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Pieter Groenemeijer and Thilo Kühne

Abstract

A climatology of tornadoes (over land and water) is presented, based on the European Severe Weather Database (ESWD), which contains reports of 9529 tornadoes. With the exception of a few small countries, tornadoes have been reported from all regions of Europe. The highest density of tornado reports is in western and central Europe. ESWD tornado reports increased strongly from 1995 to 2006 as a result of increased data collection efforts, followed by a decrease that likely has a meteorological nature. There is strong underreporting in the Mediterranean region and eastern Europe. The daily cycle of tornadoes over land (sea) peaks between 1500 and 1600 (0900 and 1000) local time. The Mediterranean annual maximum is in autumn and winter, while regions farther north have a maximum in summer. In total, 822 tornado fatalities have been recorded in the ESWD, which include 10 tornadoes with more than 20 fatalities. The average annual number of tornado fatalities in Europe is estimated to be between 10 and 15. The F2 and F3 tornadoes are responsible for the majority of the fatalities.

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Anja T. Rädler, Pieter Groenemeijer, Eberhard Faust, and Robert Sausen

Abstract

A statistical model for the occurrence of convective hazards was developed and applied to reanalysis data to detect multidecadal trends in hazard frequency. The modeling framework is based on an additive logistic regression for observed hazards that exploits predictors derived from numerical model data. The regression predicts the probability of a severe hazard, which is considered as a product of two components: the probability that a storm occurs and the probability of the severe hazard, given the presence of a storm [P(severe) = P(storm) × P(severe|storm)]. The model was developed using lightning data as an indication of thunderstorm occurrence and hazard reports across central Europe. Although it uses only two predictors per component, it is capable of reproducing the observed spatial distribution of lightning and yields realistic annual cycles of lightning, hail, and wind fairly accurately. The model was applied to ERA-Interim (1979–2016) across Europe to detect any changes in lightning, hail, and wind hazard occurrence. The frequency of conditions favoring lightning, wind, and large hail has increased across large parts of Europe, with the exception of the southwest. The resulting predicted occurrence of 6-hourly periods with lightning, wind, and large hail has increased by 16%, 29%, and 41%, respectively, across western and central Europe and by 23%, 56%, and 86% across Germany and the Alps during the period considered. It is shown that these changes are caused by increased instability in the reanalysis rather than by changes in midtropospheric moisture or wind shear.

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Bogdan Antonescu, David M. Schultz, Alois Holzer, and Pieter Groenemeijer

Abstract

The social and economic impact of tornadoes in Europe is analyzed using tornado reports from the European Severe Weather Database between 1950 and 2015. Despite what is often assumed by the general public and even by meteorologists and researchers, tornadoes do occur in Europe and they are associated with injuries, fatalities, and damages, although their reported frequencies and intensities are lower compared with the United States. Currently, the threat of tornadoes to Europe is underestimated. Few European meteorological services have developed and maintained tornado databases and even fewer have issued tornado warnings. This article summarizes our current understanding of the tornado threat to Europe by showing the changes in tornado injuries and fatalities since the 1950s and by estimating for the first time the damages associated with European tornadoes. To increase awareness of tornadoes and their threat to Europe, we propose a strategy that includes 1) collaboration between meteorological services, researchers, and the general public toward a pan-European database; 2) development of national forecasting and warning systems and of pan-European convective outlooks; and 3) development by decision-makers and emergency managers of policies and strategies that include tornadoes.

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Tomáš Púčik, Pieter Groenemeijer, David Rýva, and Miroslav Kolář

Abstract

The environments of severe and nonsevere thunderstorms were analyzed using 16 421 proximity soundings from December 2007 to December 2013 taken at 32 central European stations. The soundings were assigned severity categories for the following hazards: hail, wind, tornado, and rain. For each of the soundings, parameters were calculated representing the instability, vertical wind profile, and moisture of the environment. The probability of the various hazards as a function of CAPE and 0–6-km bulk shear (DLS) is quite different for each of the hazards. Large hail is most likely for high CAPE and high DLS, a regime that also supports severe wind events. A second severe wind regime exists for low CAPE and very high DLS. These events are mostly cold season events. Storms with significant tornadoes occur with much higher DLS than storms with weak or no tornadoes, but with similar CAPE. The 0–1-km bulk shear (LLS) does not discriminate better than DLS between weak and significant tornadoes. Heavy rain events occur across a wide range of DLS, but with CAPE above the median for nonsevere thunderstorms and are most likely when both absolute humidity in the boundary layer and relative humidity in the low- to midtroposphere are high. LCL height does not discriminate well between the intensity categories of tornadoes, but higher LCL heights were associated with a higher probability of severe hail. Storm relative helicity shows similar results to DLS, but with more overlap among intensity categories.

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Tomáš Púčik, Christopher Castellano, Pieter Groenemeijer, Thilo Kühne, Anja T. Rädler, Bogdan Antonescu, and Eberhard Faust

Abstract

By 31 December 2018, 39 537 quality-controlled reports of large hail had been submitted to the European Severe Weather Database (ESWD) by volunteers and ESSL. This dataset and the NatCatSERVICE Database of Munich RE jointly allowed us to study the hail hazard and its impacts across Europe over a period spanning multiple decades. We present a spatiotemporal climatology of the ESWD reports, diurnal and annual cycles of large hail, and indicate where and how they may be affected by reporting biases across Europe. We also discuss which hailstorms caused the most injuries and present the only case with hail fatalities in recent times. Additionally, we address our findings on the relation between hail size to the type of impacts that were reported. For instance, the probability of reported hail damage to roofs, windows, and vehicles strongly increases as hail size exceeds 5 cm, while damage to crops, trees, and greenhouses is typically reported with hailstone diameters of 2–3 cm. Injuries to humans are usually reported with hail 4 cm in diameter and larger, and number of injuries increases with increasing hail size. Using the NatCatSERVICE data, we studied economic losses associated with hailstorms occurring in central Europe and looked for long-term changes. The trend in hail losses and the annual number of hail loss days since 1990 to 2018 are compared to that of meteorological conditions favorable for large hail as identified by ESSL’s Additive Regression Convective Hazards model. Both hail loss days and favorable environments show an upward trend, in particular since 2000.

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Mateusz Taszarek, John Allen, Tomáš Púčik, Pieter Groenemeijer, Bartosz Czernecki, Leszek Kolendowicz, Kostas Lagouvardos, Vasiliki Kotroni, and Wolfgang Schulz

Abstract

The climatology of (severe) thunderstorm days is investigated on a pan-European scale for the period of 1979–2017. For this purpose, sounding measurements, surface observations, lightning data from ZEUS (a European-wide lightning detection system) and European Cooperation for Lightning Detection (EUCLID), ERA-Interim, and severe weather reports are compared and their respective strengths and weaknesses are discussed. The research focuses on the annual cycles in thunderstorm activity and their spatial variability. According to all datasets thunderstorms are the most frequent in the central Mediterranean, the Alps, the Balkan Peninsula, and the Carpathians. Proxies for severe thunderstorm environments show similar patterns, but severe weather reports instead have their highest frequency over central Europe. Annual peak thunderstorm activity is in July and August over northern, eastern, and central Europe, contrasting with peaks in May and June over western and southeastern Europe. The Mediterranean, driven by the warm waters, has predominant activity in the fall (western part) and winter (eastern part) while the nearby Iberian Peninsula and eastern Turkey have peaks in April and May. Trend analysis of the mean annual number of days with thunderstorms since 1979 indicates an increase over the Alps and central, southeastern, and eastern Europe with a decrease over the southwest. Multiannual changes refer also to changes in the pattern of the annual cycle. Comparison of different data sources revealed that although lightning data provide the most objective sampling of thunderstorm activity, short operating periods and areas devoid of sensors limit their utility. In contrast, reanalysis complements these disadvantages to provide a longer climatology, but is prone to errors related to modeling thunderstorm occurrence and the numerical simulation itself.

Open access
Mateusz Taszarek, John T. Allen, Pieter Groenemeijer, Roger Edwards, Harold E. Brooks, Vanna Chmielewski, and Sven-Erik Enno

Abstract

As lightning-detection records lengthen and the efficiency of severe weather reporting increases, more accurate climatologies of convective hazards can be constructed. In this study we aggregate flashes from the National Lightning Detection Network (NLDN) and Arrival Time Difference long-range lightning detection network (ATDnet) with severe weather reports from the European Severe Weather Database (ESWD) and Storm Prediction Center (SPC) Storm Data on a common grid of 0.25° and 1-h steps. Each year approximately 75–200 thunderstorm hours occur over the southwestern, central, and eastern United States, with a peak over Florida (200–250 h). The activity over the majority of Europe ranges from 15 to 100 h, with peaks over Italy and mountains (Pyrenees, Alps, Carpathians, Dinaric Alps; 100–150 h). The highest convective activity over continental Europe occurs during summer and over the Mediterranean during autumn. The United States peak for tornadoes and large hail reports is in spring, preceding the maximum of lightning and severe wind reports by 1–2 months. Convective hazards occur typically in the late afternoon, with the exception of the Midwest and Great Plains, where mesoscale convective systems shift the peak lightning threat to the night. The severe wind threat is delayed by 1–2 h compared to hail and tornadoes. The fraction of nocturnal lightning over land ranges from 15% to 30% with the lowest values observed over Florida and mountains (~10%). Wintertime lightning shares the highest fraction of severe weather. Compared to Europe, extreme events are considerably more frequent over the United States, with maximum activity over the Great Plains. However, the threat over Europe should not be underestimated, as severe weather outbreaks with damaging winds, very large hail, and significant tornadoes occasionally occur over densely populated areas.

Open access
Tomáš Púčik, Pieter Groenemeijer, Anja T. Rädler, Lars Tijssen, Grigory Nikulin, Andreas F. Prein, Erik van Meijgaard, Rowan Fealy, Daniela Jacob, and Claas Teichmann

Abstract

The occurrence of environmental conditions favorable for severe convective storms was assessed in an ensemble of 14 regional climate models covering Europe and the Mediterranean with a horizontal grid spacing of 0.44°. These conditions included the collocated presence of latent instability and strong deep-layer (surface to 500 hPa) wind shear, which is conducive to the severe and well-organized convective storms. The occurrence of precipitation in the models was used as a proxy for convective initiation. Two climate scenarios (RCP4.5 and RCP8.5) were investigated by comparing two future periods (2021–50 and 2071–2100) to a historical period (1971–2000) for each of these scenarios. The ensemble simulates a robust increase (change larger than twice the ensemble sample standard deviation) in the frequency of occurrence of unstable environments (lifted index ≤ −2) across central and south-central Europe in the RCP8.5 scenario in the late twenty-first century. This increase coincides with the increase in lower-tropospheric moisture. Smaller, less robust changes were found until midcentury in the RCP8.5 scenario and in the RCP4.5 scenario. Changes in the frequency of situations with strong (≥15 m s−1) deep-layer shear were found to be small and not robust, except across far northern Europe, where a decrease in shear is projected. By the end of the century, the simultaneous occurrence of latent instability, strong deep-layer shear, and model precipitation is simulated to increase by up to 100% across central and eastern Europe in the RCP8.5 and by 30%–50% in the RCP4.5 scenario. Until midcentury, increases in the 10%–25% range are forecast for most regions. A large intermodel variability is present in the ensemble and is primarily due to the uncertainties in the frequency of the occurrence of unstable environments.

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Pieter Groenemeijer, Christian Barthlott, Ulrich Corsmeier, Jan Handwerker, Martin Kohler, Christoph Kottmeier, Holger Mahlke, Andreas Wieser, Andreas Behrendt, Sandip Pal, Marcus Radlach, Volker Wulfmeyer, and Jörg Trentmann

Abstract

Measurements of a convective storm cluster in the northern Black Forest in southwest Germany have revealed the development of a warm and dry downdraft under its anvil cloud that had an inhibiting effect on the subsequent development of convection. These measurements were made on 12 July 2006 as part of the field campaign Prediction, Identification and Tracking of Convective Cells (PRINCE) during which a number of new measurement strategies were deployed. These included the collocation of a rotational Raman lidar and a Doppler lidar on the summit of the highest mountain in the region (1164 m MSL) as well as the deployment of teams carrying radiosondes to be released in the vicinity of convective storms. In addition, an aircraft equipped with sensors for meteorological variables and dropsondes was in operation and determined that the downdraft air was approximately 1.5 K warmer, 4 g kg−1 drier, and therefore 3 g m−3 less dense than the air at the same altitude in the storm’s surroundings. The Raman lidar detected undulating aerosol-rich layers in the preconvective environment and a gradual warming trend of the lower troposphere as the nearby storm system evolved. The Doppler lidar both detected a pattern of convergent radial winds under a developing convective updraft and an outflow emerging under the storm’s anvil cloud. The dryness of the downdraft air indicates that it had subsided from higher altitudes. Its low density reveals that its development was not caused by negative thermal buoyancy, but was rather due to the vertical mass flux balance accompanying the storm’s updrafts.

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Pieter Groenemeijer, Tomáš Púčik, Alois M. Holzer, Bogdan Antonescu, Kathrin Riemann-Campe, David M. Schultz, Thilo Kühne, Bernold Feuerstein, Harold E. Brooks, Charles A. Doswell III, Hans-Joachim Koppert, and Robert Sausen

Abstract

The European Severe Storms Laboratory (ESSL) was founded in 2006 to advance the science and forecasting of severe convective storms in Europe. ESSL was a grassroots effort of individual scientists from various European countries. The purpose of this article is to describe the 10-yr history of ESSL and present a sampling of its successful activities. Specifically, ESSL developed and manages the only multinational database of severe weather reports in Europe: the European Severe Weather Database (ESWD). Despite efforts to eliminate biases, the ESWD still suffers from spatial inhomogeneities in data collection, which motivates ESSL’s research into modeling climatologies by combining ESWD data with reanalysis data. ESSL also established its ESSL Testbed to evaluate developmental forecast products and to provide training to forecasters. The testbed is organized in close collaboration with several of Europe’s national weather services. In addition, ESSL serves a central role among the European scientific and forecast communities for convective storms, specifically through its training activities and the series of European Conferences on Severe Storms. Finally, ESSL conducts wind and tornado damage assessments, highlighted by its recent survey of a violent tornado in northern Italy.

Open access