this approach enables a more thorough approach for examining how ETCs, and in particular, their precipitation and wind gusts, will change in the future. Initial results from this analysis will be presented.
How to cite: Sinclair, V., Bouvier, C., Cornér, J., van den Broek, D., Köhler, D., and Ekblom, M.: Future changes to extra-tropical cyclones: combining CMIP6 projections and idealised modelling., EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-691, https://doi.org/10.5194/ems2024-691, 2024.
Extra-tropical continental summer heatwaves often occur under persistent anticyclones or blocking. Here, we study the heatwave–blocking relation under future global warming projected by CMIP6 SSP585 experiment. We define both recent-past and future heatwaves using detrended temperature anomalies and recent-past thresholds. We employ an optimized blocking index that best correlates with heatwave frequency (Pearson correlation of 0.7). The index is from Dole and Gordon 1983, which identifies persistent positive geopotential anomalies. With detrended temperature and recent-past thresholds, we find a significantly steepened (flattened) heatwave–blocking slope over Europe (Greenland) under future global warming. The steepened heatwave–blocking relation over Europe could be a result of depleted soil moisture and enhanced land–atmosphere coupling, which enhance the capacity of blocking in driving more heatwaves. The flattened heatwave–blocking relation over Greenland is likely because the melting of ice or snow and the absorption of latent heat, which limit blocking’s capacity in driving heatwaves. Considering the contribution to heatwave frequency change, the heatwave–blocking slope change likely dominates over decrease in blocking frequency. Despite the statistically significant change in heatwave–blocking slope, we find statistically insignificant change in heatwave–blocking correlation. In other words, the ratio of heatwave variability explained by blocking will not change significantly. We also find the same type of blocking found in the recent past, i.e., identified with the same amplitude threshold and duration threshold, continues to be most relevant to future heatwaves. We revisit previous evidences that suggest future heatwaves being caused by different circulation patterns, in order to understand more about the reasons behind the apparent discrepancy. Studies on future heatwave frequency should focus on the heatwave–blocking relation change, and the thermodynamical factors behind.
How to cite: Chan, P. W., Zhang, J., Catto, J., and Collins, M.: Significant slope change but insignificant correlation change between heatwaves and blocking under future global warming, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-373, https://doi.org/10.5194/ems2024-373, 2024.
Climate models aim at representing as closely as possible the statistical properties of the climate components, including extreme events on which fine-tuning data may be less available. This is a fundamental requirement to correctly project changes in their dynamics due to anthropogenic forcing. In order to estimate how much models can be trusted, we must evaluate how closely they match observations. We need algorithms capable of selecting, processing and evaluating relevant dynamical features of the climate components. This has to be reiterated efficiently for large datasets such as those issued from the Coupled Model Intercomparison Project 6 (CMIP6). In this work, we use Latent Dirichlet Allocation (LDA), a statistical soft clustering method initially designed for natural language processing, to extract synoptic patterns from sea-level pressure data. We propose to use LDA as a tool for extracting new information from model data and evaluating their performance.
LDA allows for learning a basis of decomposition of maps into objects called "motifs". From the ERA5 sea-level pressure data, the method robustly extracts a basis of motifs that are interpretable objects at synoptic scale, i.e. cyclones or anticyclones associated to locations. Pressure data can be projected onto this basis, yielding motif weights that contain local information about the large-scale atmospheric circulation. LDA decomposition is efficient and sparse: most of the information of a given map is contained in few motifs. It is therefore possible to decompose any map in a limited number of easy-to-interpret synoptic objects. This allows for a variety of new angles for statistical analysis.
The weights statistics can be used to characterize general and extreme event-specific dynamics in reanalysis and model data. By comparing the statistics obtained from reanalysis data with those obtained from a selection of CMIP6 models, we can quantify errors on each localized circulation pattern and identify model-specific and model-independent errors. We find that, thought models make higher errors on extreme events cases such as heatwaves and cold spells, on average, large-scale circulation patterns are well predicted by the models.
By projecting model simulations of several future scenarios, we can also measure changes in time of synoptic patterns predicted by the models. These changes can be general, season-specific or extreme event-specific. We find several predicted changes in motif weights statistics, some of which are consistent with currently observed changes in synoptic configuration statistics. By comparing the amplitude of the changes in different shared socio-economic pathways, we can link some of these changes to anthropogenic climate forcing, at least according to the models.
How to cite: Malhomme, N., Podvin, B., Faranda, D., and Mathelin, L.: Atmospheric circulation representation in CMIP6 models for extreme temperature events using Latent Dirichlet Allocation, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-562, https://doi.org/10.5194/ems2024-562, 2024.
The present study investigates the uncertainties in future projections of summer convective afternoon rainfall (CAR), a key meteorological feature in Taiwan that significantly contributes to over 40-50% of the total summer rainfall in Taiwan. The analyses employ a dynamical downscaling approach utilizing the Weather Research and Forecasting Model (WRF) and the High-Resolution Atmospheric Model (HiRAM). The projections were driven by four different sea surface temperature (SST) categories derived from CMIP5 model simulations under the Representative Concentration Pathway (RCP) 8.5 scenario. All projections indicate a consistent decrease in CAR frequency but an increase in intensity by the end of the 21st century. However, the extent of changes in CAR varies across four simulations with different degrees of Pacific SST warming. These variations in potential future changes in CAR frequency and intensity are linked to variations in daytime thermal instability, local inland wind convergence, and moisture flux convergence over Taiwan. The study further investigates the relationship between local thermodynamic conditions and projected large-scale atmospheric circulation patterns, emphasizing a decrease in East Asian monsoonal low and a decrease in subtropical Pacific high in the future. The study also clarifies the potential impact of ENSO on modulating the changes in CAR activities over Taiwan under global warming, where La Niña-like circulation patterns induce more CAR activities over Taiwan compared to El Niño-like circulation patterns. In summary, the study offers valuable insights into the potential implications of climate change on CAR events in Taiwan, highlighting the importance of understanding the underlying physical mechanisms of the factors governing CAR event characteristics.
How to cite: Huang, W.-R., Chien, Y.-T., Cheng, C.-T., Hsu, H.-H., and Koralegedara, S. B.: Exploring Uncertainty in Future Changes of Convective Afternoon Rainfall in Taiwan: The Influence of Sea Surface Temperature, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-59, https://doi.org/10.5194/ems2024-59, 2024.
Extratropical cyclones are essential for redistributing energy, moisture, and momentum from lower latitudes to higher latitude regions. Although extratropical cyclones during the winter season are relatively well studied, less is known about summer cyclones and their moisture sources. Therefore, this research focuses to enhance our understanding of how summertime extratropical cyclones in the Northern Hemisphere shape the characteristics of the global water cycle. More specifically, the study focused on determining the moisture sources of these storms and analyses how precipitating air parcels are transported to the cyclone center.
For this purpose, 8-day backward trajectories were calculated for a subset of the 20 % most intense summertime cyclones over the North-Atlantic and for all air parcels within the vicinity of cyclone center, using the Lagrangian Analysis Tool LAGRANTO. Subsequently, moisture uptakes along the trajectories of precipitating air parcels were identified using the moisture source diagnostic WaterSip. Using this approach, we find that the bulk of the precipitation associated with summertime cyclones falls close to the cyclone center within the WCB, mainly during the intensification phase. The origins of this moisture correspond to areas of high evaporation, with significant hotspots over the Gulf Stream region and its northeastern extension, and continental sources for cyclones in the Labrador Sea. During the early stages of cyclone development, moisture is delivered to the cyclone center, and this changes to more remote sources when intensity is increasing and when precipitation is at its peak. Local evaporation becomes again more dominant as cyclones generate less precipitation and start to decay. Therefore, the source distance is largest during the intensification phase and decreases thereafter. We lastly discuss differences between extratropical cyclones that undergo an extratropical transition.
How to cite: Stoffels, R., Weijenborg, C., and Benedict, I.: Moisture sources of summertime intense extratropical cyclones in the North-Atlantic, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-707, https://doi.org/10.5194/ems2024-707, 2024.
Cyclones handle extreme weather events across the Mediterranean and its neighboring continents, affecting the lives of hundreds of millions. Despite many studies addressing Mediterranean cyclones (MCs) in the last decades, their correct simulation and prediction remain a significant challenge to the present day. This may be attributed to the large variability between MCs, which differ greatly from each other in many aspects. Past classifications of MCs are primarily based on geographical and seasonal separations, yet recent advances and the appearance of “Medicanes” – devastating tropical-like Mediterranean cyclones - emphasize the need for a dynamical classification, focusing on the cyclone deepening mechanisms. A variety of processes alternately govern Mediterranean cyclones' evolution, including diabatic and adiabatic processes, topographic influences, surface temperature anomalies, and land-sea contrasts. Fortunately, each process bears a distinct signature on the potential vorticity (PV) field. Therefore, a PV perspective is called upon to distinguish among the driving mechanisms of the different “types” of Mediterranean cyclones. Here, a combined cyclone tracking algorithm is used to detect and track Mediterranean cyclones in ECMWF ERA5 from 1979-2020. Cyclone-centered, upper-level isentropic PV structures in the peak time of each cyclone track are classified using the Self Organizing Map (SOM). The SOM analysis reveals 9 classes of Mediterranean cyclones, with distinct cyclone characteristics, lifecycles, associated hazards, and long-term trends. Though classified by upper-level flow structures, each class shows different flow structures down to the surface. Unique synoptic, thermal, dynamical, seasonal, and geographical features indicate dominant processes in the evolution of each Mediterranean cyclone subset. Furthermore, the tropopause-surface coupling is explored and reveals the importance of topographically induced Rossby-wave breaking to the generation of the most intense Mediterranean cyclones. These results enhance our understanding of Mediterranean cyclones' predictability, by linking predictable large-scale Rossby wave formations and life cycles to under-predicted cyclonic variability and associated hazards.
How to cite: Givon, Y., Hess, O., Flaounas, E., Catto, J. L., Sprenger, M., and Raveh-Rubin, S.: Process-based classification of Mediterranean cyclones using potential vorticity, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-439, https://doi.org/10.5194/ems2024-439, 2024.
Mediterranean tropical‐like cyclones (TLCs) are damaging weather systems, which form over the Mediterranean Sea, resembling tropical cyclones. These cyclones can drive important socio‐economic losses in coastal areas. However, due to their small size and the relatively recent investigation of these cyclones, there is currently no robust categorization of which Mediterranean cyclones can be considered TLC. . Therefore, in this work, we propose a method to differentiate cyclones that attain actual tropical‐like characteristics in part of their lifetime, as they develop a warm core through intense convective processes. Several warm‐core cyclones in the Mediterranean, which were analyzed in the literature, are studied using ERA5 reanalysis, to identify the environment where they develop and distinguish tropical‐like cyclones from non‐tropical warm‐core cyclones. Initially, the cyclone phase space is analyzed to distinguish the cyclones that have a symmetrical deep warm core. Subsequently, the temporal evolution of several parameters is considered, including the distance between the area of maximum tangential wind speed and the cyclone center. The main results of this study show that part of the analyzed cyclones have features similar to tropical cyclones. Some differences are observed between the cyclones analyzed: one category of cyclones develops in areas of moderate‐low baroclinicity and intense convective processes, as occurs in tropical cyclones. Another group of cyclones develops in a strongly baroclinic environment with weak convective processes and intense vertical wind shear, as occurs in warm seclusions. Two cyclones, showing similarities with polar lows, are also identified. The results of this study propose a key to identify the Mediterranean cyclones that have tropical‐like characteristics.
How to cite: Gutiérrez-Fernández, J., Miglietta, M. M., González-Alemán, J. J., and Gaertner, M. Á.: A New Refinement of Mediterranean Tropical‐LikeCyclones Characteristics, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-360, https://doi.org/10.5194/ems2024-360, 2024.
The release of condensation latent heat associated with water phase changes is the most important non-adiabatic energy source in tropical cyclones (TCs) besides sea surface temperature, and will affect the energy conversion process inside TCs. Based on the derived moist atmospheric energy tendency equation in cylindrical coordinates, the numerical simulation results were used in this paper to analyze the distribution and evolution characteristics of the multi-scale energy of Typhoon "Meranti".
The results showed that the high-value center of symmetric kinetic energy was always located at the Radius of maximum wind(RMW), distributed in a columnar shape and centrifugally inclined, whereas the high-value areas of asymmetric kinetic energy at the vortex scale and sub-vortex scale were mainly located in the strong updraft area within the eyewall and the low-level inflow area. At the strongest stage of the typhoon "Meranti", there also existed large kinetic energy in the middle-level inflow layer and the high-level outflow layer away from the vortex center. The high values of symmetric moist available potential energy were mainly distributed in the warm core and wet core within the eyewall, and their vertical stretching increased with the enhancement of the warm core structure and low-level moisture transport.
The moist available potential energy of symmetric vortices and asymmetric perturbations at each scale was generated by the non-adiabatic heating process at the corresponding scale, and was further converted into kinetic energy at the corresponding scale through vertical heat transfer.The research in this paper will help to analyze how the multi- water phase affect the energy and intensity of tropical cyclones from the energy perspective.
How to cite: Huang, H., Wang, J., Wang, T., Wang, Z., Jiang, C., Fan, Y., and Wang, X.: The Distribution and Evolution of Multi-scale Moist energy within the Typhoon "Meranti", EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-769, https://doi.org/10.5194/ems2024-769, 2024.
This research discusses the influence of tropical cyclone (TC) activities in the summer tropical northwest Pacific on the East Asian midlatitude westerly jet through statistical analysis and numerical experiments, and the mechanism is also analyzed.
The statistical results indicate that TC activities in the northwest Pacific can induce abnormal westerlies in the midlatitude region of East Asia, and the axis of the westerly jet shifts toward regions with larger anomalous westerlies induced by TCs.
Numerical research on the case of TC Maria shows that the thermal effects of the abnormal disturbances near Japan induced by TC activities in the tropical northwest Pacific can result in abnormal temperatures in the midlatitude region of East Asia. Under the constraint of thermal wind relation, the abnormal westerlies form and the position of the westerly jet changes, correspondingly. During this process, the abnormal phase of the P-J teleconnection induced by TC activities plays a major role in causing the subsidence motion in the midlatitude region of East Asia, leading to abnormal changes of the atmospheric temperatures, which affect the meridional temperature gradient in the midlatitude region. Due to the vertical variation of zonal winds and horizontal temperature gradients in the midlatitude region satisfying the thermal wind relationship overall, the zonal winds in the midlatitude region undergo abnormal vertical variations correspondingly, which further changes the position of area with maximum value of the East Asian westerly, and the meridional position of westerly jet axis changes as well.
Results of this study demonstrate that TC-induced remote disturbances have thermal effects and can also affect midlatitude systems.
How to cite: Wang, T., Wang, J., Huang, H., Wang, X., and Jiang, C.: Impacts of Thermodynamics Disturbances Induced by Tropical Cyclone on the East Asian Subtropical Westerly Jet, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-890, https://doi.org/10.5194/ems2024-890, 2024.
The weather-dependent planning and decision-making benefit greatly from subseasonal to seasonal (S2S) weather predictions made for up to six weeks ahead. At this timescale anomalies in the extratropical stratospheric circulation, which can last for several weeks in the Northern Hemisphere during winter, are known to affect climate at the surface and can extend the predictability of tropospheric weather conditions. Although state-of-the-art models can to some extent capture the surface impacts of such stratospheric variability and have demonstrated enhanced skill at the subseasonal timescales after major stratospheric events, the causal link between the events and the sources of predicted signals in real-time forecasts is difficult to establish.
We performed relaxation (nudging) experiments to uncover sources of predicted signals in the forecasts, specifically focusing on forecasts of high impact weather events, such as cold spells in the winter season. Nudging of winds and temperatures towards a reference state, usually towards observations is a technique commonly used to constrain model behavior. In addition to this traditional approach where nudging is done towards observations and climatology, we nudge towards control forecast ensemble members predicting anomalous behavior in the stratosphere. By comparing these nudged experiments with the control forecast ensemble, we identify the cause of predicted signal in real-time situations when observations may not yet be available. Therefore, the main benefit is that the methodology can be applied in operational practice as an additional tool to interpret forecast behavior. This approach also provides more accurate quantification of the signal-to-noise ratio because, when nudging is done towards observations, the predicted signal is artificially enhanced by the elimination of model biases. Understanding the conditions associated with enhanced sub-seasonal predictability allows to distinguish between signals associated with remote sources and cases when the signal has no clear origin.
How to cite: Statnaia, I. and Karpechko, A.: Understanding enhanced sub-seasonal predictability of cold spells with nudged experiments., EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-189, https://doi.org/10.5194/ems2024-189, 2024.
Europe has been identified as a heatwave hotspot, with numerous temperature records having been broken in recent summers. These temperature extremes exact a heavy toll on society; approximately 60,000 heat-related deaths have been estimated for the summer of 2022 alone. With projections showing extreme temperatures becoming more frequent, intense and longer in duration, there is a pressing need to further develop heat-warning systems to help protect, in particular, the most vulnerable members of society. Here we evaluate the skill of daily temperature related mortality forecasts in the context of current numerical weather prediction model capabilities. We consider the summers of 2022 and 2023 as case studies and find that temperature related mortality can, on average, be forecast skilfully up to lead times of approximately one week for these two summers, although we also note the increased predictability in south-western Europe in late-July 2022 coinciding with record breaking temperatures. We further discuss the implications that the non-linear relationship between temperature and temperature related mortality has on temperature related mortality forecast spread and errors, concluding that further developments in forecasting capabilities for extreme temperature events are of key importance for improving temperature related mortality forecasts. Finally, we highlight the implication of these results in a warming climate. In the absence of meaningful adaptation measures or considerable advances in numerical weather prediction capabilities, temperature related mortality forecasts will be associated with larger errors owing to increased climatological temperatures. We emphasise that continued work on understanding the predictability of temperature extremes and temperature related mortality is vital for the further development of heat-warning systems.
How to cite: Holmberg, E., Quijal-Zamorano, M., Messori, G., and Ballester, J.: The predictability of temperature related mortality in the summers of 2022 and 2023, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-408, https://doi.org/10.5194/ems2024-408, 2024.
The characteristics of summertime extreme rainfall events (ERE) according to the weather patterns in South Korea and compares the extreme rainfall model simulation skills depending on the use of cumulus parameterization has been studied. The WRF simulations with and without the a cumulus parameterization scheme (Kain–Fritsch cumulus scheme in this study) (CU_ON and CU_OFF experiment, respectively) are conducted over a 3 km high-resolution domain. The ERE-occurring days are clustered into four representative weather patterns (northern cyclonic circulation, frontal pattern between low and high, southwestern extratropical cyclone, and dominant positive geopotential height patterns) according to the 850 hPa geopotential height anomaly. As the occurrence dates of observed Clusters 1 and 2 overlap with a significant portion of the Korean summer monsoon season (Changma period), their rainfall is characterized by continuous low-intensity rainfall. In contrast, relatively high-intensity, short-duration rainfall occurs mainly in Clusters 3 and 4. The WRF experiments generally describe the clustered weather patterns well. For CU_ON, the spatial distribution of the daily rainfall anomaly composite in Clusters 1 and 2 is well depicted, but the overall rainfall intensity is underestimated. CU_ON better reproduces the Clusters 1 and 2 type rainfall characterized by long-duration rainfall than CU_OFF. The observed rainfall events exceeding 20 mm h−1 intensity with a short-duration are reproduced better in CU_OFF than in CU_ON, showing reasonable performance for sub-daily time-scale rainfall in Clusters 3 and 4. Hence, the CU_ON well depicts the continuous low-intensity ERE type, while CU_OFF captures the ERE type where high-intensity rainfall with a short duration occurs relatively frequently.
How to cite: Ahn, J.-B. and Seo, G.-Y.: Characteristics of rainfall simulation over South Korea by summertime weather patterns according to the use of cumulus parameterization, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-452, https://doi.org/10.5194/ems2024-452, 2024.
Extreme cold events in East Asia have significant socio-economic impacts, and accurate prediction is crucial for effective risk management. Understanding cold surges requires consideration not only of their intensity but also their duration. Some cold surge events are notable for their prolonged duration. Meanwhile, cold surges accompanied by abrupt temperature fluctuations have become increasingly frequent in recent years. These drastic cold surges pose substantial risks to human lives and energy supply. In this study, cold surge events in Korea are classified into two types based on their temporal characteristics. Prolonged type cold surges last about three weeks, while drastic type cold surges are defined by a difference of 8˚C or more between the lowest and highest temperatures within a three-week span. It is observed that a trough over East Asia stagnates due to a blocking high over the northeastern Pacific under negative Arctic Oscillation conditions, leading to prolonged cold surges. Conversely, drastic cold surges occur in association with the Madden Julian Oscillation (MJO). Specifically, when the MJO phase spans through 8-1-2-3, warm conditions over East Asia are followed by cold surges. The predictability of these features in the Korea Integrated Model (KIM) has been investigated. KIM demonstrates skillful prediction capability by capturing both types of cold surges within a 2-week lead time. However, as the forecast lead time increases, the predictability of drastic type cold surges tends to surpass that of prolonged type. This is attributed to the better predictability of tropical convection compared to blocking over the northeastern Pacific. This study will provide detailed descriptions of the dynamical characteristics of the two types of cold surges and their subseasonal predictability sources.
How to cite: Yeo, S.-R. and Seol, K.-H.: Two types of cold surges of Korea and their predictability in Korea Integrated Model (KIM) , EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-630, https://doi.org/10.5194/ems2024-630, 2024.
The properties and distributions of precipitation are often determined by specific synoptic patterns. Hence, the objective identification of corresponding impact patterns is an important field of research for improving rain forecasting. However, the identification of the weather patterns producing intense rainfall is much more challenging. Since they are violent and local, impact patterns tend to be meso- or smaller-scale systems and are often incompletely presented or only presented in limited regions. In this paper, a deep learning network with a feature cross-fusion module, FConvNeXt, was proposed to address this difficulty and showed great potential. Four major patterns corresponding to intense rainfall in the Beijing–Tianjing–Hebei Region were studied. Statistical testing showed that FConvNeXt performed better than ConvNeXt and ResNet and that the model could identify the weak synoptic forcing type, the subtropical high-pressure type, and the low-vortex pattern with high accuracy. Furthermore, a strictly independent 2021 dataset was tested, and FConvNeXt maintained an equal if not even slightly better performance in spite of a decrease in the subtropical high-pressure type. Meanwhile, the study showed that the accuracy in identifying the upper-level trough type is the lowest for the three deep learning methods, which maybe because the northeast vortex was intercepted in the limited region, making it difficult to distinguish from the shallow upper-level trough type. This study is useful for improving the fine objective of forecasting intense rainfall.
In summary, in contrast to previous objective classifications on large-scale weather systems in large regions, this study explored the objective classification of meso- and small-scale weather patterns that correspond to heavy rainfall and flash flooding within a limited region. Advanced deep learning models were employed that showed significant potential for this application. Furthermore, a new cross-fusion feature extraction module was proposed that improved the accuracy of the LVT classification within a limited region. Moreover, the study introduced a pre-training model to improve the training speed, which improved the accuracy and significantly shortened the training time.
How to cite: Zhong, Q. and Jing, L.: Identify Patterns of Flash Heavy Rainfall in Limited Area by FConvNeXt, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-538, https://doi.org/10.5194/ems2024-538, 2024.
Blocking anticyclones embedded in highly amplified Rossby waves and flanked by stationary cut-off cyclones can cause devastating extreme events. However, often the magnitude of such extremes is difficult to predict on medium- to subseasonal lead times. In this presentation we discuss the synoptic evolution of two recent extreme events: The heat wave in western North America in June 2021 and the extreme rainfall in the Eastern Mediterranean in September 2023. For both cases we show that the downstream development triggered by the interaction of synoptic weather systems with the upper-tropospheric wave guide in regions far upstream ultimately lead to highly amplified Rossby Waves. Thereby, the interaction of diabatic outflow due to latent heat release in ascending air streams with the jet is key and the exact phasing and timing of this interaction represents a predictability barrier with regard to the magnitude of downstream ridge and associated extreme events. Therefore, probabilistic weather forecasts are only able to predict the extremity of the events once the complex interaction of synoptic activity is captured. Thus, the sequence of individual weather events limits the predictability of the magnitude of extremes linked to highly amplified Rossby waves. We conclude that a correct assessment of highly amplified Rossby waves for weather prediction and climate projection requires large ensembles in order to capture the rare sequence of interactions causing such extremes. Also, an accurate representation of the physical and dynamical processes across spatiotemporal scales in particular of moist processes on synoptic weather scales would help, however, there are indications that the predictability barrier is intrinsic in nature.
How to cite: Grams, C. M., Oertel, A., Quinting, J. F., Magnusson, L., Deinhard, M., Dorrington, J., Hauser, S., Wandel, J., Balmaseda, M., and Vitart, F.: How synoptic weather activity and interaction with the extratropical wave guide matter for the prediction of blocking and associated extremes, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-640, https://doi.org/10.5194/ems2024-640, 2024.
The spatiotemporal variability of atmospheric blockings over the Euro-Atlantic region and their influence on the occurrence of the persisting extreme temperature conditions in Poland namely heat waves and cold spells during the period 1978-2023 was analysed. Blockings were identified at 500 hPa geopotential level, using the meridional geopotential gradient method, supplemented with the quantile filter and persistence filter. Heat waves (HWs) and cold spells (CSs) were defined as sequences of at least 3 days with the maximum air temperature above 30°C or below -10°C, respectively.
The climatology of Euro-Atlantic blocking occurrence in the zonal belt between 45 and 75 degrees in the northern hemisphere exhibits high spatiotemporal variability. Blocking structures are most frequent in the spring (MAM) occurring with a 10% frequency in the arched belt extending from the west of the British Islands through the North Sea, south Scandinavia, to the east of the Baltic Sea. In summer (JJA) they are most frequent in the area spanning from south Finland to west Russia, as well as over Greenland. During winter (DJF) blockings are most commonly located over the southeast Atlantic (west of France) and extending northeastward up to southern Scandinavia with their occurrence being least frequent in autumn (SON).
The mean annual number of HW days ranges from one day in the southern mountainous regions and in northern Poland to more than four days in the central and western parts. However, during the extremely hot summer of 2015, the number of HW days exceeded 20 in central and southern Poland. CSs have become rarer, with the average seasonal number of CS days amounting to 1-2 days in recent decades, and only in singular seasons such as 1986/1987, more than 10 days fulfilled the criteria of CS in northeastern Poland. The occurrence of HWs in Poland is constantly accompanied by blocking situations, most often located northeast of Poland, while the winter CSs are associated with the blockings located over the North Atlantic and northern Scandinavia.
How to cite: Bednorz, E. and Tomczyk, A. M.: Impact of Euro-Atlantic blocking on the occurrence of heat waves and cold spells in Poland, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-115, https://doi.org/10.5194/ems2024-115, 2024.
The study concerns the determination of the characteristics of bioclimatic conditions, as well as the synoptic situations related to the occurrence of thermal stress conditions, in Poland. The study was based on daily data obtained from the Institute of Meteorology and Water Management – National Research Institute from the period 1966–2020 for 37 synoptic stations in Poland. Based on the obtained data, values of the Universal Thermal Climate Index (UTCI) were calculated. The occurrence of heat stress increases from the north to the south, corresponding with the variability of influx of solar radiation, and is modified by factors at a smaller spatial scale. The results of this paper evidently point to the cooling effect of the waters of the Baltic Sea. In circulation conditions favouring strong and very strong heat stress, e.g. in two of the designated circulation types (T1 and T2), the occurrence of an expansive high-pressure ridge in the Atlantic-European area is typical, stretching from the region of the Azores High towards the north-east, with a secondary high developed within its boundaries. In the third of the designated circulation types (T3), the high-pressure area extends from the Azores eastwards, reaching the Black Sea. Each of the three circulation patterns associated with the unfavourable biometeorological conditions of very strong and extreme cold stress in Poland is characterised by strong pressure centres formed in the Euroatlantic region, triggering the airflow from the northern (T4) or eastern (T5, T6) sector. The study revealed high variability of bioclimatic conditions in Poland, both in temporal and spatial terms. The lowest UTCI was recorded in the north-east of Poland and at the east coast of the Baltic Sea. The highest index values were observed in south-western and western regions of the country. High spatial variability of UTCI related to regional variability of climatic conditions in Poland permitted the designation of bioclimatic regions characterised by the different occurrence of heat stress, particularly in the cool season of the year. Regions in the south-west and west of Poland proved the most favourable in bioclimatic terms, with the highest number of days with no thermal stress. In these regions, the highest UTCI values were observed, while the lowest were recorded in the northeast of Poland and at the east coast of the Baltic Sea. Among unfavourable biometeorological conditions, the ones causing hypothermia have so far occurred more frequently than the ones causing overheating of the human organism.
How to cite: Tomczyk, A. M., Bednorz, E., and Szyga-Pluta, K.: The occurrence of bioclimatic conditions in Poland – bioclimatic regions and impact of regional baric systems., EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-483, https://doi.org/10.5194/ems2024-483, 2024.
Understanding extreme precipitation events (EPEs) and their underlying dynamical processes and moisture transport patterns is essential to mitigate EPE-related risks. In this study, we investigate the dynamics of 82 EPEs (≥ 100 mm∙day-1) over the territory of Ukraine in the recent decades (1979-2019), of which the majority occurred in summer. The EPEs are identified based on precipitation observations from 215 meteorological stations and posts in Ukraine. The atmospheric variables for the case study analysis of selected EPEs and for climatological composites and trajectory calculations were taken from ERA5 reanalyses. Moisture sources contributing to the EPEs in Ukraine are identified with kinematic backward trajectories and the subsequent application of a moisture source identification scheme based on the humidity mass budget along these trajectories. The large-scale atmospheric circulation associated with EPEs was studied for a selection of representative EPEs in all seasons and with the aid of composites of all events per season. Results show that EPEs in summer occur all across Ukraine, but in other seasons EPE hotspots are mainly in the Carpathians and along the Black and Azov Seas. All EPEs were associated with a surface cyclone, and most with an upper-level trough, except for the winter events that occurred in situations with very strong westerly jets. Isentropic potential vorticity anomalies associated with EPEs in Ukraine show clear dipole structures in all seasons, however, interestingly with a different orientation of these anomaly dipoles between seasons. The analysis of moisture sources revealed a very strong case-to-case variability and often a combination of local and remote sources. Oceanic sources dominate in winter, but land evapotranspiration accounts for 60-80% of the moisture that rains out in EPEs in the other seasons. Taken together, these findings provide novel insight into large-scale characteristics of EPE in Ukraine, in a region with a unique geographical setting and with moisture sources as diverse as Newfoundland, the Azores, the Caspian Sea, and the Arctic Ocean.
How to cite: Agayar, E., Aemisegger, F., Armon, M., Scherrmann, A., and Wernli, H.: Precipitation extremes in Ukraine from 1979 to 2019: Climatology, large-scale flow conditions, and moisture sources, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-88, https://doi.org/10.5194/ems2024-88, 2024.
A sequence of extreme cold events occurred across mid-high-latitude Eurasia and western North America during winter 2020/2021. Here, we explore the causes and associated mechanisms for the extremely cold temperatures using both ERA5 observations and large-ensemble atmospheric model simulations. Experiments were conducted with observed ocean surface boundary conditions prescribed globally, and regionally to discern the specific influence of Arctic, tropical Pacific and North Pacific drivers. Increased likelihood of daily cold extremes in mid December to mid January are found in Eurasian midlatitudes in response to reduced Arctic sea ice. Tropical sea surface temperature anomalies, more specifically the La Niña pattern, increased probability of extreme cold over high-latitude Eurasia in early January to early Febraury. Both reduced Arctic sea ice and La Niña increased the probability of daily cold extremes over western North America in late January to late Febraury. We conclude that a combination of reduced Arctic sea ice, La Niña, and a sudden stratospheric warming in January 2021 were factors in the February 2021 extreme cold-wave that caused huge societal disruptions in Texas and the Southern Great Plains. Although the magnitude of the simulated cold extremes are relatively small when compared with observed anomalies, the Arctic and Pacific Ocean surface conditions in winter 2020/21 increased the probability of cold days as cold or colder than observed by approximately 17%~43%. Extreme cold events occurred across Eurasia and North America during winter 2020/21 were made more likely by a combination of reduced Arctic sea ice, La Niña, and a sudden stratospheric warming.
How to cite: Zhang, R.: Arctic and Pacific Ocean Conditions Were Favourable for Cold Extremes over Eurasia andNorth America during Winter 2020/21, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-66, https://doi.org/10.5194/ems2024-66, 2024.
The North Atlantic eddy-driven jet (EDJ) is the main driver of winter weather in Europe and is typically described by its latitude or strength. Here, we show that the relationship between the EDJ and European winter temperature extremes can be better understood by using a multiparametric perspective that accounts for additional aspects of the EDJ structure (tilt, zonal elongation, etc.). First, we identify four regions where the influence of the EDJ on extreme temperatures is different: Scandinavia, Central Europe, Eastern Europe, and Western Mediterranean (WMED). Overall, the main mechanism leading to extreme event occurrence is the anomalous horizontal advection induced by blockings during cold spells and enhanced westerlies during warm events. In the case of the WMED region, diabatic processes play though a principal role in the occurrence of warm events. Additionally, the circulation anomalies and radiative fluxes involved in both processes generate asymmetric effects in minimum and maximum temperatures, leading to more intense cold than warm events. These extreme events are accompanied by different EDJ configurations entailing perturbed EDJs during cold spells and strong tilted EDJs during warm events, but with distinguishable characteristic depending on the region. In almost every region, the probability of cold and warm events is increased when considering the combined effects of more than two EDJ parameters, suggesting an oversimplification of traditional approaches based on a single EDJ parameter. More importantly, our results derived from logistic regression models highlight the relevance of EDJ parameters others than latitude and intensity to drive the largest changes in the odds of extremes.
How to cite: Garcia-Burgos, M., Ayarzagüena, B., Barriopedro, D., and Garcia-Herrera, R.: Jet Configurations Leading to Extreme Winter Temperatures Over Europe, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-598, https://doi.org/10.5194/ems2024-598, 2024.
Keeping oscillation of low frequency of 30~60 days, Butterworth band-pass filter method was used to process the NCEP/NCAR reanalysis data. Based on the application of the low-frequency synoptic map, low frequency features of the two extreme low temperature events were analyzed in order to reveal the characteristics of the low frequency systems during these two events. The results show that in early 2008, large-scale atmospheric systems including blocking-high and upper-level jet stream all featured a distinct 30-60-day oscillation. The positive (negative) anomaly of geopotential height was closely coincided with the low frequency high (low) pressure of the low frequency systems, and the center of positive zonal wind anomaly was consistent with the high value center of low frequency zonal wind. Meanwhile, the positive phase of the AO favored the strengthening of the Middle East jet and the maintenance of the blocking high, resulting in durative low temperature in south China. The 30-60-day oscillation features of the weather systems including upper-level jet and blocking high were not so obvious during “overlord”-level cold wave in 2016. However, the low pressure of low frequency can describe the generating and developing of the polar vortex. Under north air stream at the front of blocking high ridge guidance, the rapid invasion of strong cold air in the middle of polar vortex caused temperature in China drop fast. The low-frequency synoptic map reflected the phase transition of AO before and after the cold wave. The phase of AO was positive in later December 2015 while negative in early January 2016. Then the polar cold air invaded southern China, which can be conclude as the main cause of the sharp drop in temperature. The low-frequency flow field showed the phase transition of AO lagging behind the synoptic flow field about two days during the two events.
How to cite: Li, X., Li, Y., and Zhang, J.: Low-frequency features during the two typical extreme cold events in China, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-288, https://doi.org/10.5194/ems2024-288, 2024.
The North Atlantic Oscillation (NAO) has been confirmed to be closely related to the weather and climate in many regions of the Northern Hemisphere; however, its effect and mechanism upon the formation of dust events (DEs) in China have rarely been discussed. By using the station observation dataset and multi reanalysis datasets, it is found that the spring dust aerosols (DAs) in North China (30-40° N, 105- 120° E), a non-dust source region, show high values with a strong interannual variability, and the spring DAs in North China are significantly correlated with the previous winter's NAO. According to the nine spring DEs affected significantly by the negative phase of the preceding winter's NAO in North China during 1980-2020, it is shown that before the outbreak of DEs, due to the transient eddy momentum (heat) convergence (divergence) over the DA source regions, the zonal wind speed increases in the upper-level troposphere, strengthening the zonal wind in the middle-lower levels through momentum downward transmission. Simultaneously, there is transient eddy momentum (heat) divergence (convergence) around the Ural Mountains, which is favorable for the establishment and maintenance of the Ural ridge, as well as the development of the air temperature and vorticity advections. The combined effects of temperature and vorticity advections result in the Siberian Highs and Mongolian cyclone to be established, strengthen, and move southward near the surface, guiding the cold air from high latitudes southward, and is favorable for the uplift and transmission of DAs to North China downstream. Simultaneously, the changes in upstream transient eddy flux transport can cause both energy and mass divergence in North China, resulting in diminishing winds during DEs, which would facilitate the maintenance of dust aerosols here and promote the outbreak of DEs. This study reveals the impact of transient eddy flux transport on the dusty weather anomalies modulated by the NAO negative signal in North China, which deepens the understanding of the formation mechanism of DEs in China.
How to cite: Li, Y. and Li, X.: Influence of the previous North Atlantic Oscillation (NAO) on the spring dust aerosols over North China, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-289, https://doi.org/10.5194/ems2024-289, 2024.
Severe dust storms are frequent throughout the year in the Middle East. While dust events are more active during winter and spring in the north, their maximum activity with the greater intensity occurs in the summer in the southwest of the region. The Middle East is affected by frontal dust storms mainly in winter and “Shamal” dust storms mostly in summer.
Surface and satellite observations as well as ERA5 reanalysis data were used to describe the formation and evolution of a winter shamal dust storm that occurred in February 2017. It initiated in central western Iraq and dust was transported southeastward towards the Persian Gulf, impacting all the countries in the region up to the Oman Sea.
The SEVIRI Dust RGB product shows dust mobilization at 07 UTC (10 LT) in several point areas west of the Euphrates River and over Mesopotamia, in Iraq. A dense dust plume is then advected between the Zagros Mountains, to the North and East, and the high plains of Saudi Arabia, to the South and West, reaching the Persian Gulf in the first hours of February 18. The next day, dust spreads throughout the Persian Gulf and surrounding countries. It resulted in the widespread reduction of horizontal visibility and impaired air quality, as reported by the region's Synop/METAR surface observations and air quality stations.
The large-scale upper-level processes leading to this event started days before with a strong amplification of an anticyclonic Rossby wave break in the Polar Jet over the North East Atlantic, with large penetration poleward of subtropical air up to Scandinavia and cold air advection equatorward over Iberia on February 11. At a late dissipative stage and downstream displacement, the RWB resulted in a closed ridge over southeastern Europe and Turkey and a trough downstream over the study area. A strong pressure gradient was established at low levels between high pressures centered over southeastern Europe and the Black Sea and low pressures over the Persian Gulf with minima over the Arabian Peninsula and over Iran to the north of the Zagros Mountains. Strong northerly winds, imposed by the pressure gradient, accelerated downslope in the lee of the mountains in southern Turkey. The analysis suggests the formation of a low-level jet to the south of the mountains. During the morning hours of February 17 it mixed with the air over the surface, transferring momentum and initiating dust deflation in source areas of northern Iraq and Syria. Dust plumes were transported southeastward at heights below 2 km, as shown by CALIPSO profiles in the first half of February 18, within the PBL. On February 19, the dust plume was mixed all over the Persian Gulf basin.
How to cite: Karbasi, S., Chham, E., and G. Orza, J. A.: Meteorological drivers of a winter shamal dust storm over the Middle East, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-940, https://doi.org/10.5194/ems2024-940, 2024.
This study characterizes and contrasts Rossby wave activity during the 25 sudden stratospheric warming (SSW) and 31 strong polar vortex (SPV) events that occurred in the period 1979–2021. While the events are tied to a decrease or increase, respectively, in background flow, it is less clear how the associated properties of Rossby waves change, e.g., how their phase speed is affected. The goal is to identify the specific tropospheric and stratospheric waves exhibiting anomalous behavior during these events. Applying space-time spectral analysis to ERA5 reanalysis data allows us to assess both the wavenumber and the zonal phase speed of the waves.
We find that SSW events are associated with a reduced phase speed of Rossby waves, first in the stratosphere and then in the troposphere, while SPV events are tied to a concomitant increase of phase speed across vertical levels. These phase speed anomalies become significant around the event and persist for 2-3 weeks thereafter. In the stratosphere, both SSW and SPV events are dominated by the change in the background flow, manifested as a systematic reduction or increase, respectively, in eastward propagation of Rossby waves across most wavenumber.
In the troposphere, the effect of the background flow is complemented by changes in wave properties, with a shift towards higher wavenumbers during SSW events and towards lower wavenumbers during SPV events. This opposite response between SSW and SPV events is also visible in the meridional heat and momentum flux co-spectra, which highlight from a novel perspective the connection between stratospheric Rossby waves and upward propagation of waves.
How to cite: Schutte, M., Domeisen, D. I. V., and Riboldi, J.: Opposite spectral properties of Rossby waves during weak and strong stratospheric polar vortex events, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-118, https://doi.org/10.5194/ems2024-118, 2024.
Abstract：The intraseasonal reversal of the temperature over East Asia and the sudden stratospheric warming (SSW) are two abrupt events during the winter of 2020/21. The characteristics of these two events and their relationship are the focused issues in this paper. In winter 2020/21, there was a transition from cold to warm over East Asia. The surface air temperature (SAT) in East Asia suddenly changed from cold to warm with long duration. The cold period lasted from 21 November 2020 to 11 January 2021, while the warm period lasted from 19 January to 28 February 2021. And the transition of the cold to the warm was from 12 to 18 January 2021.There are significant differences of the atmospheric circulations between the cold and warm periods in the upper and lower troposphere. During the cold period, both of Ural Mountain High(UH) and Siberian High(SH) were stronger than normal, the Westerly Jet (WJ) was weakened and located to the area farther southerly than usual, the northerly wind anomalies and the cold advection occurred in East Asian; vice versa during the warm period. The mutations of the atmospheric circulations in the upper and lower levels were obvious during the transition period, especially for UH and SH. A strong SSW event occurred in the stratosphere at the beginning of 2021. As a result, the stratospheric polar vortex was largely weakened and deviated to the areas over Ural Mountains and nearby, the polar westerly wind turned to easterly wind and the temperature gradient reversed. The abnormal changes of the temperature and the geopotential height over the Ural Mountains area in the stratosphere were closely related to the SSW. These abnormal signals caused by the SSW gradually transmitted from stratosphere to troposphere over the Ural Mountains and the polar region, leading to the weakening and collapse of tropospheric UH and further caused the transition from cold to warm in East Asia. The SSW may be the most critical factor leading to the turning from cold to warm over East Asia during the winter of 2020/21.
How to cite: Yan, H.: Possible impact of sudden stratospheric warming on the intraseasonal reversal in winter, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-217, https://doi.org/10.5194/ems2024-217, 2024.
The penetration of upper-level troughs into low latitudes and their amplification and thinning accompanying the equatorward breaking of Rossby Waves (RWB) has been observed to trigger heavy precipitation events and massive dust storms over subtropical areas like North Africa and the Middle East. Conversely, the poleward extension of a subtropical ridge structure as part of the amplified wave is associated with heat waves and drought events in mid-latitudes.
We present a long-term (2000- 2022) analysis of RWB in the Polar Jet Stream (PJ) by analyzing the large-scale and irreversible overturning of high-PV contours on the 330K isentropic surface. The identification of RWBs and their anticyclonic or cyclonic character is followed by the further geometrical analysis of the contours: the length of the overturning; the centroid and area of the 2 PVU tongues extending equatorward and the subtropical ridges built poleward as part of the amplified wave train; the location and timing of the maximum penetration equatorward and poleward of 2 PVU contours on the isentropic surface over the time; the equivalent latitude of the 2 PVU contours; and the ratio short-circle length at the equivalent latitude to the great-circle distance along the PVU contour.
There is a clear seasonal dependence of the preferred southernmost locations where RWBs penetrate, both in latitude and longitude. Locations and seasonality coincide with those of dust outbreaks and/or precipitation. The equivalent latitude has a strong seasonal component with a superimposed small but significant upward trend, which is more prominent in summer, suggesting a poleward retreat of the polar incursions.
How to cite: Garcia Orza, J. A. and Kaplan, M. L.: An analysis of the equatorward penetration of Rossby Wave breaks in the Polar Jet stream in the Northern Hemisphere, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-994, https://doi.org/10.5194/ems2024-994, 2024.
The zonal inhomogeneities of the land surface are known to strongly modulate the large-scale atmospheric circulation, especially in the northern hemisphere. This is related both to the mechanical forcing induced by high orography and to the thermal contrast between water and land. In a warming world, it is known that such a thermal contrast is changing but its impacts on the large-scale atmospheric circulation are poorly constrained. Recently, it has been shown that a reduced winter thermal land-sea contrast is projected to reduce the amplitude of planetary waves, with a predominant control of the Asian-Pacific land-sea contrast at the global scale.
Goal of the present study is to investigate the impacts of an enhanced summer thermal land-sea contrast on the planetary circulation. In particular, the role of soil moisture in modulating such contrast is considered with special attention. In fact, despite the importance of the land-atmosphere coupling mechanisms, that involve feedbacks with atmospheric circulation, clouds, precipitation and surface fluxes, there are still fundamental gaps in their understanding. These gaps result in an incorrect representation of the global hydrological cycle in CMIP6 models. For example, CMIP6 models are characterized by large biases in the water vapor trend representation in arid and semi-arid regions, which might be related to a poor representation of soil moisture and its impacts on the overlying and downstream atmospheric dynamics.
By analyzing the CMIP6 model spread in soil moisture, surface air temperature and upper-tropospheric geopotential height, we aim to quantify the variability of the planetary circulation in a range of realistic soil moisture configurations. Climate data records of satellite products of surface soil moisture can also be used to constrain the soil moisture variability observed in the last decades. In the second part of the project, numerical simulations with an Earth Model of Intermediate Complexity will shed light on the link between soil moisture distribution, land-sea summer contrast and planetary circulation.
How to cite: Meroni, A. N., D’Andrea, F., and Pasquero, C.: Impacts of the changing summer thermal land-sea contrast on the northern hemisphere planetary circulation, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-21, https://doi.org/10.5194/ems2024-21, 2024.
During the summer of 2016, several heavy rainfall events over the middle and lower reaches of the Yangtze Basin (MLY) were regulated by the 10–20-day quasi-biweekly oscillation (QBWO). The characteristics and mechanisms of the QBWO associated with the MLY rainfall events were examined using ERA-Interim reanalysis data. In addition to the biweekly oscillation in the western North Pacific subtropical high in the lower troposphere, The QBWO of the MLY rainfall was closely linked with upstream intraseasonal potential vorticity (PV) anomalies generated over the eastern slope of the Tibetan Plateau (TP) due to topographic lateral friction. The PV budget analysis demonstrates that the horizontal PV advection and subsequent topographic friction with a four-day phase-lag between them dictated the QBWO of PV anomalies around the eastern TP. The TP-generated PV anomalies then migrate downstream to facilitate the development of the anomalous circulation over the MLY. Importantly, the strongest of the six heavy rainfall events occurred in the end of June 2016, and it was attributable to a TP Vortex (TPV) in conjunction with a Southwest China Vortex (SWCV). The eastward-moving TPV merged vertically with the SWCV over the eastern Sichuan Basin due to the positive vertical gradient of the TPV-related PV advection, leading the lower-tropospheric jet associated with moisture transport to intensify greatly and converge over the downstream MLY. The merged TPV–SWCV specially facilitated the upper-tropospheric isentropic-gliding ascending motion over the MLY. With the TPV-embedded mid-tropospheric trough migrating continuously eastward, the almost stagnant SWCV was re-separated from the overlying TPV, forming a more eastward-tilted high-PV configuration to trigger stronger ascents including isentropic-gliding, isentropic-displacement, and diabatic heating-related ascending components over the MLY, thus resulting in the most intense rainfall.
How to cite: Mao, J. and Zhang, G.: Quasi-biweekly Oscillation of the 2016 Summer Rainfall over the Yangtze Basin in Relation to Intraseasonal Potential Vorticity around the Eastern Tibetan Plateau, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-47, https://doi.org/10.5194/ems2024-47, 2024.
This study investigates the combined impact of the El Niño–Southern Oscillation (ENSO) and Arctic Oscillation (AO) on the variability of winter temperatures in South Korea at the subseasonal time scale. This is achieved by analyzing hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF) database. Six composite cases are selected based on thresholds for each index: El Niño with positive AO (EP), El Niño with negative AO (EN), La Niña with positive AO (LP), La Niña with negative AO (LN), positive AO only (PA), and negative AO only (NA). Results from reanalysis data indicate that these climate factors can serve as predictors for predicting South Korea's temperature up to 4 weeks in advance. The study confirms that ENSO significantly influences the strength of AO's impact on temperature anomalies, depending on whether they are in phase or out of phase. For instance, during El Niño periods, there's a notable increase in mean temperature anomalies due to positive geopotential height (GPH) anomalies and warm temperature advection over South Korea in the EP case. Conversely, during La Niña periods, there's a significant decrease in mean temperature anomalies due to negative GPH anomalies and cold temperature advection over South Korea in the LN case. The ECMWF S2S hindcast exhibits reasonable ability to replicate circulation patterns over East Asia up to 3 weeks in advance. It also adequately predicts weekly mean temperature anomalies over South Korea in the EP, LN, and PA cases. This suggests that the combination of ENSO and AO indices can contribute to improved subseasonal forecasting of winter temperatures in South Korea.
How to cite: Lee, W.-S., Kim, G., and Yang, S.: Combined Effect of ENSO and AO on Winter Temperatures of the Korean Peninsula on Subseasonal Time Scales, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-68, https://doi.org/10.5194/ems2024-68, 2024.
Extreme cold winters have been projected to decrease in the future, although their impacts on society are still significant. The goal of this study is to assess whether climate change affects the atmospheric mechanisms leading to cold winters.
We first explore the dynamics of 15-day winter cold spells in France, as observed since 1950. We find that the most extreme events tend to have the same atmospheric circulation pattern, consisting of an eastward-shifted NAO- dipole. We calculate an atmospheric index that characterizes this dipole. Then, using a stochastic weather generator with importance sampling, we show that this is a sufficient condition to trigger extremely cold temperatures in France, and that it performs better than a classical North Atlantic Oscillation index. This suggests that a dipole of atmospheric circulation is a necessary and sufficient condition leading to extreme cold spells in France.
We use this atmospheric index to select the CMIP6 models that best reproduce the identified dynamics leading to extreme cold spells of 15 days. Using a stochastic weather generator with importance sampling, we run simulations of worst-case winter cold spells from 2015 to 2100, following different emission trajectories for the selected models. 15-day winter cold spells in France will reach less extreme temperatures at the end of the century, especially in the case of a high-emission scenario (SSP5-8.5). However, the simulated ensembles of extreme cold spells do not show the same warming trend as the mean temperature, and very extreme cold spells are still possible in the near future. The atmospheric circulation prevailing during these events is analyzed and compared with the circulation observed during previous events.
How to cite: Cadiou, C. and Yiou, P.: Assessing changes in the intensity and dynamics of extreme cold spells in France from CMIP6, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-263, https://doi.org/10.5194/ems2024-263, 2024.
The Interactive Atlas of the IPCC AR6 Working Group 1 is used to analyse the geographical differences of climate projections within Europe. For this purpose, the multi-model and multi-scenario ensemble is evaluated from global climate model simulations (available via the CMIP6, i.e. Coupled Model Intercomparison Project Phase 6 of the World Climate Research Programme) including four different SSP-RCP scenario pairs (i.e. from immediate rapid mitigation and effective adaptation, SSP1-RCP2.6, to no mitigation with highly challenging adaptation, SSP5-RCP8.5). The study provides key information so the regional and national adaptation strategies for different socio-economic sectors can be built and/or updated accordingly.
This study focuses on temperatures extremes, e.g. the monthly frequency of heat days with daily maximum temperature above 35 °C. The target periods cover two decades on the medium- and long-term, i.e. mid-century (2041-2060) and late-century (2081-2100), respectively, and the reference period defined as the last two decades of the historical simulation period (1995-2014). Several zonal and meridional segments were defined over Europe, along which the projected changes are analysed with a special focus on sea cover, continental, and topography effects. Furthermore, the consequences of different scenarios are also compared. The results clearly show that greater radiative forcing change implies more severe health effects via the more frequent heat stress events. However, substantial differences can also be identified from south to north as well, as from west to east.
Acknowledgements: Research leading to this study has been supported by the European Climate Fund (G-2309-66801), the Hungarian National Research, Development and Innovation Fund (under grants PD-138023 and K-129162), and the National Multidisciplinary Laboratory for Climate Change (RRF-2.3.1-21-2022-00014).
How to cite: Pongracz, R., Divinszki, F., and Kis, A.: Analysis of zonal and meridional differences in projected changes of extreme temperature within Europe using CMIP6 simulation data under different scenarios, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-626, https://doi.org/10.5194/ems2024-626, 2024.
The tropics are the most active regions of convective activity on earth, which plays a crucial role in global atmospheric energy and water cycle. Climate change due to increases of greenhouse gas concentrations has led to a substantial increase in intense convection and extreme precipitation. We use simulations from a global cloud-system resolving model, Nonhydrostatic Icosahedral Atmosphere Model (NICAM), to study the future changes of extreme convection include overshooting deep convection (ODCs) and the most intense convection (MICs) at the end of the 21st century. Combining TRMM satellite observations with ERA5 reanalysis data, we find that the NICAM well reproduces the spatio-temporal distributions of TRMM observed extreme convection and atmospheric environment. The results show that future extreme convection will show a globally increasing trend with climate warming. However, the trends vary either between different convective property thresholds or between different regions. For example, the future occurrences of ODCs with cloud top height above 15.5 km, 16.9 km and 18.3 km scaled by the global temperature increase will increase by 7%/K, 27%/K and 90%/K, respectively, over ocean where the atmosphere becomes warmer and wetter in a warming world. The corresponding changes are -1%/K, 10%/K and 37%/K over land where the atmosphere is hotter but drier. In the other hand, the frequency of MICs will increase significantly in the Atlantic and central Pacific, while decrease slightly in central and northern Africa. Specifically, the frequency of MICs in the Northern Hemisphere will increase significantly in boreal summer and decrease in boreal winter. Nevertheless, the GCRM simulations also show some discrepancies compare to the observations, i.e., the simulated convection over northeast of the South America was significantly less, which still need a further improvement. However, GCRM will be a crucial tool for studying global climate change and will undoubtedly provide important assistance for us to better address future climate change.
How to cite: Wu, X., Fu, Q., Jia, D., and Kodama, C.: The Future Responses of Tropical Extreme Convection to Climate Change based on GCRM simulation, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-209, https://doi.org/10.5194/ems2024-209, 2024.
A cluster of severe extratropical cyclones (Dudley, Eunice and Franklin) hit North-Western Europe within one week in February 2022, and caused widespread damage and fatalities by strong wind gusts and high accumulated precipitation amounts. These cyclones developed over the North-Atlantic within a baroclinic environment with strong jet streams accompanied by atmospheric rivers. Extratropical cyclone clustering is counterintuitive because individiual cyclones reduce large-scale temperature gradients and baroclinicity that are essential for their growth. We hypothesise that diabatic heating through latent heat release enhances the baroclinic environment favourable for secondary cyclogenesis. To quantify the influence of latent heat release on this baroclinic environment, we performed idealised model experiments with the Open Integrated Forecast System (OpenIFS) from the European Center for Medium Range Weather Forecastst (ECMWF). The latent heat of vaporisation constant was enhanced and reduced by 50 percent respectively.
The control experiment captured the location, speed and direction of the jet stream, and the path and intensities of the individual cyclones well. The model results show that reduced latent heating weakens the jet stream strength, while enhanced latent heating strengthens the jet stream strength. The baroclinic environment responds similarly — i.e., the meridional temperature gradient decreases with reduced latent heating and increases with enhanced latent heating. We plan to apply the isentropic slope diagnostic for baroclinicity to quantify the diabatic contributions of latent heating to the baroclinic environment. We also plan to discuss the effects of latent heating on the individual cyclones. With this case-study we explore diabatic heating as a pathway for extratropical cyclone clustering.
How to cite: Batelaan, T. J., Weijenborg, C., and Steeneveld, G.-J.: The Role of Latent Heating in a North-Atlantic Baroclinic Environment Conducive to Extratropical Cyclone Clustering, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-634, https://doi.org/10.5194/ems2024-634, 2024.
The Best Track from China Meteorological Administration (CMA) tropical cyclone database as well as the observation data of high level and surface stations, minute precipitation, radar, satellite and reanalysis data were used to analyze both periodic characteristics and cause of continuous heavy rainfall induced by the “north going” landing Typhoon Yagi(1814) in Shandong. The results show: During the period under the influence of Yagi, the torrential rain has distinct phases. The precipitation of typhoon body and the cold air and typhoon interaction are much stronger than those of two other stages. The precipitation formed by the typhoon body has larger range but weaker intensity than that of the heavy rain phase when the cold air interacted with the typhoon. The typhoon body precipitation phase has low-centroid tropical precipitation features, while the cold air and typhoon interaction phase shows high-centroid frontal precipitation features. During the northward movement of Yagi, the water vapor transport conditions were significantly abnormal compared with those of normal years, which was favorable for persistent heavy rainfall. During the typhoon body precipitation stage, the main heavy precipitation area is located in the forward direction of the typhoon, while in the cold air interacted with the typhoon stage, the main heavy precipitation area is located in the direction of the typhoon and the spiral rain band on the right side of Yagi. The top of low-level jet, vertical wind shear in the low and middle troposphere and convergence line above ground surface together determine where the main heavy precipitation area occurs. The moving direction of typhoon is consistent with both the long axis direction of isentropic potential vorticity (IPV) and the moving direction of high value center of IPV. During the typhoon movement, the IPV has a significant increase, which is caused by the transfer from the higher-level atmosphere that has high potential vorticity (PV) cold air to the lower level atmosphere. During the later period of Yagi landfall, the spiral rain band developed, while the mesoscale convection characteristics were significant. This is related to frontogenesis induced by the cold air invasion as well as high-altitude terrain. On the one hand, the frontogenesis in the cold air and typhoon interaction phase is obvious. On the other hand, the high-altitude terrain in the central and eastern regions of Shandong which plays an uplifting role and the frontogenesis of the weather system are conducive to the convective system enhancement, therefore promote the spiral rain band to develop.（This paper has been published in Plateau Meteorology）
How to cite: Sun, S., Chen, B., Sun, J., Sun, Y., Diao, X., and Wang, Q.: Periodic Characteristics and Cause Analysis of Continuous Heavy Rainfall Induced by Typhoon Yagi (1814) in Shandong, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-258, https://doi.org/10.5194/ems2024-258, 2024.
Recent several studies have focused on the predictability of tropical cyclone track forecast. As a response to the question issued by Landsea and Cangialosi (2018) about "the approaching limit of predictability for tropical cyclone (TC) track prediction is near or has already been reached", Zhou and Toth (2020) (short for ZT20) and Yu et al. (2022) (short for Y22) have found that the limit of predictability for TC track prediction has not been reached both in Atlantic (ATL) and Western North Pacific (WNP) basins. However, the predictabilities are different in two basins, as ZT20 found that 1 day's improvement can be obtained through 10 years in ATL, while Y22 found that 2 days' improvement can be obtained through 15 years in WNP. To reveal the causes of this difference, the predictability of TC track in WNP is first investigated under the same framework as ZT20. Then important parameters that determined the predictabilities are found and analyzed. Results suggested that the growth rate of true track forecast error in WNP is higher than that in ATL, indicating a lower predictability in WNP. Further explorations suggested that TCs in WNP basin have averagely larger sizes, stronger intensities, lower-latitude locations, and poorer forecast skills of their guided flows. All these factors contribute to the larger track forecast error growth rate. Moreover, it is pointed out that as the improvement of forecast skills over years mainly due to the reduction of initial analysis errors, although a lower predictability is found in WNP, the forecast skill improvement in WNP is faster than that in ATL.
How to cite: Zhou, F. and Ye, Y.: Reasons for Different Predictability of Tropical Cyclone Tracks in the Western North Pacific and Atlantic Oceans, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-39, https://doi.org/10.5194/ems2024-39, 2024.
