AS1.14
Orals: Wed, 26 Apr | Room M1
A large number of intense cyclones occur every year in the Mediterranean basin, a relatively small and densely populated region, but also a worldwide climate-change hotspot. Given their importance for the variability of the regional climate and its extremes, Mediterranean cyclones have lately attracted much of attention, especially due to the broad range of severe socio-economic and environmental impacts that they produce.
This talk aims at summarizing the concentrated knowledge of the last decade on the dynamics, climatology and relevant impacts of Mediterranean cyclones. We will especially focus on the processes that take place in different spatiotemporal scales triggering cyclogenesis and turning Mediterranean cyclones into catastrophic storms. We will also discuss the role of the unique regional geographical features therein, along with the influence of the latitudinal location of the Mediterranean basin. Finally, we will discuss the different subtypes of Mediterranean cyclones that develop in the region, devoting special attention to medicanes, i.e. cyclones with tropical characteristics and subjects of numerous recent studies. Througout the talk, research perspectives that advance the field of Mediterranean cyclones as a whole will be highlighted, along with current trends in community efforts within the framework of MedCyclones COST Action that address relevant topics to the complex dynamics of Mediterranean cyclones and consequent severe socio-economic impacts.
How to cite: Flaounas, E.: Mediterranean cyclone dynamics and climatology: current knowledge and research perspectives, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-6278, https://doi.org/10.5194/egusphere-egu23-6278, 2023.
Previous studies showed that the midlatitude atmospheric circulation generally shifts poleward in response to climate change induced by increased greenhouse gas concentration, including the midlatitude storm track and the eddy-driven jet. The magnitude of this shift varies widely between different climate models and depends on the season, hemisphere and longitude. In this study we aim to reexamine the connection between the shifts of the sensible eddy heat flux and the eddy-driven jet in response to climate change and the role of diabatic heating and latent eddy heat flux in this relation. Our approach is to use the constraints of the zonally averaged heat and momentum budgets in order to connect the eddy-driven jet latitude to the heat budget terms. First, we examine the relation between the eddy-driven jet latitude and the eddy heat flux latitude in the inter-model spread of CMIP6 models. We find that the latitudinal separation between the eddy heat flux and eddy-driven jet depends on the amount of diabatic heating in the midlatitude midtroposphere, which varies widely between different models. This relation is explained based on the heat and momentum budgets.
Next, we use an idealized general circulation model with interactive water vapor and full radiation. We customized the model with different levels of saturation vapor pressure by increasing CO2 concentration and by increasing the humidity factor in the Clausius-Clapeyron relation. We found that in both the cases the atmospheric circulation responds in a similar way and the heat budget terms shift upward and poleward, signifying an upward and poleward shift of the storm track. We found that when the diabatic heating rises upward and strengthens enough over the midlatitude mid-troposphere in response to climate change, the adiabatic cooling by the Ferrel cell rising branch balances the diabatic heating and an equatorward shift of the eddy driven jet and the Ferrel cell is observed. These results provide further insight to the relation between the responses of the midlatitude circulation and the poleward energy flux terms to climate change.
How to cite: Ghosh, S., Lachmy, O., and Kaspi, Y.: The latitudinal shift of the midlatitude atmospheric circulation in response to climate change and the role of midlatitude diabatic heating, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-1801, https://doi.org/10.5194/egusphere-egu23-1801, 2023.
Cyclones carry heat and moisture that impact local conditions along their path. Cyclones with different origins can, however, have different life cycles and cause different impacts. To quantify differences in the thermodynamic evolution of cyclones originating from different latitudes during wintertime, we separate the cyclones according to their origin (cyclogenesis location): midlatitude (ML) cyclones originating in the North Atlantic and high-latitude (HL) cyclones originating in the Nordic Seas and Barents Seas. It is found that HL cyclones generally carry lower thermodynamic energy as they originate in a cold environment. In contrast, ML cyclones have much higher thermodynamic energy throughout their lifecycle, even though they lose a large amount of heat as they travel long distances from their origin towards the Arctic. For a given region in the high latitudes (e.g., the Barents Sea), the mean vertical profiles of temperature and moisture from the HL group are colder and drier compared to the ones from the ML group, but the maximum values in the HL group can reach those of the ML group. Further analysis for the top 10% warmest profiles in the HL group suggests that these HL cyclones form in a preconditioned warm and moist environment. The precondioning is set up by the large-scale circulation with influences from the upstream North Atlantic. Under special conditions, the formation of high latitude cyclones in a preconditioned warm and moist environment can lead to extreme warming events in the deep Arctic like the one during New Year’s 2015/16.
How to cite: Tao, D., Li, C., Davy, R., He, S., Michel, C., and Rosendahl, A.: The thermodynamic differences between winter cyclones from midlatitudes and high latitudes, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-1890, https://doi.org/10.5194/egusphere-egu23-1890, 2023.
Extratropical cyclone airstreams, such as warm conveyor belts (WCBs), are linked to strong precipitation along with latent heat release at low levels and, thus, changes in the low-level PV distribution. Previous studies have shown significant changes in PV anomalies in a future climate under the RCP8.5 scenario, which are also associated with changes in strong near-surface winds. However, the source of these PV anomalies is still unclear, especially at upper levels. Based on the 1% strongest winter-cyclones in the North Atlantic (NA) region over the two periods 1990-2000 and 2091-2100, we adopt a Lagrangian perspective to investigate such changes in CESM Large Ensemble simulations.
Backward trajectories are computed to explicitly identify the contributions of diabatic processes to future changes in cyclone-associated PV anomalies. Moreover, the role of specific airstreams in PV generation/destruction is examined with Lagrangian composites.
The results show a sinificant change in the mean trajectory properties 24 hours before the maximum cyclone intensity at low and upper levels. This period of 24 hours is taken to construct Lagrangian composites at 700 hPa and 250 hPa, which provide insights into changes in WCB and dry intrusion (DI) airstreams. We further analyze these airstrem changes by constructing cross sections downstream (WCB regime) and at the equatorward side (DI regime) of the cyclone center.
In general, increased diabatic heating along backward trajectories amplifies positive PV anomalies near the cyclone center at both lower and upper levels in a warmer future climate. More specifically, a poleward and upward shift of the WCBs with a larger PV production at middle levels are is found. DIs near the cyclone center are projected to be responsible for stronger PV production at low levels to the south of the cyclone center. At upper levels, the decreased PV anomaly to the south of the cyclone center results from a combined effect of a decreased climatological PV in the NA region and a shift in the origin of the air masses. The increasing importance of diabatic processes in a wamer climate suggests that a better representation of these processes in climate models is necessary to reduce uncertainties.
How to cite: Dolores-Tesillos, E. and Pfahl, S.: Future changes in North Atlantic winter cyclones in CESM-LE simulations from a Lagrangian-composite perspective, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-6851, https://doi.org/10.5194/egusphere-egu23-6851, 2023.
Mediterranean cyclones are extratropical cyclones, typically of smaller size and weaker intensity than other cyclones that develop over the main open ocean storm tracks. Nevertheless, Mediterranean cyclones can attain high intensities, even comparable to the ones of tropical cyclones, and thus cause large socio-economic impacts in the densely populated coasts of the region. After cyclogenesis takes place, a large variety of processes are involved in the cyclone’s development, contributing with positive and negative potential vorticity (PV) changes to the lower-tropospheric PV anomalies in the cyclone center. Although the diabatic processes that produce these PV anomalies in Mediterranean cyclones are known, it is still an open question whether they occur locally within the cyclone itself or remotely in the environment (e.g., near high orography) with a subsequent transport of high-PV air into the cyclone center. This study introduces a Lagrangian method to determine the origin of the lower-tropospheric PV anomaly, which is applied climatologically to ERA5 reanalysis and to 12 monthly simulations, performed with the IFS model. We define and quantify so-called "cyclonic" and "environmental" PV and find that the main part of the lower-tropospheric PV anomaly (60%) is produced within the cyclone, shortly prior (-12 h) to the cyclones’ mature stage. Nevertheless, in 19.5% of the cyclones the environmental PV production near the mountains surrounding the Mediterranean basin plays a significant role in forming the low-tropospheric PV anomaly, and therefore in determining the intensity of these cyclones. The analysis of PV tendencies from the IFS simulations reveals that the major PV production inside the cyclone is typically due to convection and microphysics, whereas convection and turbulent momentum tendencies evoke most of the positive PV changes in the environment.
How to cite: Scherrmann, A., Flaounas, E., and Wernli, H.: Origin of low-tropospheric potential vorticity in Mediterranean cyclones, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2195, https://doi.org/10.5194/egusphere-egu23-2195, 2023.
Explosive cyclones (ECs), defined as developing extratropical cyclones that experience pressure drops of at least 24 hPa in 24 hours, are impactful weather events which occur along highly populated coastal regions in the eastern United States. These storms occur due to a combination of atmospheric and surface processes, such as jet stream intensification and latent heat release. Even though previous literature has elucidated the role of these processes in EC formation, the sources of interannual variability that impact seasonal EC frequency are not well known. To analyze the sources of interannual variability, we track cases of ECs and dissect them into two spatial groups: those that formed near the east coast of North America (coastal) and those in the North Central Atlantic (high latitude). The frequency of high-latitude ECs is strongly correlated with the North Atlantic Oscillation, a well-known feature, whereas coastal EC frequency exhibits a growing relationship with an atmospheric wave-train emanating from the North Pacific in the last 30 years. This wave-train pattern of alternating high-and-low pressure resulted in heightened upper-level divergence and baroclinic instability along the east coast of North America. Using a coupled model experiment, we show that the tropical Pacific Ocean and North Pacific oceans are the main driver of this atmospheric wave train and the subsequent enhancement seasonal baroclinic instability in the North Atlantic.
How to cite: Stuivenvolt-Allen, J., Wang, S. S.-Y., Chikamoto, Y., Meyer, J., and Johnson, Z.: Growing Pacific Linkage with Western North Atlantic Explosive Cyclones, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9393, https://doi.org/10.5194/egusphere-egu23-9393, 2023.
Extreme precipitation drives a series of natural disasters such as floods, flash floods, landslides, or crop losses. These disasters directly impact people's lives, their homes, and their food security. Located at the edge of the subtropics, on the northern edge of the Sahara desert, Morocco is particularly vulnerable to extreme precipitation. Indeed, between 1951 and 2015, Morocco experienced more than 35 major floods, which resulted in significant material and human losses. Understanding the spatio-temporal characteristics of extreme precipitation is key to better predicting and mitigating the risks associated with extreme precipitation events (EPEs). Yet, the spatio-temporal distribution and physical drivers of extreme precipitation in Morocco remain poorly understood. To address this gap, we apply temporal and spatial clustering methods to precipitation data from the ERA5 database as well as from observational databases to identify the main drivers of EPEs in Morocco. We find that Morocco exhibits five spatially coherent regions in terms of EPE timing, corresponding to mixed influences of large-scale extratropical and tropical weather systems. Indeed, EPEs in northern regions are caused by weather patterns similar to the negative phase of the North Atlantic Oscillation (NAO), associated with strong upper air flow enhanced by Greenland blocking and Rossby wave breaking (RWB). By contrast, extreme precipitation in southern regions is associated with tropical-extratropical interactions. There, EPEs are linked to an intense water vapor transport from the tropics and a relatively weak upper air flow.
How to cite: Chaqdid, A., Tuel, A., EL Fatimy, A., and EL Moçayd, N.: Extreme rainfall events in Morocco: spatio-temporal characteristics and climate drivers, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-1812, https://doi.org/10.5194/egusphere-egu23-1812, 2023.
Cyclone clustering, the swift succession of multiple extratropical cyclones in a geographically confined region during a short period of time, constitutes a large fraction of European weather extremes. The idea that several cyclones follow a similar track dates back to the centennial concept of cyclone families of Bjerknes and Solberg. To investigate the dynamical causes of cyclone clustering, it is necessary to diagnose the occurrence of cyclone clustering and to determine their characteristics. So far, most diagnostics focused either on local impact or on a statistical analysis of storm recurrence. While the first cannot be applied globally, the latter is difficult to relate to individual events. We therefore use a novel method to globally detect cyclone clustering that is closer to the original concept of Bjerknes and Solberg, where extratropical cyclones follow similar tracks within a given time period.
How to cite: Weijenborg, C., Spengler, T., and Priestley, M.: Global climatology of cyclone clustering in present and future climates, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8826, https://doi.org/10.5194/egusphere-egu23-8826, 2023.
The risk from individual natural hazards (such as extratropical cyclones) can be large, but the aggregate loss over yearly timescales is significantly greater. For example, wind damage from the three major European windstorms in February 2022 caused more than €3.5 billion of insured losses.
This study proposes a random sum modelling framework for understanding the correlation between aggregate risks that occur from compound events. By considering the frequency and intensities of compound events as random variables, the framework provides an expression for correlation between two random sums (which each represent different types of loss from compound events).
The framework shows that this correlation will generally increase monotonically towards one as the dispersion (clustering) of the number of events increases. Under certain conditions, the correlation will always monotonically increase with dispersion.
The framework has been illustrated by applying it to annual sums from 1980-2020 using wind speed and precipitation as proxy measures for insured loss. This is calculated from ERA5 reanalysis data which includes 39587 storm events and covers the European region and Atlantic Ocean (from 30°N 100°W to 75°N 40°E).
The framework performs well, capturing the general behaviour of the correlation, with large positive correlation over the N. Atlantic Ocean and weaker correlations over European land regions.
How to cite: Jones, T., Stephenson, D., and Priestley, M.: A framework for understanding the correlation between aggregated losses of compound events, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-14446, https://doi.org/10.5194/egusphere-egu23-14446, 2023.
Austral summer precipitation increased by ~25% from 1902-2021 in southeastern South America (SESA; southern Brazil, Paraguay, Uruguay, and northern Argentina); one of the largest observed precipitation trends globally. Previous studies have attributed the SESA precipitation increase to the Atlantic multidecadal variability (AMV), stratospheric ozone depletion, or greenhouse gas (GHG) forcing. While models simulate SESA precipitation increases due to anthropogenic forcing over the projection interval, a precipitation trend as large as that observed is almost never captured in model simulations over the historical period. We first assessed this model behavior by characterizing the relationship between the South American low-level jet and SESA precipitation using analyses of low-level moisture fluxes within the jet region. We use these fluxes as a metric for predicting SESA precipitation in observations, reanalysis, and simulations from phase 6 of the Coupled Model Intercomparison Project from 1951-2014 (the period of overlap between reanalysis data and historical simulations). We find that while the simulated relationships between the jet and SESA precipitation are consistent with observations, only ~2% of the historical simulations yield a SESA precipitation trend that falls within the observed uncertainty range. Therefore, the simulated low-level jet dynamics do not appear to resolve the muted CMIP6 estimates of 20th- to 21st-century SESA precipitation trends. We subsequently characterize the influence of large-scale dynamics on the SESA precipitation trend. We take particular interest in quantifying the roles of the AMV, stratospheric ozone depletion, and GHG forcing on the SESA precipitation trend across seasons and through time in observations compared to CMIP6. These large-scale dynamics are interpreted in terms of the unexplained portions of the historical trend and interpreted in terms of their representation in the CMIP6 models.
How to cite: Varuolo-Clarke, A., Smerdon, J., and Williams, P.: The mystery of multidecadal precipitation trends in southeastern South America, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9454, https://doi.org/10.5194/egusphere-egu23-9454, 2023.
Posters on site: Wed, 26 Apr, 10:45–12:30 | Hall X5
The widening of the Southern Hemisphere tropical meridional circulation has been attributed to various forcings from increased greenhouse gases, ozone depletion and natural variability. While climate models can reproduce some characteristics of this observed change, there is some uncertainty in the operating mechanisms and driving regions setting the edge of the tropical circulation. Here we examine the impacts of systematic model biases of the atmosphere-only Unified Model onto the simulation of the Southern Hemisphere tropical extent. We utilise nudging experiments with prescribed sea-surface temperatures, where potential temperature and horizontal winds are relaxed back to reanalysis for a 20-year period. Specifically, experiments with regionally-defined bias correction aide to determine the influence of remote model biases on the tropical width. The experiments are applied to different tropical width metrics previously identified to measure the boundary between the tropical to extratropical circulation. We uncover a more consistent improvement of the location of the Hadley cell edge by correcting Southern Hemisphere extratropical circulation biases, than tropical ones. The analysis is further expanded to the range of atmosphere-only model simulations of the Coupled Model Intercomparison Project Phase 6 (CMIP6). We explore the relationships between tropical and extratropical biases and the models’ representation of the Hadley cell.
How to cite: Freisen, P., Arblaster, J., Jakob, C., and Rodriguez, J.: Tropical and extratropical circulation biases and the Southern Hemisphere Hadley cell width, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-10041, https://doi.org/10.5194/egusphere-egu23-10041, 2023.
European windstorms are among the natural hazards with the highest economic losses. We investigate the impact of European windstorms under recent and future climate conditions at high spatial resolution. With this aim, we use hourly surface wind data at 30 km resolution from ERA5 reanalysis for 1959-2021, and 3-hourly surface wind data at 12.5 km resolution from 60 different global-to-regional climate model (GCM-RCM) chains from EURO-CORDEX (EUR-11). The windstorm activity is compared in 30-year periods from the historical events (1976-2005) to the future events (under RCP8.5 scenario) at global warming levels (GWL) of +2°C and +3°C. We apply different indices (meteorological index and loss index) to quantify the severity of windstorms and to estimate the corresponding impacts. For the historical period, storm Wiebke in 1990 (storm names as used by the German Weather Service DWD) caused the highest loss for Central Europe, followed by storm Lothar in 1999. The United Kingdom and Germany are countries in Central Europe that have the highest loss index (more vulnerable to the European windstorms). The results from the EURO-CORDEX ensemble show only small changes in windstorm activity between the historical period and the different GWLs, but display decadal variability.
How to cite: Alifdini, I., Moemken, J., and Pinto, J. G.: European windstorm risk at the regional scale under recent and future climate conditions, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13929, https://doi.org/10.5194/egusphere-egu23-13929, 2023.
European winter storms are a significant threat to communities, public infrastructure, and private and commercial properties. On seasonal timescales, potential predictability was evidenced in recent state-of-the-art seasonal hindcast suites e.g., the UK Met Office’s GloSea5. Related positive and potentially usable forecast skill for frequency and intensity measures were based on pre-season model initialisation around the beginning of November for the following core winter (DJF) season’s assessment.
This study expands on these findings by analysing extended lead times of seasonal forecast into autumn and late summer before the winter season. Here, in a systematic way, a multi-model ensemble of hindcasts is analysed to evaluate current models’ capability to forecast the seasonal activity for initialisations from September to November. First results indicate potential predictability precursors already from the September initialisations for storm frequencies. These results vary from model to model though. The presentation will discuss differences between models as well as lead times for both, storm frequency and intensity.
How to cite: Leckebusch, G. C., Degenhardt, L., Berrie, E., Ng, K. S., and Spreitzer, E.: Assessing the boundaries of seasonal forecast skill for European winter storms from different hindcast suites, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-14695, https://doi.org/10.5194/egusphere-egu23-14695, 2023.
Windstorms pose continual risk to Europe. Among their associated hazards, strong near-surface winds can be particularly damaging, threatening infrastructure, life and billions of pounds in insured losses. Insurers (and reinsurers) therefore need to prepare for the potential cost of extreme windstorms. Storm severity indices (SSIs) have been developed to quantify the potential losses associated with windstorm winds. Here, the most extreme windstorms that could potentially occur in the current climate are estimated using seasonal forecast data together with a cyclone-tracking algorithm, and their potential losses quantified using an SSI. As maximum wind gusts, the typical input variable for SSIs, are not available in the seasonal forecast dataset, a method is developed to calculate SSIs using wind speed data and a bias correction used to convert to SSI values representative of those obtained when calculated using wind gusts. Nearly 700 extended winter seasons of forecast data are analysed, representing a much larger sample of potential windstorms compared to that available from reanalysis or observational products. The storm track is reasonably well represented in the seasonal forecast data: spatial features are similar to those in a reanalysis, but there exists a slight poleward bias and underestimation of number of storms per season (maximal underestimation of around 10%). Additionally, distributions of SSI values for several countries in Europe are similar in the forecast data and reanalysis. Together, these suggest that the seasonal forecast data is suitable for analysing windstorm statistics and informing on potential extreme storms. We give estimates of worst-case storms, and worst-case seasons, that are identified in the forecast data and compare to those seen in a reanalysis, highlighting the potential insurance loss implications.
How to cite: Maddison, J., Catto, J., Siegert, S., and Hansen, S.: Estimating worst-case European windstroms, and worst-case seasons, using seasonal forecasts., EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-14410, https://doi.org/10.5194/egusphere-egu23-14410, 2023.
European windstorms are a frequent and damaging natural hazard that can cause loss of human life and damage to property and infrastructure. As there is a high degree of uncertainty in climate projections, it is crucial to understand the physical risks and economic losses at regional and local scales associated with European Windstorms. In this study, we develop a simple model to estimate historical windstorm losses over the European region. The model uses winds from the ERA5 reanalysis and different exposure datasets based on countrywide total insured property values, gross domestic product, and historical population density.
We find that the estimated losses associated with major historical storms in North-western Europe and estimated average EU-wide losses are comparable to the reported estimates and those from propriety vendor models. However, estimated losses from windstorms in France and Germany are lower than reported. Differences in the estimated losses are attributed to the contrasts in the regional-level exposure within and between different exposure datasets. We also tested the sensitivity of regional-level vulnerabilities and find that accounting for regional-level vulnerability differences slightly improves the biases in countrywide losses. Further, we also find that the major contribution to the estimated losses comes from the United Kingdom, France, and Germany for most of the storm seasons, and thus it is important to correctly represent the exposure and vulnerabilities over these countries. The ability of the model to estimate reported losses is also limited by the representation of the winds in ERA5, which has limited skill in representing the hazard footprint, especially for specific storms such as the Great October Storm of 1987.
Keywords: Losses, Windstorms, Climate Change, Natural Hazards
How to cite: Kumar, D., Shaffrey, L., Dixon, R., Bloomfield, H., Bates, P., and Hillier, J.: High-resolution loss modeling for European Windstorms, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12391, https://doi.org/10.5194/egusphere-egu23-12391, 2023.
Storm surges in the German Bight can have great destructive potential. This includes devastating floods, structural damage to infrastructure, and even loss of life. The most important driver of storm surge events in the German Bight is strong winds from north-westerly directions, often related to intense extra-tropical cyclones travelling from the North Atlantic into the North Sea region.
Making use of an objective, impact-oriented identification and tracking scheme, we analyse storm events related to storm surges in the German Bight. This particular version of the tracking algorithm includes the so-called Storm Surge Severity Index (SSSI) and is used as a complementary tool in operational forecasting by the German Federal Maritime and Hydrographic Agency (BSH). The SSSI takes wind speed and direction into account and intends to quantify storm surge risk in the German Bight. However, to date, the SSSI has never been systematically evaluated for past storm surge events. To fill this gap and to prove that the SSSI can be used as a proxy for storm surge risk, we analyse the relationship between SSSI values of past storm events and the associated water levels recorded in the German Bight using ERA5 atmospheric reanalysis data. Moreover, we analyse potentially storm surge-relevant storms in a multi-model ensemble of global climate model simulations to assess potential future changes in storm surge risk in the German Bight.
How to cite: Schaffer, L., Becker, N., Schenk, L., Hinrichs, C., Ditzinger, G., Schade, N. H., Befort, D. J., and Kruschke, T.: Objective assessment of storm surge risk in the German Bight – historical events and future climate change, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7236, https://doi.org/10.5194/egusphere-egu23-7236, 2023.
In the mid-latitudes, synoptic-scale phenomena like high and low-pressure systems generate the variability of the regional-scale weather system. To identify the weather variability of extra-tropical region storm track activity has been analyzed based on observations since the mid-nineteenth century. After early-stage research that directly counted the movement of cyclones, the time filtering method based on grid analysis has been used for an isolated disturbance with periods of 2~7 days. This bandpass filtering method has the advantage of being able to examine the distribution and the variability of the storm track spatially in vertical and horizontal space.
In this study, we confirm the storm track activity in the East Asia region using the dynamical down-scale results from CORDEX (COordinated Regional climate Downscaling EXperiment) East Asia projects. We verify the reproducibility and confirm the temporal change in the storm track activity from various RCM data. In addition to the historical period, we examine the difference in storm track intensity over future climate change scenarios. Through this, we also discuss the role of added value from RCM.
This work was funded by the Korea Meteorological Administration Research and Development Program under Grant KMI2022-01210.
How to cite: Byun, U.-Y., Chang, E.-C., Kim, J., Cha, D., Ahn, J.-B., and Min, S.-K.: Extratropical storm track activity change in future climate change scenarios, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-10466, https://doi.org/10.5194/egusphere-egu23-10466, 2023.
Windstorms (major winter storms) are one of the most important natural hazards in Europe. Despite the large observed socioeconomic losses, the impact of windstorms and its decadal variability is not yet fully understood. This study aims to assess the loss potentials associated with European windstorms and the variability in the wind speed climatology across Europe. We use the 12,500-years LAERTES-EU (LArge Ensemble of Regional climaTe modEl Simulations for EUrope) RCM ensemble to study the spatio-temporal distribution and variability of windstorms over Europe. LAERTES-EU is validated against reanalysis data (ERA5) and available ground-based station observations. The associated windstorm losses are estimated by computing statistics of extreme wind speeds and related indices. Different loss indices are validated using historical loss data from the insurance sector. The results reveal that the loss index (LI) is a good proxy for the estimation of potential losses associated with windstorms across Europe in winter. The derived statistics of extreme windstorms such as return periods (RP) show hardly any change in the severity and frequency of windstorms during the covered period 1900-2028, but a strong decadal variability is apparent.
How to cite: Sethunadh, J., Pinto, J. G., Ludwig, P., Feldmann, H., and Ehmele, F.: Decadal variability of extreme winds and potential storm losses in Europe using large RCM ensembles, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13914, https://doi.org/10.5194/egusphere-egu23-13914, 2023.
Extratropical cyclones produce extreme surface wind speeds and heavy rainfall which can individually and jointly influence impacts and potentially produce large aggregate impacts. Within this study, we assess the UKCP 12-member ensemble of local convection-permitting 2.2 km climate projections. We quantify the likelihood of cyclones producing large footprints of both extreme winds and rainfall over the UK in a control (1981-2000) and future (2061-2080, RCP8.5) climate simulation. Following this, we characterise the convective and frontal drivers of wet and windy conditions within cyclones, and identify the characteristics of cyclones, their tracks and interactions with the jet stream that contribute to the occurrence of large, combined footprints in the control and future simulations. The future simulations project an increased probability of extratropical cyclones producing extremely wet and windy conditions in the same storm, as well as an increase in the land area covered by such conditions. In both the control and future simulations, combined wet and windy extremes largely occur close to cold and warm fronts, likely due to the warm conveyor belt which produces heavy rainfall (due its ascent over the frontal boundaries) and high winds (when occurring within a region of tight pressure gradients). Cyclone composites reveal that the largest changes in joint extremes are closely located within the sector of cyclones where we expect to see the warm conveyor belt, suggesting their change arises partly through the response of this shared driver rather than being a simple consequence of increased rainfall due to thermodynamics. In further analysis, we identify favourable conditions and cyclone characteristics that lead to cyclones producing large rainfall and wind footprints over the UK.
How to cite: Manning, C., Kendon, E., Fowler, H. J., Catto, J. L., Chan, S. C., and Sansom, P.: Drivers of large footprints of extreme winds and rainfall and their projected future changes, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-13180, https://doi.org/10.5194/egusphere-egu23-13180, 2023.
In the semi-enclosed basin of the Mediterranean Sea, a wide variety of cyclone mechanisms are known to develop, including baroclinic waves coming from the Atlantic, Mediterranean cyclones originating from the cut-off of baroclinic waves, Warm Seclusions, Tropical-Like Cyclones (TLC), Rapid-Cyclogeneses (RC) and Intense Mediterranean Cyclones (IMC). Depending on the cyclone's type, the characteristic frequency of appearance can vary, ranging from tens per month to around 1-1.5 per year, as in the TLC case. RCs are among the rarest and probably most intense and destructive cyclone events that can develop in nature; they usually originate at high latitudes, during wintertime, and mainly over the sea, preferring areas with high Sea Surface Temperature (SST) gradients. It is generally accepted that these events are described by quick drop of pressure, close to 1hPa/hr for 24 hours, within the eye of the cyclone. Several recent studies investigated the formation of RC’s over Mediterranean Basin (MB). RCs formation is an extremely complicated process, and in the MB it is mostly driven by dry air intrusions from the stratosphere and by the trigger of Atmospheric Rivers.
Using ERA5 dataset, we firstly conducted a physical and dynamical analysis of the most intense cyclone events occurred in the Mediterranean basin in the period 1979-2020, identifying factors which triggered, generated and contributed to the intensification of such events. According to Sanders’ and Gyakum’s definition of Bergeron, a parameter which describes RCs’ deepening rate and varies from 28mb/(24h) at the pole to 12 mb/(24h) at latitude 25°N, we were able to classify them in the three aforementioned categories. With the help of EOF analysis, we outlined synoptic configuration more likely to drive the phenomena, highlighting the role of SCAND index and NAO-. Moreover, we have investigated the deepening with a new promising approach involving the use of 6 hours timespans, in order to single out the cyclones with higher gradients of pressure and faster evolution in semi enclosed basins. Further analysis is being undertaken to determine the cyclones’ phase and their main morphological characteristics, as well as their correlation with atmospheric rivers and SST anomalies exhibited by the Central Mediterranean Basin.
How to cite: Carniel, C. E., Ferretti, R., Ricchi, A., Curci, G., Miglietta, M. M., Reale, M., Serafini, P., Wellmeyer, E. D., and Zardi, D.: Explosive Cyclones in the Mediterranean Sea exploiting ERA5 dataset: detection, classification, statistical and synoptic analysis of their occurrance, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-9411, https://doi.org/10.5194/egusphere-egu23-9411, 2023.
The coastal region of southern and southeastern Brazil, which is part of the South Atlantic Ocean basin, is a genesis region for subtropical cyclones and, therefore, is susceptible to weather changes caused by these systems. The first named subtropical cyclone in the South Atlantic basin was Anita in 2010. Since then, some studies on subtropical cyclones have been carried out, but there are still several questions to be investigated. Thus, this study aims to: (a) describe the main physical mechanisms of genesis of the subtropical cyclones that were named in the South Atlantic Ocean between 2010 and 2021 and (b) identify the value of the Genesis Potential Index (GPI) between the pre-cyclogenesis and the phase in which these systems acquire subtropical characteristics. The rationale for analyzing the CPI is that we want to identify a possible pattern that helps in operational weather forecasting. The main database used in the study is the ERA5 reanalysis. Of the 14 cyclones studied, only two systems did not have cyclogenesis with subtropical characteristics, but acquired it 24 hours after cyclogenesis. The results indicate that 5 cyclones have a genesis associated with mid-level troughs in the atmosphere, and 9 with blocking patterns (cutoff low type). As most of the cyclones studied occur in an environment with blocking structure, this indicates that the condition of weak vertical wind shear is an important factor for subtropical cyclones. As the GPI does not show a standard value in the 14 cyclones studied, between pre-cyclogenesis and the moment when these systems become subtropical, as it varies from 0.35 in the Deni genesis to 22.71 in the Anita genesis, perhaps it is not possible to use it with a threshold in operational practices. The authors thank Programa de P&D regulado pela ANEEL e empresa Engie Brasil Energia e a Companhia Energética Estreito for the financial support.
How to cite: Ribeiro, J. G. M., da Paz, G. T., Reboita, M. S., Gozzo, L. F., Ferreira, G. W. D. S., and da Rocha, R. P.: Analysis of the Genesis Potential Index in Subtropical Cyclones off the Coast of Brazil, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-716, https://doi.org/10.5194/egusphere-egu23-716, 2023.
Extreme winds account for more than half of Norway’s insurance claims related to natural hazards [1]. Quantifying windstorm-damage relations is crucial to prepare for and mitigate the effects of future wind events. However, there has never been an attempt to quantify windstorm-damage relations at the municipality level in Norway. The work in hand employs four different damage functions at the municipality level of Norway. Along with the newly proposed modified Prahl damage function [2], an ensemble means of the damage estimates are tested for 356 municipalities in Norway. We evaluate the damage functions in terms of forecast accuracy. The spatial distribution of losses suggests severe damages along the west coast of Norway. Further inland in Norway, there are seldom any losses due to Norway’s unique topography and demography. The losses above the 99.7th percentile in each municipality constitute 85% of total national loss, and we focus on this extreme loss class. A significant agreement between the observed and estimated losses at the municipality and national levels indicates that the damage functions are suited for forecasting storm-induced damages. The damage functions are also able to successfully reconstruct the spatial spread and pattern of losses caused by very extreme windstorms.
References
[2] B.F. Prahl et al., Applying stochastic small-scale damage functions to German winter storms, Geophysical Research Letters 39, (2012)
How to cite: Jaison, A., Sorteberg, A., Michel, C., and Breivik, Ø.: Storms and associated damages in Norway, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4675, https://doi.org/10.5194/egusphere-egu23-4675, 2023.
Cyclones are essential elements of the climate and of the water cycle in the Mediterranean. The most intense of them lead to natural disasters because of their violent winds and extreme rainfall, which can cause significant damage to the territories bordering the Mediterranean (coast and mountain ranges). Reliable forecasts of cyclones are therefore essential to better anticipate and prevent their societal impact. However, their predictability is often limited by their particularities: smaller cyclones with a shorter life cycle than in the North Atlantic, complex topography, interactions with the relatively warm sea and air masses laden with dust from the Sahara.
We investigate the predictability of Mediterranean cyclones in a systematic framework using an ensemble prediction system. A reference dataset was first obtained by tracking cyclones in the ERA5 reanalysis (1979-2021), using an algorithm developped for the North Atlantic and adapted for the Mediterranean region. We then investigated the predictability using ARPEGE ensemble reforecasts in a homogeneous configuration over 22 years (2000-2021).
We restricted the study on 500 cases, which were selected using a storm severity index based on wind gusts and adapted for the Mediterranean region. The cases were then divided in several categories following their dynamical context, their intensity and their geographical origin. The predictability of the reforecasts was finally quantified on each of those categories, using probabilistic scores on cyclone trajectories (along and cross track error) and on intensities (mean sea level pressure and storm severity index).
While past studies have been limited by the fact that regular updates of operational forecasting systems do not allow the predictability of cases to be compared with each other, the homogeneous configuration of the ARPEGE ensemble reforecasts makes it possible to systematically identify the limitation to the predictability of Mediterranean cyclones.
How to cite: Doiteau, B., Pantillon, F., Plu, M., Descamps, L., and Rieutord, T.: Investigating the predictability of Mediteranean cyclones and their severity, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5690, https://doi.org/10.5194/egusphere-egu23-5690, 2023.
"All models are wrong. Some are wrong in a useful manner.” (adapted by the authors from George Box) In this presentation, we utilize an error in the surface flux formulation of the ICON-NWP numerical weather prediction model to elucidate how cloud-radiative heating affects the intensity of idealized extratropical cyclones.
We present idealized baroclinic life cycle simulations with two versions of the global atmosphere model ICON-NWP. Both versions simulate the same cyclone when run without radiative heating, but disagree when cloud-radiative heating is allowed to affect atmospheric temperature and the cyclone evolution. In version 2.1, taking into account cloud-radiative heating leads to a weaker cyclone, while in version 2.6 a stronger cyclone results. The simulations use a new modeling technique for which only cloud-radiative heating interacts with the cyclone and clear-sky radiative heating is omitted. The technique circumvents changes in the mean state due to clear-sky radiative heating that has complicated the interpretation of previous work.
A defining difference between the two model versions is the amount of simulated low-level clouds. Compared to version 2.6, version 2.1 simulates twice as many low-level clouds and a twice as strong cooling of the planetary boundary layer by cloud-radiative heating. While the increase in low-level clouds is tied to an error in the surface flux formulation in version 2.1 that was corrected in version 2.6, the error provides an opportunity to probe the impact of cloud-radiative heating in the boundary layer (below 2 km) versus the free-troposphere (above 2 km). Sensitivity studies show that negative cloud-radiative heating in the boundary layer from the tops of low-level clouds weakens the cyclone by making the atmosphere more stable. At the same time, they show that negative cloud-radiative heating near the tropopause from the tops of high-level clouds strengthens the cyclone by decreasing atmospheric stability. The changes in stability are particularly evident in regions of upward motion.
Overall, our results indicate that the vertical distribution of clouds and their radiative heating are an important factor for the dynamics of extratropical cyclones and that model differences in the simulation of low-level clouds can translate to model differences in cyclone intensity.
How to cite: Voigt, A., Keshtgar, B., and Butz, K.: Cloud-radiative heating shapes idealized extratropical cyclones by changing atmospheric stability, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-6570, https://doi.org/10.5194/egusphere-egu23-6570, 2023.
The Mediterranean is characterized by a high extratropical cyclone activity. These cyclones are an important source for water availability in the region, but at the same time they have the potential to cause extreme weather in the form of precipitation and wind extremes. The Mediterranean is heavily affected by the ongoing anthropogenic climate change, which is expected to have a profound effect on cyclones in this area. In this study, we investigate the effects of internal climate variability and anthropogenic climate change on the characteristics of Mediterranean cyclones. The analysis is based on two simulations from the Community Earth System Model 1.2 (CESM): a seamless simulation spanning 3500 years from 1500 BCE to 2012 CE and a simulation of future RCP8.5 scenario from 2013 to 2300 CE. The simulations have a 1.9°x2.5° horizontal resolution, and cyclones are identified using an established detection and tracking algorithm. Comparison with the ERA5 reanalysis for the period 1981–2010 shows that CESM is able to realistically represent cyclone frequency on a global scale, though it slightly underestimates cyclone activity in the Mediterranean. Our results indicate that cyclone activity in the Mediterranean varies on interdecadal to centennial time scales before 1850 CE. These variations are linked to positive and negative climate anomalies and fluctuations in strength of several modes of circulation, such as the North Atlantic Oscillation. The variations caused by internal variability are, however, of smaller magnitude than the effects of future climate change on the Mediterranean cyclones. In the RCP8.5 scenario, Mediterranean cyclones will become less frequent based on our simulation, and cyclone related precipitation will decrease in addition to that, which is contrary to what is being observed in other important storm track regions, such as the North Atlantic. We hypothesize that the changes in cyclone characteristics are more pronounced in the Western Mediterranean than in the Eastern Mediterranean. Overall, the study suggests that cyclone activity in the Mediterranean is projected to leave the bandwidth of variability of the last 3500 years near the end of the century.
How to cite: Doensen, O., Messmer, M., Kim, W. M., and Raible, C.: Effects of climate variability and change on cyclones in the Mediterranean, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-6629, https://doi.org/10.5194/egusphere-egu23-6629, 2023.
Extra-tropical cyclones are key components of the atmospheric general circulation due to their ability to transport large quantities of heat, moisture, and momentum. Cyclones are an important contributor to extreme weather as their passage is associated with strong winds, and large precipitation accumulations. Here we connect a cyclone compositing scheme with regionally derived distributions of precipitation to present a framework for classifying spatially dependent extremes relative to the cyclone centre. Using this framework, cyclone composites for both average (50th percentile) and extreme (90th and 98th percentile) precipitation are derived from ERA5 reanalysis output. Composites are then partitioned into different stages of the cyclone lifecycle to assess the spatial and temporal evolution of precipitation extremes. We find that most extreme precipitation occurs within the comma-cloud structure close to the cyclone centre, with the extreme precipitation occurrence and intensity occurring in that region. Extreme precipitation is also identified to be largest during the period of deepening before the maximum cyclone intensity is reached. These regions of the cyclone correspond to places where large fractions of precipitation are above the extreme threshold. Strong spatial correlation are also seen between the average and extreme precipitation during the deepening phase for the precipitation mean, occurrence and fraction. This correlation weakens as the cyclone evolves and as the threshold used to determine extreme precipitation increases.
How to cite: McErlich, C., McDonald, A., and Renwick, J.: An assessment of extreme precipitation within cyclone composites using ERA5, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-10857, https://doi.org/10.5194/egusphere-egu23-10857, 2023.
Posters virtual: Wed, 26 Apr, 10:45–12:30 | vHall AS
Volcanic eruptions are well known to influence Earth's temperature, however, how eruptions influence the atmosphere's circulation pattern, especially on the scale of everyday weather is poorly understood. Changing Earth's temperature can affect temperature gradients which in turn could affect baroclinicity and hence high- and mid-latitude weather. Yet, to what extent volcanic eruptions do in fact exert such an influence is not clear.
To answer this, we followed two independent lines of investigation: First, we query the Greenland ice-core proxy record for Indications of increased extra-tropical cyclone frequency that correlates with evidence for volcanism. This is done by comparing the storm proxy sea salt (a substance transported to the ice sheet by wind) with the volcanological proxy sulfur. Secondly, we simulate eruptions with the MPI-ESM1.2 Earth System Model and use the TRACK algorithm to explore how extra-tropical cyclone frequency is affected in the model experiments. Both approaches suggest that volcanic eruptions impact high- and mid-latitude weather by increasing the number of extra-tropical cyclones especially at higher latitudes. A detailed interrogation of the simulated eruption scenarios suggests that this increase in cyclone frequency is associated with features such as an increase in isentropic slopes and sea-ice extent most commonly found under colder climate regimes and is the reverse of what one finds in more equable climates such as that projected for the future.
How to cite: Andreasen, L., Corner, J., Abbott, P., Sinclair, V., Riede, F., and Timmreck, C.: Volcanically induced increase in extra-tropical cyclone frequency, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12526, https://doi.org/10.5194/egusphere-egu23-12526, 2023.
