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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.

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.

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: Erner, 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.

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.

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.

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.

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.

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.

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.

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.