We invite contributions on theoretical developments of classification methods as well as on their use in various tasks of atmospheric sciences, such as climate zonation, identification and analysis of circulation and weather types, and synoptic catalogues. Climatological, meteorological, and environmental applications of circulation classifications are particularly welcome.
The session will also include presentations on statistical (empirical) downscaling as a tool for evaluation and reconstruction of historical climate, gap filling in time series, analysis of extremes and non-climatic variables. Also intercomparisons among downscaling methods and their validation belong to this session.
Contributions on teleconnections (modes of low-frequency variability) and circulation regimes are expected to cover particularly their impacts on surface weather, climate, and environment.
The contributions on climatology of cyclones and other pressure formations will include analyses of cyclone tracks, life time and intensity of cyclones, as well as analyses of anticyclones and blockings. We also invite studies on impacts of the pressure formations on the environment and society, their relationships with large scale circulation patterns, as well as analyses of their recent trends and behavior in possible future climates.
Episodes of atmospheric blocking over the Southeastern Pacific and Southweastern Atlantic oceans significantly affect the weather over South America. These blocking events have been associated with heatwaves, droughts, and abnormally wet periods. Given the significant impacts that atmospheric blockings can have on regional climate, it is important to investigate their simulation in general circulation models (GCMs) and document any inherent biases within these models. If models are unable to accurately capture blocking under present-day conditions, it undermines the confidence in their future projections of blocking frequency.
However, few studies have examined the representation of Southern Hemisphere blockings in GCMs, especially for the Southeastern Pacific and Southweastern Atlantic regions. Therefore, in this study, we investigate the representation of blocking events in a set of 13 GCMs from the Coupled Model Intercomparison Project Phase 6 (CMIP6) within the domain of the SE Pacific, with boundaries from latitudes 40°S to 65°S and longitudes from 140°W to 82.5°W, and the SW Atlantic, with boundaries from latitudes 40°S to 65°S and longitudes 80°W to 10°W. These areas are of primary interest due to their proximity to the South American continent.
Historical runs are employed to analyze blocking conditions in the recent-past climate, focusing on the period from 1985 to 2014. Geopotential height at 500 hPa (Z500) data from the Fifth Generation of the European Centre for Medium-Range Weather Forecasts Reanalysis (ERA5) is utilized to represent the observed state of the atmosphere within the specified domain covering the Southeastern Pacific and Southern Atlantic Oceans.
Overall, the analyzed GCMs generally underestimate blocking events in the Pacific sector, although the MPI-ESM1-2-HR and MPI-ESM1-2-LR models closely reproduce observed numbers, with around -10% and -9% underestimation, respectively. For the Atlantic sector, the performance of the CMIP6 models is more varied, and the models seem to have more difficulty in capturing the observed blocking events, with differences exceeding +/-25% compared to ERA5. Nevertheless, most CMIP6 models accurately capture the seasonal distribution of blocking events, with the highest occurrence in winter and the lowest in summer. Additionally, they reproduce the center position, overall area, and Z500 anomalies of the blockings.
This research results from the R&D Project from ENGIE Brasil Energia and Companhia Energética Estreito (R&D-00403-0054/2022), and regulated by the Brazilian National Electric Energy Agency (ANEEL).
How to cite: Ferreira, V., Maroneze, R., Toledo Bonfim, O., and Mortarini, L.: Atmospheric blocking events over Southeast Pacific and Southwest Atlantic oceans in CMIP6 present-day climate, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-869, https://doi.org/10.5194/ems2024-869, 2024.
Blocking anticyclones are planetary-scale phenomenon. They typically develop close to the Pacific and Atlantic jet-streams. Blocking anticyclones usually have a strong impact on weather, not only in the regions they occur, but also in their surroundings, sometimes leading to heat waves in summer and cold spells in winter (Trigo et al., 2004). The strong decrease in ice cover extent and thickness together with accompanying warming in the Arctic caused changes in frequency, persistence and mean localization of blocking anticyclones in the middle and high latitudes of Northern Hemisphere. This paper is aimed on the analysis of spatial and temporal variability of blocking events in the middle and latitudes in the sector 60W to 120E, i.e. in the region where blocking anticyclones have a strong impact on weather in Europe.
The daily values of geopotential at 500 hPa level (G500) from ERA5 dataset acquired from the ECMWF's Meteorological Archival and Retrieval System (MARS) were used. A blocking index based on the modificated Tibaldi and Molteni method was used. In the area extending from 60ºW to 120ºW and from 35ºN to 65ºN, the daily values of blocking index were calculated for each grid point and and the extend of blocking events was extablished. On this basis the frequencies of blocking episodes were assessed as the number of days per month where a blocking event can be identified in the surrounding of selected grids. Then the trends in these frequencies were identified and discussed.
This research was funded by National Science Center (NCN), grant number 2019/33/B/ST10/01136
How to cite: Wibig, J., Jędruszkiewicz, J., and Piotrowski, P.: The long-term trends in blocking events frequencies in the Euro-Atlantic sector of NH midlatitudes, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-607, https://doi.org/10.5194/ems2024-607, 2024.
Synoptic-scale Arctic cyclones are atmospheric perturbations of thousands of kilometers size associated with a low pressure that form in the Arctic or move towards this region. They live between a few days and more than a month and are characterized by strong surface winds and precipitation. The specificity of Arctic cyclones, in comparison with mid-latitude ones, is that some of them, long-lived ones, have a cold-core vertical structure. Others have a warm-core mid-latitude-like structure. Arctic cyclones form a major hazard if human activities are to be continued, even intensified, in the Arctic region. As they transport heat and humidity, they also represent a key feature to understand Arctic climate, especially in a context of global warming. In order to simulate Arctic faster warming – i.e. polar amplification – in a realistic way, it is crucial to better understand Arctic cyclones life cycles and to improve their representation in the models. The aim of our study is to evaluate this representation in the IPSL climate model by analysing HighResMIP simulations run at four different spatial resolutions (Low (LR), Middle (MR), High (HR) and Very High (VHR)) and comparing with ECMWF-ERA5 reanalysis. To do so, Arctic cyclones are detected and tracked by applying the algorithm TempestExtremes to the mean sea level pressure (MSLP) field. Different cyclones’ characteristics are analysed: their frequency, their lifetime and their intensity. Maps of genesis, lysis and track densities are also analysed. Preliminary results show that the higher the resolution, the higher the number of tracks. The model represents Arctic cyclones quite well in summer but is less skillful in winter with an overrepresentation of the most intense ones, especially above the continents and sea ice. This bias increases with the resolution. The ability of the model to represent interactions of Arctic cyclones with tropopause polar vortices and the transitions from baroclinic structure to axisymmetric cold-core structure is also investigated.
How to cite: Besson, M., Rivière, G., and Fromang, S.: Arctic cyclones and their representation in climate models, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-147, https://doi.org/10.5194/ems2024-147, 2024.
Mid-latitude cyclones play a significant role in shaping the weather and climate of Europe. Cyclones forming in the Mediterranean region and its neighbouring areas – known as Mediterranean cyclones –, typically have shorter life spans and lower pressure at their centres than their Atlantic counterparts. This study uses ERA5 reanalysis data and general circulation model (GCM) outputs of the CMIP6 project to identify potential cyclone centres during the period of 1950–2014. First, the identification algorithm locates local minima in the mean sea level pressure values, which are regridded to a spatial resolution of 1°, and are available in the main synoptic times (6-hour temporal frequency). The potential cyclone centres are then connected through subsequent timesteps to produce cyclone trajectories. The frequency of cyclones and the characteristics of these trajectories are analysed and are validated by comparing the results of reanalysis and CMIP6-simulation data. For the best performing models future trends are assessed across different Shared Socioeconomic Pathway (SSP) scenarios – representing different anthropogenic impacts and mitigation efforts. Mediterranean cyclones affect other parts of Europe, including Hungary and its vicinity. Our ultimate goal is to analyse what portion of the annual precipitation is connected to Mediterranean cyclones and their frontal systems, and how it will change in the future. For this purpose, we use precipitation data from the above-mentioned databases. The outcome of this study is important for future impact assessments. Results can serve as input for decision-makers and stakeholders as they formulate long-term strategies for their respective sectors.
Acknowledgements: The study contributes to the COST CA19109 action (MEDCYCLONES). Research leading to this study has been supported by the following sources: the Hungarian National Research, Development and Innovation Fund (under grant K-129162), and the National Multidisciplinary Laboratory for Climate Change (RRF-2.3.1-21-2022-00014).
How to cite: Dolgos, E., Pongrácz, R., and Bartholy, J.: Identifying Mediterranean cyclones in GCM simulations – validation and future predictions, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-521, https://doi.org/10.5194/ems2024-521, 2024.
Cyclone clustering, the rapid succession of multiple extratropical cyclones at a confined region during a short period of time, causes a large fraction of European weather extremes like wet spells and wind extremes. The idea that several cyclones follow a similar track dates back to the 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. However, 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, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-695, https://doi.org/10.5194/ems2024-695, 2024.
Advancements in computer technology have facilitated the objective analysis of atmospheric fronts. With objectively defined fronts at hand, it was a relatively short step to producing the climatology of their occurrence. Recent studies have focused on delineating the spatial distribution of fronts across various spatial scales and on elucidating the relationship between fronts and precipitation patterns.
Despite the considerable attention given to the link between fronts and precipitation, little emphasis has been placed on exploring their influence on surface temperature dynamics over consecutive days. Traditionally, fronts have been associated with temperature gradients aloft rather than at the surface. However, a number of small-scale (regional and local) processes play an important role in surface temperature dynamics. Surface temperature is modified by a variety of boundary-layer and surface processes, that make its dynamics in the vicinity of fronts and during their passage very complex, and therefore worth investigating. Examining the impact of fronts on surface temperature variability is important task, given their potential contribution to asymmetries in distributions of day-to-day temperature differences – a key indicator of weather variability.
This study presents a comprehensive climatological analysis of cold and warm fronts in the Euro-Atlantic region, utilizing the ERA-5 reanalysis dataset spanning from 1961 to 2020. To objectively identify fronts, we utilize the temperature frontal parameter defined as the second derivative of temperature in the direction perpendicular to the front. The change in temperature over a 24-hour period centered on the passage of a front is retained for each gridpoint over which the front passes. Through this approach, we derive the climatology of day-to-day temperature differences associated with the passage of cold and warm fronts, and we compare these results with temperature patterns in the absence of any frontal activity.
We found that strong day-to-day cooling events are significantly more likely to be associated with the passage of cold fronts in summer across most of the European continent. However, the passage of a warm front in summer leads, on average, to warming only over approximately one half of the continent, while the opposite effect can be seen for the other half. We will further analyse the whole day-to-day temperature difference distribution and its regional specifics during frontal passages, and investigate links between day-to-day temperature differences and various properties of cloud cover.
How to cite: Navrátilová, D., Kašpar, M., Huth, R., and Stryhal, J.: Atmospheric fronts and their association with day-to-day temperature differences, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-704, https://doi.org/10.5194/ems2024-704, 2024.
Large-scale, recurrent and persistent regional atmospheric circulation patterns – known as weather regimes – are increasingly used in subseasonal-to-seasonal prediction and in analysis of climate variability and change. Yet, the majority of existing studies have focused on regimes in the North Atlantic-European sector, with comparably fewer studies investigating North American regimes. An additional limitation stems from the seasonal dependence of existing North American regime classifications, which primarily focus on winter. Here, we normalize the seasonal cycle in daily geopotential height variance over North America and use empirical orthogonal function analysis combined with k-means clustering to define a new set of four year-round North American weather regimes: the Pacific Trough, Pacific Ridge, Alaskan Ridge, and Greenland High regimes. Additionally, we define a fifth ‘No Regime’ state to represent conditions closer to climatology than any of the regimes. Multiple statistical tests support the choice of four regimes, and the clustering solution is robust to various methodological choices. All four regimes typically persist for around one week, with instances of extremely long-lived regimes persisting for up to six weeks, while the No Regime state is mostly associated with regime transitions and typically persists for only a few days. Regime-associated temperature and precipitation anomalies are reported, alongside the link between regimes and the modulation of tornado activity across the United States. We also use the regimes to objectively quantify historical trends in the large-scale circulation over North America during 1979—2023, finding a large increase in the frequency and persistence of the Greenland High regime during summer, particularly since 2007.
How to cite: Lee, S., Tippett, M., Polvani, L., and Malloy, K.: A New Year-Round Weather Regime Classification for North America, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-993, https://doi.org/10.5194/ems2024-993, 2024.
Weather regimes are recurrent and quasi-stationary large-scale atmospheric circulation patterns, typically linking to surface weather. They have been defined over multiple regions and used in a range of practical applications, including forecasting, the study of atmospheric circulation trends and climate model evaluation. Despite their widespread use, the extent to which regimes reflect physical modes of the atmosphere is seldom investigated. Similarly, the statistical and physical connections between weather regimes defined over different geographical regions have been largely left unstudied. Here, we consider two widely-used sets of weather regimes: North American and Euro-Atlantic regimes. By leveraging dynamical systems theory, we find that when the atmospheric flow is assigned to a regime, it displays persistent characteristics and a lifecycle-like temporal evolution. We further find that these characteristics are enhanced when the atmospheric flow displays a comparatively strong projection onto the cluster-mean of the regime to which it is assigned (while the reverse is true for a weaker projection). We argue that this is evidence supporting the physical relevance of both North American and Euro-Atlantic weather regimes. We next consider the connection between the two sets of regimes. Specific pairs of regimes show a close visual and statistical correspondence. Moreover, the joint analysis of the two sets of regimes can provide medium-range statistical predictability for anomalies in their occurrence frequencies. Conditioning on North American weather regimes also results in anomalies in both the large-scale circulation during specific Euro-Atlantic regimes, and the associated European surface weather. We conclude that there is a benefit in conducting joint analyses of North American and Euro-Atlantic weather regimes, as opposed to considering the two in isolation.
How to cite: Messori, G., Lee, S., and Dorrington, J.: Dynamical life-cycle and teleconnections of North American and Euro-Atlantic weather regimes, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-1006, https://doi.org/10.5194/ems2024-1006, 2024.
The rainy season in most of southern Africa (SAF), defined here as Africa subcontinent poleward of 10°S, is associated with the summer monsoon regime, occurring in the region between October and March with seasonal peak during December-January-February (DJF) months. The quality of this season, particularly the peak monsoon (DJF), is of great importance as it rains little during the rest of the year over most SAF. In this study, the influence of El Niño-Southern Oscillation (ENSO) episodes, El Niño (EN) and La Niña (LN), on the summer monsoon circulation, rainfall and the frequency of extreme rainfall events over SAF is analyzed with seasonal and monthly resolution, using data from monthly GPCC products version 2022 and three different daily datasets (CPC version 1.0, CHIRPS version 2.0 and GPCP version 2022) for the 1982-2019 period. Extreme events are defined as 3-day mean precipitation above the 90th percentile. The mean frequencies of extreme events are determined for each month and season, and for each ENSO category episodes (EN, LN, and neutral), and the differences between EN and neutral years and LN and neutral years are computed through anomaly composites superimposed with either outgoing long-wave radiation (OLR) or eddy streamfunction at either low or upper levels, to understand the circulation mechanisms associated with ENSO episodes anomalies. Although some little differences exists, the pattern of significant signals in the frequency of extreme events is found to be well coherent with that of seasonal total rainfall anomalies in both the EN and LN episodes. The analysis also shows that some anomalies, which are consistent and important during part of the season, are smoothed out in a seasonal analysis. All daily datasets considered here show similar results, but that of CHIRPS seems to outperform better the well known and expected pattern, probably due to its high resolution. There are changes of anomalies within the summer monsoon season, with positive (negative) anomalies associated with EN episodes in spring (summer) over almost all SAF, associated with a low-level cyclonic (anticyclonic) anomalies to the north-eastern (southwestern) SAF, connected with a Rossby wave train generated in central south Pacific, which passing over south America/south atlantic ocean suffer from it less interference in comparison with the same wave in late summer. The opposite is almost true for LN events. To our knowledge, such results have never been reported for SAF elsewhere before, and those suggest the prevalence of likely regional processes over remote influences during part of the season. Similar results has been found early for South America. These results have important implications for seasonal predictions improvement, and rain dependent activities.
How to cite: Silverio, K. and Ambrizzi, T.: ENSO impacts on summer monsoon seasonal aggregate and daily rainfall characteristics in southern Africa, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-32, https://doi.org/10.5194/ems2024-32, 2024.
Shape of mathematical curves describing local weather statistics are systematically influenced by large-scale conditions and geographical factors, and we present results suggesting that it is possible to downscale this kind of information directly. We downscale cumulative probability functions for daily precipitation and intensity-duration-frequency (IDF) curves for estimating return values of intense sub-daily rainfall. Recent progress in reanalyses such as ERA5 has made it possible to use wet-day frequency and mean precipitation intensity as predictors to compute local variations in wet-day frequency and mean intensity respectively, and we use a simple and approximate formula from earlier studies with these two parameters to specify the shape these curves. Downscaling the shape of such curves may be referred to as ‘downscaling climate’ if we regard ‘local climate’ as the statistical description of various weather parameters. This approach is distinct from the more traditional approach ‘downscaling weather’, where one seeks to estimate particular local states for instance on a day-by-day basis. We also present work on downscaling the shapes of pdfs and IDFs for large multi-model ensembles for the application in climate change adaptation efforts. Our eanalysis is accompanied by an evaluation of both methodology and the global climate models' (GCMs) ability to reproduce observed large-scale climatic variability in terms of the salient spatio-temporal covariance structure. When it comes to providing future regional climate projections, we also emphasise that it is important to combine different strategies for downscaling, e.g. regional climate models (RCMs) and empirical-statistical downscaling (ESD) that are based on different assumptions, for getting robust future regional climate projections.
How to cite: Benestad, R. and Parding, K. M.: Recent progress in downscaling key parameters describing local daily precipitation statistics in the Nordic countries, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-149, https://doi.org/10.5194/ems2024-149, 2024.
The study presents an evaluation and selection of global climate models for downscaled projections over Spain. The Spanish Meteorological Service (AEMET) is responsible for the elaboration of regionally downscaled climate projections over Spain according to the second National Adaptation Plan to Climate Change (PNACC-2 2021-2030). Regionally downscaled climate projections allow for increased spatial resolution compared to those provided by global climate models. AEMET develops regionally downscaled climate projections over Spain by applying empirical-statistical downscaling (ESD) methods to projections from a set of global climate models. Regionally downscaled climate projections are necessary for conducting impact and vulnerability studies that require data at high spatial resolution.
The present work has two objectives. The first one is to evaluate and select a reduced ensemble of global climate models from CMIP6 in order to downscale their projections. The second objective is to compare different ESD methods in order to choose the best performing method for downscaling daily data of the following climatic variables of interest: maximum temperature, minimum temperature, and accumulated precipitation. The evaluation of global climate models was carried out using the GCMeval tool [1]. The comparison of ESD methods is based on the analysis of indicators such as bias, coefficients of temporal and spatial correlation, root mean square error, and future trends, and it was carried out using pyClim-SDM [2], the statistical downscaling software developed at AEMET. The results indicate the selection of eleven global climate models for downscaling and the choice of specific methods for each climatic variable: regression-analogues (MLR-ANA) for temperatures and eXtreme Gradient Boost (XGB) for precipitation. Additionally, bias correction using Quantile Delta Mapping is recommended to improve projections. This research lays a robust groundwork for future studies on climate change in Spain, highlighting its implications for the population, socio-economic sectors, and ecosystems.
Ultimately, the key findings from the downscaling of projections over Spain using the representative ensemble of global climate models from CMIP6 and employing the resulting downscaling techniques from the prior assessment are presented.
[1] Parding, K. M. et al., (2020), GCMeval – An interactive tool for evaluation and selection of climate model ensembles. Climate Services, Volume 18, 100167. https://doi.org/10.1016/j.cliser.2020.100167
[2] Hernanz, A., et al. (2023), pyClim-SDM: Service for generation of statistically downscaled climate change projections supporting national adaptation strategies. Climate Services, Volume 32, December 2023, 100408. https://doi.org/10.1016/j.cliser.2023.100408
How to cite: Correa, C., Hernanz, A., and Rodríguez-Guisado, E.: Evaluation of ESD methods for climate change projections over Spain, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-650, https://doi.org/10.5194/ems2024-650, 2024.
Several global-scale atmospheric reanalyses (ARs) have been developed since the mid-1990s providing outputs that reach back into the pre-satellite era. These datasets, despite their deficiencies, are routinely used in climate research, prompting the need for validation and inter-comparison studies. Here, we compare synoptic-scale atmospheric circulation in an ensemble of ARs, namely, in the NCEP/NCAR Reanalysis 1 (NCEP1), ECMWF ERA-40 reanalysis, the Japanese 55-year reanalysis (JRA-55), ECMWF Reanalysis v5 (ERA5), and two ARs that assimilate only a limited number of surface variables—NOAA-CIRES-DOE Twentieth Century Reanalysis version 3 (20CRv3) and ECMWF twentieth century reanalysis (ERA-20C).
A survey on the accordance of daily circulation types (CTs) identified in daily mean sea level pressure, 700 hPa and 500 hPa geopotential height fields is carried out for boreal (DJF) and austral (JJA) winters. The CTs are independently calculated in many 30° × 20° (longitude × latitude) overlapping regions by a k-means algorithm, each region centred on one of the total of 9,360 grid points that cover the Earth from 80°S to 80°N at the step of 2.5° × 2.5°.
Not surprisingly, marked differences can be found between ARs in the pre-satellite era, particularly at the surface and over the Southern Hemisphere. Although the accordance of ARs tends to increase over time, there are several regions in which significant differences persist into the 21st century (central South America, sub-equatorial Africa, northern parts of the Indian subcontinent, western parts of China, and Antarctica).
In the presentation, methodological issues (number of CTs, size of classification domain, choice of circulation variable, choice of classification method, and type of normalization of fields prior to their classification) will be discussed, and a physical interpretation of inter-reanalysis differences will be provided.
How to cite: Stryhal, J.: The accordance of synoptic-scale circulation in atmospheric reanalyses and its methodological sensitivity, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-274, https://doi.org/10.5194/ems2024-274, 2024.
Classifications of circulation patterns have been widely used in synoptic climatology for decades. Recently, this concept has been extended towards spatially sliding classifications, i.e., large sets (arrays) of classifications conducted independently for individual grid points.
We make use of spatially sliding classifications in an attempt to explain asymmetry of statistical distributions of day-to-day temperature difference (DTD). Over most of Europe, negative skewness of DTD prevails in summer, while positive skewness dominates in winter. The asymmetry is reflected in small temperature increases occurring more often than small temperature decreases in summer, the opposite holding for winter. Unlike for temperature itself, mechanisms governing the asymmetry of DTDs have only been hypothesized but investigated neither in sufficient detail nor on scales larger than local.
We employ the Jenkinson-Collison (JC) method to classify atmospheric circulation for grid points covering Europe and North Atlantic. The version of the JC method with 11 types (8 directional, 2 (anti)cyclonic, and one with weak pressure field) is adopted. For each grid point, we identify circulation types with the largest asymmetry of small DTDs (i.e., with the largest difference between the number of small day-to-day warmings and coolings). ‘Small’ is defined here as the inner 50% of values of a DTD distribution. Although the circulation types most conducive to the DTD asymmetry vary regionally, there is a pronounced tendency for the anticyclonic type, the type with a weak pressure field, and types with warm advection from the southern quadrant to contribute to the DTD asymmetry in summer. In winter, the anticyclonic type and types with cold northerly to easterly advection largely contribute to the DTD asymmetry.
Next, we analyze surface energy budget, and particularly radiation and energy fluxes, for the circulation types conducive to the DTD asymmetry at selected representative grid points. We demonstrate that it is indeed radiative processes that strongly contribute to the prevalence of small day-to-day warmings in summer and of small day-to-day coolings in winter.
We use ERA5 reanalysis as input dataset, with grid step of 1.25° x 1.25°. Sea level pressure is used as a circulation variable. Maximum temperature in summer and minimum temperature in winter are used to describe DTD. Analyzed period is 1940-2022.
How to cite: Huth, R., Stryhal, J., Sedlák, P., and Krauskopf, T.: Spatially sliding classifications of atmospheric circulation patterns: a tool to explain skewness of day-to-day temperature differences, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-197, https://doi.org/10.5194/ems2024-197, 2024.
Daily weather types over Serbia were obtained using the objective weather typing system of Jenkinson and Collison (1977), which is based on the Lamb weather types (Lamb 1972). The daily atmospheric circulation that affects Serbia was characterized using a set of indices associated with the direction and vorticity of the geostrophic flow over an area that covers the majority of Europe and the Mediterranean Sea. To obtain the circulation types, the daily values of the sea level pressure (SLP) and the geopotential height at 500 hPa for the period 1961-2020 were retrieved from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis (Kalnay et al. 1996) on a 2.5° × 2.5° longitude-latitude grid.
Composite maps of ten circulation weather types (CWT): northerly (N), northeasterly (NE), easterly (E), southeasterly (SE), southerly (S), southwesterly (SW), westerly (W), northwesterly (NW), cyclonic (C) and anticyclonic (A), were constructed for the period 1961-2020 for all seasons. The C and AC weather types are based on the severity of the geostrophic vorticity. Each of the CWT has a distinct underlying synoptic pattern that produces the expected type and direction of flow over the study area. The relative frequencies of the circulation types were computed. It was found that the AC type with 25.2% was the most common type, followed by the C type with a frequency of 14.6%.
Multiple Linear Regression (MLR) models were used to establish the relationship between atmospheric circulation and number of days without precipitation. The models were calibrated for the first 40 years and validated for the remaining 20 years. Using ten circulation weather patterns (eight pure directional, cyclonic, and anticyclonic) as predictor variables, good results were obtained for modelling of number of days without precipitation in Serbia. Almost 52% of the variability could be explained by ten circulation types in Belgrade.
References
Jenkinson AF, Collison FP (1977) An initial climatology of gales over the North Sea. Synoptic Climatology Branch Memorandum 62, Meteorological Office, Bracknell, UK
Kalnay E, Kanamitsu M, Collins W, Deaven D, Gandin L, Iredell M, et al (1996) The NCEP/NCAR 40-year reanalysis project. Bull Am Meteorol Soc 77:437–470
Lamb HH (1972) British Isles Weather Types and a Register of Daily Sequence of Circulation Patterns, 1861–1971. Geophysical Memoir 116, HMSO, London (UK), 85 pp
How to cite: Tošić, I., Filipović, L., and Putniković, S.: Influence of atmospheric systems on dry conditions in Serbia, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-1145, https://doi.org/10.5194/ems2024-1145, 2024.
In Croatia, drought causes the highest economic losses inflicting serious damages, especially in the agricultural and water management sectors. Only in 2003, the drought damage was 90 %. Even though drought can occur in every season, the greatest damage caused by drought occurs in the warm part of the year, particularly in summer, when the absence of precipitation is combined with high air temperatures. For spatial and temporal comparison of drought events in different parts of the world and different periods throughout history in meteorological practice the standardized precipitation index, SPI is the most often used. Accordingly, in this analysis, the SPI index is used for detecting drought events in Croatia in previous decades.
The products of the COST733 action “Harmonization and Applications of Weather Types Classifications for European Regions”, both catalogue and software, were applied to the Croatian area to improve previous knowledge and practice, and to develop a classification that is intended to be the most appropriate to capture precipitation conditions in Croatian regions. For this purpose, Croatia is divided into five regions to acknowledge the orography and climate diversity of the Croatian area which lead to different weather conditions within the same atmospheric circulation. The efficacy of different objective classifications is examined utilizing Croatian meteorological station data.
Selected classifications are compared and their ability to represent drought dynamics in Croatia is evaluated. The goal is to identify classifications that are best suited to capture spatiotemporal drought variability in the Croatian area. For this purpose, the percentage anomalies approach is used for quantifying the relationship between occurrence frequencies of weather types and exceedances/undershootings of specified SPI-thresholds. Further, the analysis aims to identify the main weather patterns responsible for drought events and spatial variations in drought impacts and manifestations among Croatian regions are examined as well. Moreover, temporal variations concerning circulation-drought relationships are also analyzed.
How to cite: Marinovic, I. and Beck, C.: Drought in Croatia in the context of weather types, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-743, https://doi.org/10.5194/ems2024-743, 2024.
Understanding wind climatology and examining wind speed and direction patterns using available measurements, is vital across various sectors, including urban planning, transportation, environmental monitoring, and safety management. Surface air pressure distribution, categorized into distinct weather types, significantly influences surface air flow. This airflow can bring benefits like a refreshing sea breeze on a hot summer evening. However, wind can also pose significant challenges during severe conditions, particularly in the coastal region of Croatia. Therefore, accurately classifying wind characteristics in this region is highly beneficial for effective management and preparedness.
In this work, we used surface data observations from main and climatological meteorological stations over a ten-year period (2013–2022) to classify both strong wind events and characterize their seasonality, and also coastal circulation.
Intense synoptic systems, such as warm and cold fronts associated with low-pressure systems, as well as the passage of these systems, contribute to episodes of strong winds in the region. Two prevalent local winds, bora and jugo, though distinct in nature and originating from different synoptic systems, can both pose challenges to transportation, maritime activities, and even threats to both people and properties. Those winds are dominant in the colder part of the year. The bora wind occurs during anticyclonic weather types in cases of sudden outbreaks of cold air from north or northeastern part of the Europe, and jugo prevails when there is cyclonic weather type, more specifically it is formed on the front side of the cylone above Medditerranean.
The eastern coast of the Adriatic Sea is characterized by numerous islands and complex topography along the coastline. In warmer part of the year when the synoptic conditions are not intense, an etesian wind known as the maestro prevails in this area. This consistent wind pattern is crucial for temperature regulation during high daytime temperatures and plays a significant role in maritime navigation. Coastal circulation forms under similar synoptic situations, influencing the diffusion of aerosols, temperature regulation, and convection triggering. Finally, the complex indentation of the Croatian coastline, with many islands, necessitates examination of island circulation and its potential interference with the main coastal circulation.
How to cite: Kožul, E. and Odak Plenković, I.: Wind climatology of surface wind patterns in the coastal region of Croatia, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-834, https://doi.org/10.5194/ems2024-834, 2024.
Extreme weather events and their resulting impacts threaten all levels of society. Climate change can amplify the frequency and intensity of associated hazards. One of the key-challenges for decision-makers in civil protection is adapting to the changing landscape of weather-induced impacts driven by climate change. It is therefore essential to assess and estimate the changing conditions for extreme weather events under climate change.
This study investigates the changing landscape of regional extreme weather events in the Alpine region by utilizing weather circulation type classification through its relationship with weather-induced potential extreme events. Therein, large-scale weather types take the role of relevant precursors for regional extreme events. The local-scale potential impact events that are associated with prevailing weather types are derived by using percentile-based methods on high-resolution, gridded precipitation data. ERA5 mean sea level pressure and the classification scheme GWT ('Gross-Wetter-Typen') are used to derive 18 classes of cyclonic and anti-cyclonic weather types, representing a prevailing circulation on a daily basis for the whole Alpine region. Subsequently, unsupervised hierarchical clustering (Agglomerative clustering) is used to evaluate overlaps between cluster families in order to derive a subset of 'high-impact' weather types. The relationship between those weather types and local-scale extreme events is further characterized by analyzing percentile-based indicators from station data and high-resolution observational data. Finally, we extend our analysis by applying found relationships to state-of-the-art climate models from the Coupled Model Intercomparison Project 6 (CMIP6) to investigate the changing landscape for extreme weather events under different climate change scenarios.
Our findings indicate that a subset of weather types, related to specific cyclonic circulation patterns, is mostly responsible as precursor for extreme precipitation events. Those patterns furthermore show increases in frequency under the scenario SSP3-7.0, that are consistent across the climate model ensemble. Associated changes of precipitation totals suggest increases in intensity, however these intensity changes are not as conclusive due to large inter-model spread.
How to cite: Lehner, S., Enigl, K., Crespi, A., Pittore, M., and Haslinger, K.: Changing landscape of regional extreme weather events by intersecting large-scale weather types and local-scale potential impacts for the Alpine region, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-746, https://doi.org/10.5194/ems2024-746, 2024.
Atmospheric blocking is associated with high-impact weather events in mid and high latitudes. The specific location of the blocking system is of major importance for the type of meteorological extreme developed. The studies on blocking and its effects in the European continent are based on the differences in atmospheric mass between high, middle, and low latitudes and the strong reduction of the zonal winds. They are mainly based on the Tibaldi and Molteni (1990) blocking index or adapted versions of it, in which meridional gradients of geopotential height fields at 500 hPa integrated in longitudinal intervals are calculated.
In this work, we consider an alternative to those approaches, focusing on identifying the atmospheric centers of action and their relative positions and intensities using the geopotential height at 500 hPa. After filtering the local minima and maxima using a gradient criterion, the remaining maxima and minima are grouped according to proximity and intensity. The identification of omega and dipole (Rex) blocks at any instant is made by a selection algorithm that uses the relative positions of the action centers as well as the geopotential contours, identifying the blocking type, shape, and area of influence.
We have analyzed some of the most well-studied heat-wave events over Europe (August 2003, July 2019) and a blocking situation over the western Eurasian border which caused one of the strongest African dust outbreaks impacting Europe (March 2022).
Algorithm performance is crucial in various computational applications, from scientific simulations to real-time data processing. In this work, we focus on improving the performance and speed of the algorithms through parallelization techniques aimed to produce a climatology of blocking situations in the Northern Hemisphere. Specifically, we are exploring the use of OpenMP and MPI libraries for C, widely used in high-performance computing (execution time and scalability efficiency.
How to cite: Hernandez, V., Migallon, H., and G. Orza, J. A.: Design and acceleration of new procedures for the automated identification of atmospheric blocking situations, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-979, https://doi.org/10.5194/ems2024-979, 2024.
The Mediterranean region is known for the formation of an area of low atmospheric pressure during the cold half of the year and the formation of cyclones. Creation of cyclones in this area is influenced by both cold air advections along the cold fronts of the Atlantic cyclones, the East European Maximum, and high pressure ridges coming from the Azores Maximum located to the west. The formed cyclones over the waters of the Mediterranean Sea have an impact on a vast territory covering southern Europe, the Middle East and the northern parts of Africa, which defines the zone of Mediterranean climates. Their frequency, mode, trajectories, speed of movement and duration of retention over a given territory determine to varying degrees the intra-annual rainfall regime, as well as the individual economic activities related to them.
For the purposes of this article, data from synoptic maps at a height of 500 hPa were used, as well as average monthly precipitation data for 10 meteorological stations - four located in Northern Bulgaria (Vidin, Pleven, Ruse, Varna) and six in Southern Bulgaria: Burgas , Sliven, Plovdiv, Sofia, Kyustendil and Sandanski. They are collected from NIMH monthly newsletters.
Pisarski's (1955) classification scheme was used as the basis for the types of trajectories of Mediterranean cyclones. The monthly, seasonal and annual total number of Mediterranean cyclones is calculated. The main areas of cyclogenesis and their percentage ratios in relation to the total number of formed cyclones have been established. For each trajectory, the shares of cyclone formation areas relative to the total number of Mediterranean cyclones formed were calculated. The monthly and seasonal distribution of cyclones passing along the different trajectories, as well as average, minimum and maximum duration, are presented.
In order to analyze the influence of Mediterranean cyclones on the precipitation in Bulgaria, the correlation coefficient (r) between the precipitation amounts for the 10 selected stations and the frequency of occurrence of the formed cyclones was used.
How to cite: Popov, H. and Georgieva, A.-M.: Frequency and trajectories of the Mediterranean cyclones and their influence on precipitation in Bulgaria during the months of September-April for the period 2010-2020, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-914, https://doi.org/10.5194/ems2024-914, 2024.
Various classifications of circulation types have long been used to assess changes and variability of atmospheric circulation. As a result, several dozen different manual classifications were created, which were based on various methodological assumptions. Although they are now increasingly replaced by automatic classifications (often having the characteristics of objective classifications), the mentioned classifications are still used for many analyses. And sometimes, due to the subjectivity of the assessment of synoptic situations, they better explain specific weather conditions, including extreme ones. This also applies to Central Europe, where due to its location, weather and climate, it is characterized by great variability and transience. This was also one of the reasons why several classifications were created for this region.
The work attempts to assess the variability of atmospheric circulation over Central Europe for a long period of 120 years (1901-2020) based on available manual classifications of circulation types.
Five traditional classifications of Osuchowska-Klein, Niedźwiedź, CHMU, Pecely and the well-known Grosswetterlagen classification were considered here. Simultaneously the automatic classifications developed by Lityński, Ustrnul and Řehoř were also taken into account independently. A comparison of the occurrence of circulation types or groups of circulation types between the analyzed classifications was made.
The results sometimes showed large discrepancies between individual types of circulation defined in the analyzed classifications. The analysis showed that their causes should be sought in the diversity of mesoscale circulation conditions in Central Europe, but also in a different methodological approach taken into account in individual classifications. Taking into account other synoptic materials used in the development of individual classifications may also have some impact on the results obtained.
How to cite: Wypych, A., Ustrnul, Z., Řehoř, J., and Mika, J.: Variability of atmospheric circulation over Central Europe in the light of manual classifications, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-933, https://doi.org/10.5194/ems2024-933, 2024.
The temperature behavior in mountainous regions at mid-latitudes is particularly susceptible to the effects of global warming. Within this context, it is crucial to comprehend and quantify the influence of changes in synoptic patterns at higher resolution scales. This study focuses on a Pyrenean valley (north-east of Iberian Peninsula) prone to the formation of cold air pools (CAPs). It is established that CAPs predominantly form under stable conditions. However, deep and enclosed valleys frequently encounter decoupling situations, leading to unexpected variations in climatological temperature profiles. Consequently, understanding how global warming will impact temperatures at various altitudes in the context of Elevation Dependent Warming becomes increasingly challenging.
In this study, two temperature transects have been utilized: one covering a closed valley prone to CAP formation and one in an open valley with drainage. Each transect is composed of six temperature and humidity sensors taking data every 30 minutes, covering a range of heights of approximately 1200 m between altitudes of 1000 m and 2200 m. The sensors were set up in 2011 and have been recording data since then, which allows for a first approximation of climate trends in mid-latitude mountain ranges.
In contrast with applying purely statistical relationships based on correlations, in this case, a causal approach has been adopted to quantify how the anticipated synoptic variability is transmitted to the differences between the vertical temperature profiles previously defined. After constructing some causal relationships among variables, this has been utilized to infer changes in CAP behavior under different climate change scenarios. This methodology offers a robust framework for assessing the potential impacts of global warming on mountainous regions, providing valuable insights for future climate adaptation and mitigation efforts.
How to cite: Miró, J. R., Casellas, E., Peña, J. C., and Pepin, N.: The understanding of global warming influences on the temperature behaviour on mountainous areas, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-332, https://doi.org/10.5194/ems2024-332, 2024.
