Understanding and modelling of atmospheric hazards and severe weather phenomena

This session will welcome all technical and scientific contributions devoted to increase our understanding of atmospheric phenomena that might represent a hazard for people, property and environment. Studies devoted to enhance physical understanding of severe weather phenomena (for example deep convection or intense straight lines winds) are of particular interest even if the severe weather phenomena are not linked directly to a specific hazard (multihazard events).
Embracing the proposal given by the organizers, this year will be particularly welcome contributions dealing (directly or indirectly) with severe droughts in Europe or connecting droughts events and atmospheric hazards.
Moreover, in line with the spirit of EMS, we would encourage contributions devoted to underline the intercultural aspects of methods and findings, and to point the attention not only to the physical and meteorological characteristics of atmospheric hazards and severe droughts, but also to their relevance in a changing climate, including possible impacts on human activities and the environment.
Contributions dealing with studies of specific episodes (case studies) will be welcome, provided they further increase physical understanding and are representative at least for the area where these events took place.
Particularly welcome will be contributions incorporating both numerical and conceptual modelling to improve our understanding of severe weather phenomena.
In general we will encourage the exchange of expertise and experiences related to the various topics connected to hazardous atmospheric phenomena and severe weather events. For this reason an interdisciplinary approach will be particularly welcome.
Potential topics for this session include i.a.:
• Severe droughts;
• Flash-floods and heavy rain events;
• Hail;
• Freezing rain, icing and intense snow falls;
• Cold/heat events, even those occurring at small time scales;
• Fog;
• Tornadoes, waterspouts, derechos and downbursts;
• Severe wind storms;
• Intense Mediterranean cyclones;
• Tropical like cyclones;
• Lightning;
• Polar lows, their evolution and impacts;
• Severe katabatic or foehn winds;
• Gap and orographic flows;
• Breaking of gravity waves, as well as severe turbulence;
The above-listed topics are of course not exclusive and the session’s Conveners eagerly anticipate papers on new ideas and approaches and novel understanding covering all aspects of atmospheric hazards and severe weather events.

Conveners: Fulvio Stel, Arne Spekat | Co-conveners: Dario Giaiotti, Mario Marcello Miglietta, Victoria Sinclair, Sante Laviola, Jordi Mazon
Lightning talks
| Tue, 07 Sep, 09:00–12:30 (CEST)

Lightning talks: Tue, 07 Sep

Chairpersons: Fulvio Stel, Jordi Mazon, Victoria Sinclair
Antonio Ricchi, Giovanni Liguori, Leone Cavicchia, Mario Marcello Miglietta, Davide Bonaldo, Sandro Carniel, and Rossella Ferretti

The Mediterranean basin is the formation site of a vast number and type of cyclones. Among these, we can occasionally identify intense vortices showing tropical characteristics, called Tropical-Like Cyclones (TLC). Their development has been studied in several case studies, showing the influence of synoptic scale upper level forcings and mesoscale features, such as the sea surface temperature and the characteristics of the air masses on the formation area. The importance of Sea Surface Temperature (SST) consists in modulating the intense latent and sensible heat fluxes, which control the development of the TLC. For tropical cyclones, one of the most studied factors in recent years is the ocean heat content in the formation basin of these storms. We plan here to extend this analysis to TLC. Besides innovative studies with coupled atmosphere-waves-ocean numerical models, a simpler approach for investigating the sole effect of the ocean heat content consists of adopting a simplified ocean description by varying the local characteristics of the Ocean Mixed Layer (OML). In this work we use the WRF (Weather Research and Forecasting system) model, in standalone (atmospheric) mode, with 3 km grid spacing, forced with GFS-GDAL (0.25°x0.25° horizontal resolution) and SST initialization provided by the MFS-CMEMs Copernicus dataset. Two case studies of TLC are examined here, namely ROLF (06-09/11/2011) and IANOS (14-19/09/2020). The ocean is simulated with an OML approach, with SST updated at each iteration as a function of the atmospheric heat fluxes and with an average mixed layer deph (MDL) provided by the MFS-CMEMS dataset. For each TLC studied, the MDL is modified by increasing and decreasing its depth by 10 mt, 30 mt, 50 mt . The preliminary results show how the structure of the MDL influences  the intensity of the cyclone but also the structure and precipitation both in terms of quantity and location. 

How to cite: Ricchi, A., Liguori, G., Cavicchia, L., Miglietta, M. M., Bonaldo, D., Carniel, S., and Ferretti, R.: On the Ocean Mixed Layer influence on the genesis of Mediterranean Tropical-Like cyclones, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-424, https://doi.org/10.5194/ems2021-424, 2021.

Stefano Federico, Albert Comellas Prat, Rosa Claudia Torcasio, Leo Pio D'Adderio, Stefano Dietrich, and Giulia Panegrossi

On September 14th, 2020 a depression originated on the Libyan coasts generated a Mediterranean tropical-like cyclone (hereafter referred to as Medicane Ianos), which moved northward until it hit, with its northernmost cloud bands, the southern Italian coasts and finally bent towards Greece, where it made landfall on September 18th, 2020. Heavy precipitation and flash flooding were reported, associated to huge damages to railway, houses, and four casualties.

The correct prediction, as much as possible, of the trajectory and the intensity of these events is fundamental to prevent risks to infrastructures, natural landscapes, and people. One of the ways to evaluate the performances of the numerical weather prediction models is the comparison with satellite measurements. In particular, the mature phase of Medicane Ianos, as predicted by the weather research and forecasting (WRF) model, has been compared, for the first time, with the Global Precipitation Measurement mission Core Observatory (GPM-CO) active and passive measurements. Different microphysics schemes were used in order to investigate which is the most suitable to achieve the best forecast of Medicane Ianos considering different parameters as depression localization, reflectivity, rainfall rate, integrated liquid and ice content. The results show that all the schemes identified the precipitation bands structure of the Medicane overestimating the vertical extent of the convective structures. At the same time, all the schemes predicted an excessive columnar ice water content if compared to the one estimated from satellite measurements. It has to be highlighted that the overestimation is marked on the western precipitation bands of the Medicane eye, while a better agreement is obtained for the northern bands. Similar results are obtained for columnar liquid water content, even if the quantitative estimation is closer to the GPM measurements. Finally, all the schemes located the Medicane circulation center further north-west of its actual position.

How to cite: Federico, S., Comellas Prat, A., Torcasio, R. C., D'Adderio, L. P., Dietrich, S., and Panegrossi, G.: Medicane Ianos: a comparative study between WRF model and satellite measurements, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-297, https://doi.org/10.5194/ems2021-297, 2021.

Mika Rantanen, Jani Särkkä, Jani Räihä, Matti Kämäräinen, and Kirsti Jylhä

Extremely high sea levels on the Finnish coast are typically caused by close passages of extratropical cyclones (ETCs), which raise the sea level with their associated extreme winds and lower air pressure. For coastal infrastructure, such as nuclear power plants, it is crucial to study physically possible sea level heights associated with ETCs. Such sea levels are not straightforward to determine from observational datasets only, because tide gauge records  cover about 100 years and do not necessarily capture the most extreme cases having return periods longer than 100 years.

In this study, a method for generating an ensemble of synthetic low-pressure systems is being developed to investigate the extreme sea level heights on the Finnish coast of Baltic sea. As input parameters for the method, the point of origin, velocity of the center of the cyclone and depth of the pressure anomaly need to be given. Based on the input parameters, the method forms an idealized low-pressure system using a two-dimensional Gaussian function. In order to find extreme, but still reasonable values for the input parameters, cyclone tracks from ERA5 reanalysis data will be analysed.

The ensemble of synthetic low pressure systems (i.e. the wind and pressure data) is used as an input to a numerical sea level model. As a result, we have an ensemble of simulated sea levels, from which we can determine the properties of the ETCs that induce the highest sea levels on a given location on the coast. The preliminary simulation results show that this method works well, forming a basis for studies on extreme sea levels. 


How to cite: Rantanen, M., Särkkä, J., Räihä, J., Kämäräinen, M., and Jylhä, K.: Synthetically generated low-pressure systems to support studies of sea level extremes, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-132, https://doi.org/10.5194/ems2021-132, 2021.

Hernán Bechis, Milagros Alvarez Imaz, Inés Simone, Victoria Galligani, Maite Cancelada, Franco Piscitelli, Maldonado Paula, and Paola Salio

Storms in the Mendoza province, Argentina are known for frequently producing large and severe hail. The environmental conditions and strong interaction with topography there provide unique conditions for the initiation and intensification of severe storms. The RELAMPAGO (Remote sensing of Electrification, Lightning, And Mesoscale/microscale Processes with Adaptive Ground Observations) and CACTI (Clouds, Aerosols, and Complex Terrain Interactions) field campaigns were deployed between October 2018 to April 2019 over west-central Argentina, and have collected unprecedented Intensive Observation Periods (IOPs) in the region. During the IOP number 10 on November 26, 2018, a severe hailstorm developed and moved across the observational network in the Mendoza domain. 4-cm diameter hail was reported over multiple hailpad sites and with in-situ measurements. Several soundings, mobile and fixed radar observations, and surface observations are available for this case, along with 1-min GOES-16 ABI Mesoscale Domain Sector (MDS) data coverage.

High-temporal frequency soundings and surface observations collected prior to the convection initiation are analyzed, allowing a detailed description of the storm environment. Processes leading to convective initiation over the higher terrain include the development of the upslope flow associated with a mountain-plains circulation, the weakening and ascent of the nocturnal inversion owing to diurnal heating and mixed-layer growth, and upper-level cooling related to the advance of a shortwave trough. Once the storm initiates, it moves eastward towards the lower terrain, where the higher CAPE and deep-layer shear environment support the transition into a supercell. It is after this transition that the most severe hail at the surface is observed.

How to cite: Bechis, H., Alvarez Imaz, M., Simone, I., Galligani, V., Cancelada, M., Piscitelli, F., Paula, M., and Salio, P.: Analysis of the environment of a severe hailstorm in Mendoza, Argentina during the RELAMPAGO-CACTI field campaign, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-356, https://doi.org/10.5194/ems2021-356, 2021.

Irina Statnaia, Alexey Karpechko, and Heikki Järvinen

The weather-dependent planning and decision-making benefit greatly from subseasonal to seasonal (S2S) weather predictions made for up to six weeks ahead. At this timescale anomalies in the extratropical stratospheric circulation, which can last for several weeks in the Northern Hemisphere during winter, are known to affect climate at the surface and can extend the predictability of tropospheric weather conditions. The downward influence of the stratospheric circulation anomalies on the troposphere is projected by the Northern Annular Mode (NAM). Because of the long persistence of stratospheric anomalies beyond typical weather timescale, the increase in forecast skill is possible for the regions influenced by the atmospheric circulation variability associated with NAM based on the stratospheric predictor.

In this study, we investigate the predictability of the Eurasian severe and persistent cold spells during winter and its dependence on the state of the stratosphere. We first detected the below-normal surface temperature events over northern Eurasia (cold spells) in the ERA5 re-analysis. Then, to assess the predictability of these cold spells and to evaluate the skill in the probabilistic re-forecasts we divided them into groups associated with different stratospheric circulation anomalies which took place prior to the below-normal temperature events. When the stratospheric vortex is strong it is not expected to favor cold air outbreaks in this region. Therefore, in these cases, the cold air outbreaks result from internal tropospheric dynamics and their predictability is limited by the chaotic behavior of the weather systems. On the other hand, the weakening of the vortex is characterized by a more negative NAM index. This weakening is often followed by an equatorward shift of the tropospheric jets, an increase in the frequency of occurrence of tropospheric blocking, and cold air outbreaks over northern Eurasia. In these cases, the stratospheric vortex weakening can lead to the statistically significant improvement of the skill of cold air outbreak forecasts in case if the forecast model is capable of properly representing the coupling between the stratosphere and the troposphere. We show that the predictability of cold spells in the European Centre for Medium-range Weather Forecasts (ECMWF) model is enhanced under weak vortex conditions starting from week 3 before the event. We also evaluate how the surface predictability is affected by model imperfections by comparing the predictability across different S2S models.

How to cite: Statnaia, I., Karpechko, A., and Järvinen, H.: Stratospheric influence on the predictability of cold spells, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-231, https://doi.org/10.5194/ems2021-231, 2021.

Philipp Zschenderlein and Heini Wernli

In January 2021, large parts of Spain were affected by an unusually long cold spell and exceptional snowfall associated with the winter storm Filomena. According to the Spanish weather service AEMET, snow heights of nearly 50 cm were registered in and around Madrid. During the days after Filomena, record-breaking low temperatures were measured at many stations.

Already during the days before the arrival of storm Filomena, anomalously cold temperatures at 850 hPa and night frosts at the surface prevailed over large parts of Spain. During these days in early January, the air flow towards Spain was predominantly northeasterly and advected cold air masses from Central Europe, as revealed by backward trajectories that were initialised near the surface over Spain. The land surface progressively cooled down during the days prior to the heavy snowfall, which then prevented the snow from melting when reaching the surface. Therefore, this cold spell preconditioning seems to be very important for the extreme consequences of the snowfall event.

The storm Filomena affected Spain between 8 and 10 January. It developed from a precursor low-pressure system between the Azores and Madeira. The precursor low-pressure system itself developed on 2 January 2021 between the northeastern US and Nova Scotia, rapidly intensified along a potential vorticity (PV) streamer and propagated southeastwards. Between 4 and 6 January, the cyclone, now located near the Azores, was associated with a PV cut-off and eventually decayed into multiple centres. Out of this decaying low-pressure system, Filomena developed and reached Spain on 8 January.

The most intense snowfall occurred on 9 January and affected large parts of Spain, except for southwestern Spain, where temperatures were too high and all precipitation fell as rain. Filomena was associated with an intense air mass boundary, with dry and cold air in the north and warm and humid air in the south. Equivalent potential temperature differences at 850 hPa across Spain exceeded 20 K. Along the warm frontal part of this air mass boundary, strong ascending airstreams, intensified by the dynamics of Filomena, led to cloud formation. Due to the unusually cold lowermost troposphere, snow was not melting before reaching the land surface, and the surface snow layer could therefore easily grow.

Overall, the combination of the already cold temperatures near the surface, the optimal position of the air mass boundary, and the dynamical forcing for ascent at this intense baroclinic zone associated with Filomena were essential ingredients for this extreme snow fall event to occur.

How to cite: Zschenderlein, P. and Wernli, H.: The unusually long cold spell and the snowstorm Filomena over Spain in January 2021, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-98, https://doi.org/10.5194/ems2021-98, 2021.

Eva Plavcová, Ondřej Lhotka, and Jan Stryhal

Regional Climate Models (RCMs) are powerful tools to study changes in the climate system on the regional scale. However, the reliability of their simulations has been considerably limited by the longstanding issue that climate models often fail to reproduce various aspects of the historical climate. In our study, we analyse how RCMs from the EURO-CORDEX project are able to reproduce extreme winter weather. We analyse temporal and spatial characteristics of extreme wind gust, extremely cold temperature, and extreme precipitation. Model outputs are validated against observed data from the European gridded observational database (EOBS) and the novel ERA5 reanalysis. We focus on the Central European domain (defined between 48–52°N and 10–19°E) over the 1979 – 2017 period. We investigate a set of 9 simulations of 3 different RCMs driven by 3 different global climate models which allow us to analyse the influence of driving data on the RCM’s performance. Since local climate elements are relatively tightly linked to a large-scale atmospheric circulation over Europe in winter, we also evaluate the ability of RCMs to reproduce the atmospheric circulation and its links to selected high-impact winter weather in detail. We use the classification of circulation based on the method of Sammon mapping. Investigation of these links can lead to better physical understanding of the climate and to the identification of inadequacies in simulated characteristics of the studied events. All of this is an important step forward in further improving the models and enhancing the credibility of climate change scenarios based on climate model simulations.

How to cite: Plavcová, E., Lhotka, O., and Stryhal, J.: Winter extreme weather in EURO-CORDEX climate models and their links to large-scale atmospheric circulation, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-377, https://doi.org/10.5194/ems2021-377, 2021.

Joseba Egaña, Santiago Gaztelumendi, Roberto Hernandez, and Kepa Otxoa de Alda

Episodes of very strong winds can occur in the Basque Country for different reasons. On the one hand, in the winter season (October-April) due to deep depressions that move from west to east within the intense zonal circulation, often passing through the British Isles. In the summer season (May-September) strong winds may be due to severe storms or "galernas" (coastal trapped disturbances). The most relevant and widespread damage occurs in the cold season, usually due to deep depressions or other configurations that generate a strong local pressure gradient. In fact, the most damaging situations affecting the Basque Country during the 21st century are related to deep low passages considered as explosive cyclogenesis events, the Klaus and Xynthia episodes.

This work analyses some of the most relevant wind events during the 21st century, in terms of synoptic configuration and local behaviour, using reanalysis data and observations from the Automatic Meteorological Stations (AWS) network of the Basque Country.  The final objective is to identify and classify the common behaviour among high impact weather episodes, understanding the main synoptic features and drawing general conclusions on how synoptic features condition relevant aspects at lower scales, where complex orographic effects are relevant.

In the Basque Country area, most extreme episodes correspond to intense zonal circulations where deep depressions are generated, often explosive cyclogenesis. If we look at the surface configuration, the most frequent case is that of the British low, also secondary lows to the primary one, which is located around the British Isles. Wind events generated by depressions passing through the Bay of Biscay are also frequent. In general, these situations generate southerly wind events that can sometimes turn to the west. Other less frequent situations that can generate intense winds are Atlantic lows to the west of the Iberian Peninsula, which generate southerly winds, or situations of high pressure in the Atlantic with low pressure in Europe, sometimes in France, which generate intense winds from the north and northwest. Locally, due to the complex orography and the configuration of the valleys in the Basque Country, the wind is channelled in different ways, affecting different areas depending on the direction and strength of the wind.

How to cite: Egaña, J., Gaztelumendi, S., Hernandez, R., and Otxoa de Alda, K.: Synoptic and local characteristics of severe wind episodes in the Basque Country., EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-230, https://doi.org/10.5194/ems2021-230, 2021.

Nikolay Penov, Anastasiya Stoycheva, and Guergana Guerova

Despite the continuous improvement of weather prediction fog diagnosis and forecasting remains a challenge with large economic losses for public services and in particular aviation where the cost of flight delays and rescheduling is estimated to hundreds of million euros per year. Today the operational fog forecasting is mainly done with "in-house" developed tools, which is understandable due to the fog life cycle peculiarity. The aim of this work is to investigate the fog climatology for Plovdiv, Bulgaria for the period 1991 - 2018 and to use it for calculation a threshold value of stability index, which can be implemented as an operational forecast tool. The climatology shows well-defined seasonal behavior of the fog and that the majority of the fog registrations are with horizontal visibility below 200 m. A 10-year moving average of the fog registrations time series shows a decrease after 2012. Stability index values for various visibility ranges are calculated and compared. In the last decade, there are major improvements in horizontal and spatial resolution, microphysics, and initial conditions of the Numerical Weather Prediction models.  However, fog forecasting remains a challenge due to the small scale of the phenomena and local effects, which can remain unresolved by the models. One fog case in January 2013 is selected for numerical weather prediction simulations with the WRF model for the city of Plovdiv. The reliability of the index is evaluated both with observations and model data. It was found that while the index with its site-specific threshold value well describes the fog evolution, the WRF model has large deviations in temperature compared to the observations during daytime.

How to cite: Penov, N., Stoycheva, A., and Guerova, G.: Fog climatology and stability index for Plovdiv 1991-2018, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-77, https://doi.org/10.5194/ems2021-77, 2021.

Jou Ping Hou and Li-Zhen Su

Along the coast of northwestern Taiwan, when the Meiyu front passed, there were occasional rapids caused by the terrain, and after convergence with the prevailing wind field, it caused severe precipitation. In 1987, some studies conducted by Taiwan mesoscale experiment (TAMEX) found that low-level jet (about 1 km high) under certain conditions, known as barrier jet, strongly affected the heavy rainfall in northern Taiwan. On the morning of June 2, according to the results of the study in the Meiyu frontal contact north Taiwan, in just 12 hours later to Keelung and north coast down to the super heavy rain of reason, may be related to frontal subject in northern Taiwan snowy mountains, the enhancement of barrier jets occurring near the surface height is related to the increase in barrier jets during the movement of the frontal body. The results of this study show that when the Meiyu front moves southeast, the enhancement of the convective system on the front may cause by a larger inclination angle with its front, with appropriate movement speed, through the southwest airflow in front of the Meiyu front, especially a barrier of about 1 km in height. The warm and humid air introduced by the jet is related. Strong convection will develop in a forced convergence zone off the northwestern part of Taiwan, and the convergence zone is mainly caused by a combination of sub-synoptic forcing such as low-level wind shear convergence, barrier jets, and convective feedback of non-adiabatic terms. In addition, due to the existence of the barrier jet and the frontal wind shear zone, cyclonic circulation around the jet area was generated. With the instability of the temperature gradient and the enhancement of the abnormal zone of the positive potential vorticity, it is speculated that it should be the cause of the severe precipitation in this case. Because the biggest difference between the first and the second strong precipitation may come from the complicated topographic effect and limited space, this research focuses on the reasons for the development of the first severe weather development and heavy precipitation.

How to cite: Hou, J. P. and Su, L.-Z.: A Study on the Severe Precipitation Induced by the Meiyu Front in Taiwan, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-2, https://doi.org/10.5194/ems2021-2, 2021.

Chairpersons: Fulvio Stel, Jordi Mazon
Antoine Blanc, Juliette Blanchet, and Jean-Dominique Creutin

Extreme precipitation in the Northern French Alps are mainly associated with large-scale circulations (LSCs) bringing moist air from the Atlantic Ocean and the Mediterranean Sea - two atmospheric influences that are very frequent in the climatology. In this work, we investigate what characterizes the Atlantic/Mediterranean circulations driving extreme precipitation in the Northern French Alps in comparison to "random" Atlantic/Mediterranean circulations. We focus on extreme 3-day precipitation over two medium size neighboring catchments from 1950 to 2017. Atlantic and Mediterranean circulations are identified using an existing weather pattern classification established for Southern France. Every single LSC is characterized using three atmospheric descriptors based on analogy in geopotential shapes at 500hPa over Western Europe that were introduced in previous works. They are i) the celerity, characterizing the stationary nature of a geopotential shape, and ii) the singularity and relative singularity, characterizing the resemblance of a geopotential shape to its analogs, in other words the way this geopotential shape is closely reproduced in the climatology. We add to these analogy-based descriptors a new (non analogy) descriptor accounting for the strength of the low and high pressure systems. We show that Atlantic/Mediterranean circulations driving extreme 3-day precipitation in the Northern French Alps are the Atlantic/Mediterranean circulations featuring the strongest centers of action as well as the most stationary and the most reproducible geopotential shapes - characteristics that are rare for both atmospheric influences. In the Atlantic case, these characteristics appear to be even more pronounced and rare with regard to the whole climatology, pointing LSC as an important driver of extreme precipitation. In the Mediterranean case, these characteristics appear to be more random with regard to the whole climatology, pointing a more balanced contribution between specific LSC and humidity in driving extreme precipitation.

How to cite: Blanc, A., Blanchet, J., and Creutin, J.-D.: Characterizing Large-Scale Circulations Driving Extreme Precipitation in the Northern French Alps, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-225, https://doi.org/10.5194/ems2021-225, 2021.

Kathrin Wapler and Marcus Beyer

Tornados pose a significant threat to life, property, and economy. Thus, an analysis of tornadoes is of high relevance. An understanding of historical events, e.g. regarding the characteristics of tornadic storms compared to multi-year storm statistics, may help to improve the situational awareness of future tornado events.

In this study, tornadic storms with a tornado intensity of F2 or stronger on the Fujita scale that occurred in recent years (2016 – 2020) in Germany were analyzed in detail. The four F3 tornadoes (Bützow, Affing, Bonndorf and Roetgen) and sixteen F2 tornadoes, which developed on 17 different days occurred in various parts of Germany. Most of the analysed tornadoes occurred from May to early September. The other three cases are typical winter cases that differ significantly from the summer cases in some aspects that are discussed where applicable. One case which happened in the third decade of September has characteristics form both, summer and winter, and is thus the only hybrid case. The great majority of all cases occurred during the second half of the day, most of them between 12 and 18 UTC. The most active hour was 16 to 17 UTC.

Regarding forecasting, similarities and differences of the prevailing synoptic and mesoscale conditions are assessed in addition to the convective environment of the events. Furthermore, the type of convection is analysed. The goal is to anticipate typical characteristics that enhance the threat of a potentially dangerous tornado situation. Using these findings may then help to strengthen the awareness of the forecaster. Two situations in mid- and upper-level flow are typical for the occurrence of strong tornadoes. On the majority of the analysed tornadic days, the event happened on the forward flank of a long wave trough that was slowly propagating eastward. The other typical situation is a vivid short wave trough passing rather fast over the area of interest from West to East.

Regarding nowcasting, a multi-source approach was applied to best analyse the events. For this purpose, radar reflectivity and rotation data were combined with lightning detection in order to analyse the tornadic storms with respect to storm mode and storm evolution as well as lightning and rotation characteristics. In many cases, radar radial wind data showed a persistent rotation track. The automatically detected mesocyclones had a vertical depth between 2.5 and 11 km at the time of the tornado, the diameter was above 8 km. The base of the rotation was low compared to multi-year statistics of all mesocyclonic storms. The lighting activity of the tornadic storms was high. In many cases, a lightning jump occurred between 5 and 120 minutes before the event.

How to cite: Wapler, K. and Beyer, M.: Study of strong tornadoes in Central Europe: Various aspects of Forecasting and Nowcasting, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-124, https://doi.org/10.5194/ems2021-124, 2021.

Chelsey Laurencin, Anthony Didlake, Jr., Jerry Harrington, and Anders Jensen

Squall lines have a large impact on the annual rainfall in the mid-latitudes, yet there are still unanswered questions on the mechanisms generating their precipitation structure. Precipitation processes have a large impact on the storm’s intensity, lifetime, and flooding impacts. Ice crystals are involved in modulating the distribution of latent heating throughout the storm during phase change and thus play a role in determining the thermodynamic and kinematic structure of the stormThe current study uses novel ice crystal trajectory growth (ICTG) model to examine the physical and kinematic mechanisms contributing to the ice structure of a quasi-idealized 3D simulation of a leading-line, trailing-stratiform squall line. This analysis is important for scientific understanding and accurate representation of microphysical processes in numerical models.  

Ice crystals are initialized in the leading convective line, and they are advected by the storm-relative winds while simultaneously growing by vapor deposition and riming (or sublimating). To account for variations in the ice particle size distribution, multiple trajectory simulations are conducted for crystals with varying initial diametersThe ICTG model produced a spatial distribution of ice crystals consistent with the precipitation and reflectivity structure of the simulated squall line above the melting level. Trajectory simulations using initially small crystals (< 0.05 mm in diameter) result in the crystals being transported to the forward and rear anvils and the stratiform region. There are two main paths to the stratiform region: one is characterized by sustained interaction with the mesoscale updraft, dendritic growth, and a large final particle size; the other is characterized by lack of interaction with the mesoscale updraft, prolonged sublimation or suppressed growth, and a small final particle size. Simulations using initially large crystals (0.5 mm in diameter) result in graupel-like particles that fall out primarily in the leading convective line. These findings support past studies identifying ice particle size sorting in squall lines which result in a local precipitation rate minimum occurring between the convective leading line and trailing stratiform region. The simulated patterns of ice crystal growth and trajectories also hold implications for the latent heating structure and evolution of the overall storm. 

How to cite: Laurencin, C., Didlake, Jr., A., Harrington, J., and Jensen, A.: Examining Ice Microphysical Evolution in a Quasi-Idealized Simulation of a Squall Line using an Ice Crystal Trajectory Growth (ICTG) Model, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-476, https://doi.org/10.5194/ems2021-476, 2021.

Guido Schröder

A modified lightning potential index (MLPI) for numerical models with parameterized deep convection is presented. It is based on the LPI formula of Lynn and Yair (2010). Following the idea of Lopez (2016), the quantities (e.g. vertical velocity) needed in the LPI formula are derived from the updraft of the Bechtold-Tiedtke parameterization scheme (Bechtold et al., 2014). The formula is further improved by taking into account the vertical equivalent potential temperature gradient.

The LPI and MLPI are tested in ICON with 20km resolution (ICON-20) over central Europe. A key component in the LPI is the vertical velocity. To assess its quality, the vertical velocity of the updraft in the convection scheme in ICON-20 is compared to updrafts in the convection-resolving COSMO model with 2.2 km resolution (COSMO-D2). It is shown that in ICON-20 the extension of the vertical velocity is generally broader with the maximum located in higher altitudes. In the charge separation area where the vertical velocity is relevant, the ICON-20 vertical velocity is less than in COSMO-D2. Consequently, the LPI values in ICON-20 are lower by a factor of 2 compared to COSMO-D2.

The MLPI is verified against LINET lightning data (Betz et al. 2009) over central Europe for summer 2020 and compared to LPI in COSMO-D2. The MLPI is also compared to the LPI and the lightning flash density (LFD,  Lopez, 2016), all computed in ICON-20. For the test period the MLPI outperforms the LPI and LFD. However, the quality of the LPI in COSMO-D2 cannot quite be reached.


Bechtold et al. 2014: Representing Equilibrium and Nonequilibrium Convection in Large-Scale Models. J. Atmos. Sci. 71, 734-753.

Betz et al., 2009:  LINET - An international lightning detection network in Europe. Atmos.  Res. 91 564–573.

Lopez, 2016: A Lightning Parameterization for the ECMWF Integrated Forecasting System. Mon. Wea. Rev., 144, 3057-2075.

Lynn and Yair, 2010: Prediction of lightning flash density with the WRF model  Adv. Geosci., 23, 11–16.

How to cite: Schröder, G.: A modified lightning potential index for numerical models with parameterized deep convection, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-253, https://doi.org/10.5194/ems2021-253, 2021.

Natalia Korhonen, Otto Hyvärinen, Matti Kämäräinen, and Kirsti Jylhä

Severe heatwaves have harmful impacts on ecosystems and society. Early warning of heat waves help with decreasing their harmful impact. Previous research shows that the Extended Range Forecasts (ERF) of the European Centre for Medium-Range Weather Forecasts (ECMWF) have over Europe a somewhat higher reforecast skill for extreme hot summer temperatures than for long-term mean temperatures. Also it has been shown that the reforecast skill of the ERFs of the ECMWF was strongly increased by the most severe heat waves (the European heatwave 2003 and the Russian heatwave 2010).

Our aim is to be able to estimate the skill of a heat wave forecast at the time the forecast is given. For that we investigated the spatial and temporal reforecast skill of the ERFs of the ECMWF to forecast hot days (here defined as a day on which the 5 days running mean surface temperature is above its summer 90th percentile) in the continental Europe in summers 2000-2019. We used the ECMWF 2-meter temperature reforecasts and verified them against the ERA5 reanalysis. The skill of the hot day reforecasts was estimated by the symmetric extremal dependence index (SEDI) which considers both hit rates and false alarm rates of the hot day forecasts. Further, we investigated the skill of the heatwave reforecasts based on at which time steps of the forecast the hot days were forecasted. We found that on the mesoscale (horizontal scale of ~500 km) the ERFs of the ECMWF were most skillful in predicting the life cycle of a heat wave (lasting up to 25 days) about a week before its start and during its course. That is, on the mesoscale those reforecasts, in which hot day(s) were forecasted to occur during the first 7…11 days, were more skillful on lead times up to 25 days than the rest of the heat wave forecasts. This finding is valuable information, e.g., in the energy and health sectors while preparing for a coming heat wave.

The work presented here is part of the research project HEATCLIM (Heat and health in the changing climate) funded by the Academy of Finland.

How to cite: Korhonen, N., Hyvärinen, O., Kämäräinen, M., and Jylhä, K.: Evaluating sub-seasonal heatwave reforecasts of the ECMWF over Europe, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-146, https://doi.org/10.5194/ems2021-146, 2021.

Samira Khodayar Pardo

In a changing climate an increase in frequency, intensity, and duration of heat extremes in Europe is predicted, accompanied by a decrease in mean precipitation, soil moisture and evapotranspiration. Over the last century, an increase in the duration of the heat waves and the number of hot days and warm nights was observed in Europe. One of the main “hot-spot” in this intensification of weather extremes is the Mediterranean due to the strong decrease in mean precipitation and increase in mean temperature. In the last two decades Europe has been frequently affected by extreme heat waves with record-breaking temperatures and large impacts on the natural, social system or on human health. During such heat-wave periods the daily mortality rate reached anomalies of about +70 %, causing for example additional 70,000 heat-related deaths in Europe in summer 2003. European cities are highly vulnerable to such events due to the urban heat effect.

Despite the significant impact of heat waves on society and the projected increase in the frequency and intensity of these phenomena in Europe, the local-to-regional characteristics and the physical processes that contribute to their occurrence are not yet sufficiently understood.

In this study, we investigate two relevant aspects of this extreme phenomena, (a) the evolution observed from the 1950-to-present in three different climatic zones in Europe, the Iberian Peninsula, France, and Central Europe, and (b) the sensitivity to preceding winter-spring precipitation and particularly spring soil moisture conditions.

Despite a general positive tendency towards higher number and intensity of heat waves in all climatic zones, relevant differences were also found. Wet winter seasons are correlated to a decrease in the number of heat days and heat wave events during summer periods. Furthermore, extreme dry/wet spring soil moisture conditions over the Iberian Peninsula contribute up to 30% to changes in summer temperature in central Europe. In this presentation, the above mentioned and additional key results will be further discussed in detail.

How to cite: Khodayar Pardo, S.: European heat waves: observational characterization attending to climatic zones and modelled sensitivity to spring precipitation deficit., EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-397, https://doi.org/10.5194/ems2021-397, 2021.

George Pacey, Stephan Pfahl, and Lisa Schielicke

Cold fronts provide an environment favourable for convective initiation in the mid-latitudes. Some studies also show the presence of a cold front can increase the chance of certain convective hazards, such as hail and heavy rain. Convection initiates in three locations in respect to cold fronts: ahead of the cold front in the warm sector of the cyclone, directly at the cold frontal boundary and also behind the cold front. Previous literature has typically focused on each initiation location independently, thus a comprehensive study investigating the link between cold fronts and convection is currently lacking from literature. This study seeks to better understand the climatology, scale interactions and forcing mechanisms of convection at each initiation location relative to the front (i.e., behind, at, ahead).

Automatic front detection methods are applied to reanalysis data and a convective cell-tracking dataset from the German Weather Service is used to build a climatology of cold fronts and convection between April–September. Convective cells are found to initiate most often 300–400km ahead of the cold front during the evening. Cells behind the front primarily initiate in north-western Germany and exhibit a strong diurnal cycle. On the contrary, cells at and ahead of the front initiate most frequently in southern Germany and exhibit a less prominent diurnal cycle, especially for cells at the frontal boundary. Lightning probability decreases with closing proximity to the cold front and the average number of cell initiations per day is significantly higher on days with cold fronts opposed to days without. The next stages of research will investigate the relative importance of various forcing mechanisms on the development of convective cells at different cell-front positions.

How to cite: Pacey, G., Pfahl, S., and Schielicke, L.: The interaction between cold fronts and convection in the mid-latitudes, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-343, https://doi.org/10.5194/ems2021-343, 2021.

Sante Laviola, Giulio Monte, Vincenzo Levizzani, Ralph R. Ferraro, and James Beauchamp

Hail detection is an open issue from the remote sensing point of view both from the ground and from space. Hail is extremely difficult to observe using passive and active sensing due to signal attenuation and the relatively scarce knowledge of the cloud structure in hailstorms. Several approaches have been recently proposed mainly using radar data from the ground in connection with observations in the visible and infrared from geostationary satellites. High frequency MWs aboard to airborne and satellite were also used to infer hail patterns. The potential of the 90–190 GHz frequency range when employed in the classification of cloud types and in the detection of signals from different hail sizes was recently proved by Laviola et al. 2020 and Ferraro et al. (2020) extending previous approaches to these frequencies that are now available on several platforms. MW high frequencies offer the advantage of very high sensitivity to the scattering signature from different ice particles with diameters from a few millimeters to 10s of centimeters. Thus, we are able to classify the region of convective clouds where different hail sizes are generated by identifying severity areas characterized by small ice aggregates potentially forming hail, large hail and super hail. In this work, a probability-based model originally designed for AMSU-B/MHS (Laviola et al, 2020) has been fitted to the observations of all MHS-like radiometers onboard the satellites of the GPM constellation. All MHS-like frequency channels in the 150-170 GHz frequency range were adjusted on the MHS channel at 157 GHz in order to account for the instrumental differences and tune the original model on the MHS-like technical characteristics. The novelty of this approach offers the potential of retrieving a uniform and homogeneous hail dataset on the global scale. Currently running on 10 MHS-like radiometers orbiting on the GPM constellation, the application of the hail detection model demonstrates the high potential of this generalized model to map the evolution of hail-bearing systems at very high temporal rate. The results on the global scale also demonstrate the high performances of the hail model in detecting the differences of hailstorm structure across the two hemispheres by means of a thorough reconstruction of the seasonality of the events particularly in South America where the largest hailstones are typically observed.

How to cite: Laviola, S., Monte, G., Levizzani, V., Ferraro, R. R., and Beauchamp, J.: Hail detection from high-frequency radiometers on the GPM constellation. A new prospect for a global hailstorm climatology, EMS Annual Meeting 2021, online, 6–10 Sep 2021, EMS2021-234, https://doi.org/10.5194/ems2021-234, 2021.


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