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Mid-latitude Cyclones and Storms: Diagnostics of Observed and Future Trends, and related Impacts

This session investigates mid-latitude cyclones and storms on both hemispheres. We invite studies considering cyclones in different stages of their life cycles from the initial development, to large- and synoptic-scale conditions influencing their growth to a severe storm, up to their dissipation and related socioeconomic impacts.
Papers are welcome, which focus also on the diagnostic of observed past and recent trends, as well as on future storm development under changed climate conditions. This will include storm predictability studies on different scales. Finally, the session will also invite studies investigating impacts related to storms: Papers are welcome dealing with vulnerability, diagnostics of sensitive social and infrastructural categories and affected areas of risk for property damages. Which risk transfer mechanisms are currently used, depending on insured and economic losses? Which mechanisms (e.g. new reinsurance products) are already implemented or will be developed in order to adapt to future loss expectations?

Co-organized by NH1
Convener: Gregor C. Leckebusch | Co-conveners: Jennifer Catto, Joaquim G. Pinto, Uwe Ulbrich
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Fri, 30 Apr, 13:30–15:00

Chairpersons: Gregor C. Leckebusch, Jennifer Catto, Joaquim G. Pinto

5-minute convener introduction

Ségolène Berthou et al.

Met Office convection-permitting 2.2km simulations over a European domain show 10-20% larger increases in winter mean precipitation at the end of the century compared to their 25km convection-parameterised driving model. We identify individual storms with a maximum vorticity tracking algorithm and look at storm characteristics at their time of deepest minimum sea level pressure. We show that the thermodynamical characteristics of future winter storms are getting closer to present-day autumn storms, with future winter storms showing larger values of convective available potential energy and convective inhibition and more intense rainfall in their warm sector. This suggests that embedded convection in the warm conveyor belt is a good candidate to explain the larger future intensification of rainfall per storm in the 2.2km model compared to the convection-parameterised model. Multi-model analysis is underway to identify whether these conclusions hold in other convection-permitting models.

How to cite: Berthou, S., Kendon, E., Roberts, M., Vannière, B., Belušic, D., Caillaud, C., Dobler, A., Landgren, O., Manning, C., and Vergara-Temprado, J.: Convection in future winter storms over northern Europe., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12392, https://doi.org/10.5194/egusphere-egu21-12392, 2021.

Lea Eisenstein et al.

Extratropical cyclones cause strong winds and heavy precipitation events and are therefore one of the most dangerous natural hazards in Europe. The strongest winds within these cyclones are mostly connected to four mesoscale dynamical features: the warm (conveyor belt) jet (WJ), the cold (conveyor belt) jet (CJ), cold-frontal convective features (CFC) and the sting jet (SJ). While all four have high wind gust speeds in common, the timing, location and some further characteristics typically differ and hence likely also the forecast errors occurring in association with them.

Here we present an objective identification approach for the four features named above based on their most important characteristics in wind, rainfall, pressure and temperature evolution. The main motivations for this are to generate a climatology for Central Europe, to analyse forecast error specific to individual features, and to ultimately improve forecasts of high wind events through feature-dependent statistical post-processing. To achieve, we ideally want to be able to identify the features in surface observations and in forecasts in a consistent way.

Based on a dataset of hourly observations over Europe and nine windstorm cases during the winter seasons 2017/18, 2018/19 and 2019/20, it became apparent that mean sea-level pressure tendency, potential temperature tendency, change in wind direction and precipitation (all one-hourly) are most important for the distinction between the WJ and CFC. Further adding the time (relative to storm evolution) and location (relative to the storm centre) of occurrence helps to identify the CJ. Ultimately, the identification of each feature is based on a score on a scale from 0 to 10 that reflects the various criteria for a station or grid point. Additionally, exclusion criteria for each feature are defined to rule out locations that meet some criteria (and thus have a positive score) but strongly violate others. Finally, smooth contours are drawn around each feature to define their spatial extent.

While the distinction between WJ and CFC seems to work reliably, the identification of CJ remains ambiguous and needs further parameters and exclusion criteria to avoid too large areas and overlap with other features. Furthermore, SJ and CJ are very difficult to distinguish based on surface observations alone and are therefore taken together for this preliminary analysis. Once the definition of criteria is finalised, a climatology will be compiled based on observations and the German COSMO model and forecast errors analysed for said model.

How to cite: Eisenstein, L., Knippertz, P., and Pinto, J. G.: Identifying mesoscale high-wind features within extratropical cyclones, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2248, https://doi.org/10.5194/egusphere-egu21-2248, 2021.

Suzanne L. Gray et al.

Human activity in the Arctic is expected to increase as new regions become accessible, with a consequent need for reliable forecasts of hazardous weather. Arctic cyclones are synoptic-scale cyclones developing within or moving into the Arctic region. Meso- to synoptic-scale tropopause-based coherent vortices called tropopause polar vortices (TPVs) are frequently observed in polar regions and are a proposed mechanism for Arctic cyclone genesis and intensification. While the importance of pre-existing tropopause-level features for cyclone development, and their existence as part of the three-dimensional mature cyclone structure, is well established in the mid-latitudes, evidence of the importance of pre-existing TPVs for Arctic cyclone development is more limited. Here we present a climatology and characteristics of summer Arctic cyclones and TPVs, produced by tracking them in the latest global ECMWF reanalysis (ERA5), and determine the role of pre-existing TPVs in the initiation and intensification of these cyclones.

How to cite: Gray, S. L., Hodges, K., Vautrey, J., and Methven, J.: The role of tropopause polar vortices in the intensification of Summer Arctic cyclones, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2875, https://doi.org/10.5194/egusphere-egu21-2875, 2021.

Colin Manning et al.

Extra-tropical windstorms are one of the costliest natural hazards affecting Europe, and windstorms that develop a phenomenon known as a sting-jet account for some of the most damaging storms. A sting-jet (SJ) is a mesoscale core of high wind speeds that occurs in particular types of cyclones, specifically Shapiro-Keyser (SK) cyclones, and can produce extremely damaging surface wind gusts. High-resolution climate models are required to adequately model SJs and so it is difficult to gauge their contribution to current and future wind risk. In this study, we develop a low-cost methodology to automate the detection of sting jets, using the characteristic warm seclusion of SK cyclones and the slantwise descent of high wind speeds, within pan-European 2.2km convection-permitting climate model (CPM) simulations. Following this, we quantify the contribution of such storms to wind risk in Northern Europe in current and future climate simulations, and secondly assess the added value offered by the CPM compared to a traditional coarse-resolution climate model. This presentation will give an overview of the developed methods and the results of our analysis.

Comparing with observations, we find that the representation of wind gusts is improved in the CPM compared to ERA-Interim reanalysis data. Storm severity metrics indicate that SK cyclones account for the majority of the most damaging windstorms. The future simulation produces a large increase (>100%) in the number of storms exceeding high thresholds of the storm metric, with a large contribution to this change (40%) coming from windstorms in which a sting-jet is detected. Finally, we see a systematic underestimation in the GCM compared to the CPM in the frequency of extreme wind speeds at 850hPa in the cold sector of cyclones, likely related to better representation of sting-jets and the cold conveyor belt in the CPM. This underestimation is between 20-40% and increases with increasing wind speed above 35m/s. We conclude that the CPM adds value in the representation of severe surface wind gusts, providing more reliable future projections and improved input for impact models.

How to cite: Manning, C., Kendon, E., Fowler, H., Roberts, N., Berthou, S., Suri, D., and Roberts, M.: Midlatitude cyclones in convection permitting climate simulations: the added value offered for extreme wind speeds and sting-jets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12795, https://doi.org/10.5194/egusphere-egu21-12795, 2021.

Kristine Flacké Haualand and Thomas Spengler

Many weather and climate models fail to represent the sharp vertical changes of vertical wind shear and stratification near the tropopause. This discrepancy results in errors in the horizontal gradient of potential vorticity (PV), which acts as a wave guide for Rossby waves that highly influence surface weather in midlatitudes. In an idealised quasi-geostrophic model developed from the Eady model, we investigate how variations in vertical wind shear and stratification near the tropopause affect baroclinic growth. Comparing sharp and smooth vertical profiles of wind shear and stratification across the tropopause for different tropopause altitudes, we find that both smoothing and tropopause altitude have little impact on the growth rate, wavelength, phase speed, and structure of baroclinic waves, despite a sometimes significant weakening of the maximum PV gradient for extensive smoothing. Instead, we find that baroclinic growth is more sensitive if the vertical integral of the PV gradient is not conserved across the tropopause. Furthermore, including mid-tropospheric latent heating highlights that errors in baroclinic growth related to a misrepresentation of latent heating intensity are typically much larger than those associated with the correct representation of vertical wind shear and stratification in the tropopause region. Our results thus indicate that the correct representation of latent heating in weather forecast models is of higher importance than adequately resolving the tropopause.

How to cite: Haualand, K. F. and Spengler, T.: Relative importance of tropopause structure and diabatic heating for baroclinic instability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5335, https://doi.org/10.5194/egusphere-egu21-5335, 2021.

Andrea Marcheggiani and Maarten Ambaum

Local diabatic heating and temperature anomaly fields need to be positively correlated for the diabatic heating to maintain a circulation against dissipation. Here we quantify the thermodynamic contribution of local air–sea heat exchange on the evolution of weather systems using an index of the spatial covariance between heat flux at the air–sea interface and air temperature at 850 hPa upstream of the North Atlantic storm track, corresponding with the Gulf Stream extension region. The index is found to be almost exclusively negative, indicating that the air–sea heat fluxes act locally as a sink on potential energy. It features bursts of high activity alternating with longer periods of lower activity. The characteristics of these high-index bursts are elucidated through composite analysis and the mechanisms are investigated in a phase space spanned by two different index components. It is found that the negative peaks in the index correspond with thermodynamic activity triggered by the passage of a weather system over a spatially variable sea-surface temperature field; our results indicate that most of this thermodynamically active heat exchange is realised within the cold sector of the weather systems. Finally, we will discuss the implications of our findings, including a link with meridional heat flux pulses and a novel way of understanding whether such pulses are due to enhanced correlations or enhanced variances.

How to cite: Marcheggiani, A. and Ambaum, M.: The role of heat-flux–temperature covariance in the evolution of weather systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2692, https://doi.org/10.5194/egusphere-egu21-2692, 2021.

Chris Weijenborg and Thomas Spengler

The existence of cyclone clustering, the succession of multiple extratropical cyclones during a short period of time, indicates that the baroclinicity feeding these storms undergoes longer lasting episodic cycles supporting multiple cyclones. However, the generally accepted paradigm for baroclinic instability implies that individual cyclones reduce baroclinicity to support their growth. This apparent contradiction motivates our hypothesis that some cyclones within increase baroclinicity, yielding a pathway for cyclone clustering. A case study of the extreme storm Dagmar confirms that a particular sequence of storms culminating in a severe cyclone is due to the fact that the previous storms act to maintain or increase the background baroclinity along which the succeeding storms evolved. 

Using a new cyclone clustering diagnostic based on spatio-temporal distance between cyclone tracks, we analyse cyclone clustering globally for the period 1979 until 2016. We complement this analysis with a baroclinicity diagnostic based on the slope of isentropic surfaces. With the isentropic slope and its tendencies, the relative roles of diabatic and adiabatic effects associated with extra-tropical cyclones in maintaining baroclinicity are assessed. We present a climatological analysis of where and when cyclone clustering occurs. We compare these findings to composites of clustered and non-clustered cyclones to quantify how consistent the proposed clustering mechanism is and its relation to changes in the frequency of atmospheric rivers. We complement this with an EOF analysis to investigate the variability of the clusters and how it covaries with the jet and diabatic heating.

How to cite: Weijenborg, C. and Spengler, T.: Climatology and variability of cyclone clustering, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12147, https://doi.org/10.5194/egusphere-egu21-12147, 2021.

Adi Etkin et al.

The Red-Sea Trough (RST) is a lower-level trough extending from the tropical low-pressure to the Levant. Its annual occurrence is 20%, between October and May, producing mostly dry weather, but occasionally active and causing local showers and floods. During winter the dominant synoptic system over the Levant is the Cyprus low (CL). Previous studies showed that some CLs form within pre-existing RSTs, through a tropical-extratropical interaction.

This study is the first comprehensive climatological framework of such formation events, analyzing occurrence, seasonality and the resulting rainfall in Israel. The study looked at events of new CLs formed within the domain 31°-35°N, 30°-36°E while a RST was detected within 24 hours before the event. We used the 6-hourly ERA-Interim database, with 0.75°×0.75° resolution, during 1979-2017, and identified 104 formation events, which constitute 10% of the CLs. Most events occurred during fall and early winter, as the case for the RST. Eighty-four percent of them formed during the evening or the night, and almost two thirds of the CLs disappeared temporarily at noon and regenerated afterwards. This is attributed to the sea/land diurnal oscillation. Most of the CLs that formed were found shallow with little rain, but occasionally became major storms, like "Alexa", which caused extreme snowing in Jerusalem, in December 2013.

The evolution scenarios leading to formation events were divided into four clusters, according to the synoptic situation at the 500-hPa geopotential height. The first one is characterized by a closed cyclone approaching from the southwest, often connected to active RSTs, such as the event that occurred in 2-4 November 1994. In the second, a trough is deepening from the northern sector, possibly a polar intrusion, like the "Alexa" storm. In the third, the most populated cluster, a trough is approaching from the west. A separate cluster contains four events with no upper-level support.

How to cite: Etkin, A., Ziv, B., Saaroni, H., and Harpaz, T.: Formation of Cyprus Lows within Red-Sea Trough, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7002, https://doi.org/10.5194/egusphere-egu21-7002, 2021.

Christian Passow et al.

Scientific work on European windstorms mainly focused on local damages, location (tracks), temporal evolution or the overall severity, often measured by severity indices of di erent de nitions. Each of the aforementioned windstorm properties is directly related to important characteristics within the windstorm itself, such as wind speed, duration, spatial extent or internal variability. Variation or changes within these characteristics are therefore defining aspects in the spatial and temporal evolution of windstorm. As a step towards a better understanding of such variations, we classify windstorms based on these characteristics using Quasi-Supervised K-Means clustering, a novel procedure that was specifically developed by us to cluster windstorm tracks based on a reference windstorm catalog. One of the resulting clusters, containing 300 out of more than 2000 storm tracks over the North Atlantic and Europe, includes the tracks of the 20 most severe storm events according to the XWS catalog. This cluster is further
examined to identify common characteristics of the large scale situations that determine the cluster characteristics.

How to cite: Passow, C., Ulbrich, U., and Rust, H.: Feature-based classification of European windstorms, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16372, https://doi.org/10.5194/egusphere-egu21-16372, 2021.

Oliver Krueger et al.

Global atmospheric reanalyses are commonly applied for the validation of climate models, diagnostic studies, and driving higher resolution numerical models with the emphasis on assessing climate variability and long-term trends. Over recent years, longer reanalyses spanning a period of more than hundred years have become available. In this study, the variability and long-term trends of storm activity is assessed over the northeast Atlantic in modern centennial reanalysis datasets, namely ERA-20cm, ERA-20c, CERA-20c, and the 20CR-reanalysis suite with 20CRv3 being the most recent one. All reanalyses, except from ERA-20cm, assimilate surface pressure observations, whereby ERA-20C and CERA-20c additionally assimilate surface winds. For the assessment, the well-established storm index of higher annual percentiles of geostrophic wind speeds derived from pressure observations at sea level over a relatively densely monitored marine area is used.

The results indicate that the examined centennial reanalyses are not able to represent long-term trends of storm activity over the northeast Atlantic, particularly in the earlier years of the period examined when compared with the geostrophic wind index based on pressure observations. Moreover, the reanalyses show inconsistent long-term behaviour when compared with each other. Only in the latter half of the 20th century, the variability of reanalysed and observed storminess time series starts to agree with each other. Additionally, 20CRv3, the most recent centennial reanalysis examined, shows markedly improved results with increased uncertainty, albeit multidecadal storminess variability does not match observed values in earlier times before about 1920.

The behaviour shown by the centennial reanalyses are likely caused by the increasing number of assimilated observations, changes in the observational databases used, and the different underlying numerical model systems. Furthermore, the results derived from the ERA-20cm reanalysis that does not assimilate any pressure or wind observations suggests that the variability and uncertainty of storminess over the northeast Atlantic is high making it difficult to determine storm activity when numerical models are not bound by observations. The results of this study imply and reconfirm previous findings that the assessment of long-term storminess trends and variability in centennial reanalyses remains a rather delicate matter, at least for the northeast Atlantic region.

How to cite: Krueger, O., Feser, F., Kadow, C., and Weisse, R.: Northeast Atlantic storminess in centennial reanalyses, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6082, https://doi.org/10.5194/egusphere-egu21-6082, 2021.

Richard Blender et al.

Extratropical cyclones are the primary natural hazards affecting Europe. With the release of ERA5 reanalysis data from 1950-1978 by the European Centre for Medium-Range Weather Forecasts (ECMWF), new opportunities have arisen to investigate mid-latitude cyclones in terms of climatic features and trends in longer and higher resolution. We analyze cyclones by nearest neighbor search in 1000 hPa geopotential height minima in different high resolutions for different minimum life-times. We find an intensification of North Atlantic cyclones in 1950-2019. Short-lived cyclones grow in radius and depth. In the Mediterranean, however, long-lived cyclones have weakened; but traveled also further in 1950-2019. Additionally, we illustrate relations between cyclone tracks, radii and correlated weather and climate extremes.

How to cite: Blender, R., Karwat, A., and Franzke, C.: Trend analysis of extratropical cyclones in long-term ERA5 data series (1950-2019), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8703, https://doi.org/10.5194/egusphere-egu21-8703, 2021.

Terhi K. Laurila et al.

The knowledge of long-term climate and variability of near-surface wind speeds is essential and widely used among meteorologists, climate scientists and in industries such as wind energy and forestry. The new high-resolution ERA5 reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF) will likely be used as a reference in future climate projections and in many wind-related applications. Hence, it is important to know what is the mean climate and variability of wind speeds in ERA5.

We present the monthly 10-m wind speed climate and decadal variability in the North Atlantic and Europe during the 40-year period (1979-2018) based on ERA5. In addition, we examine temporal time series and possible trends in three locations: the central North Atlantic, Finland and Iberian Peninsula. Moreover, we investigate what are the physical reasons for the decadal changes in 10-m wind speeds.

The 40-year mean and the 98th percentile wind speeds show a distinct contrast between land and sea with the strongest winds over the ocean and a seasonal variation with the strongest winds during winter time. The winds have the highest values and variabilities associated with storm tracks and local wind phenomena such as the mistral. To investigate the extremeness of the winds, we defined an extreme find factor (EWF) which is the ratio between the 98th percentile and mean wind speeds. The EWF is higher in southern Europe than in northern Europe during all months. Mostly no statistically significant linear trends of 10-m wind speeds were found in the 40-year period in the three locations and the annual and decadal variability was large.

The windiest decade in northern Europe was the 1990s and in southern Europe the 1980s and 2010s. The decadal changes in 10-m wind speeds were largely explained by the position of the jet stream and storm tracks and the strength of the north-south pressure gradient over the North Atlantic. In addition, we investigated the correlation between the North Atlantic Oscillation (NAO) and the Atlantic Multi-decadal Oscillation (AMO) in the three locations. The NAO has a positive correlation in the central North Atlantic and Finland and a negative correlation in Iberian Peninsula. The AMO correlates moderately with the winds in the central North Atlantic but no correlation was found in Finland or the Iberian Peninsula. Overall, our study highlights that rather than just using long-term linear trends in wind speeds it is more informative to consider inter-annual or decadal variability.

How to cite: Laurila, T. K., Sinclair, V. A., and Gregow, H.: Near-surface wind speeds in ERA5: Climatology, decadal variability and long-term trends, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4472, https://doi.org/10.5194/egusphere-egu21-4472, 2021.

Lisa Degenhardt et al.

The seasonal forecast of extreme events is gaining more and more interest in science, for stakeholders and the general public. The most important extreme events with a seasonal variance for Europe, including the British Isles, are winter windstorms.

This study is investigating the prediction of seasonal accumulated storm frequency and intensity based on one state-of-the-art seasonal forecast model, the UK Met Office (GloSea5 GC2) and analyses the dynamical and physical reasons for skill.

Winter (DJF) windstorm events are individually identified and tracked using 10m wind speed once exceeding the local 98th percentile. The intensity of the season is calculated via an integrated measure based on the Storm Severity Index (Leckebusch et al., 2008).  Thus, the total seasonal intensity is investigated as grid cell accumulated index over all storm events and as storm count normalised sum. The forecast skill is assessed via different skill measures (e.g. Kendall-Correlation or RPSS) and validated in a hindcast approach with ERA5 for 23 seasons (1993-2015).

This presentation will give an overview about three main topic areas: the prediction skill for storm frequency and intensity; a multi-linear regression analysis to identify dominant large-scale modes, and finally, an outlook on first results on chosen dynamical parameters influencing the skill.

This investigation shows significant positive correlations over the British Isles for all three different storm parameters (frequency and both intensity measures). The positive skill pattern of the storm intensity is shifted north-west-wards compared to the positive skill in the storm frequency results. The accumulated intensity shows slightly higher correlations as the storm frequency. The normalised intensity reveals the lowest skills but still significant values downstream of the British Isles. Hence, three different storm parameters show positive prediction over UK; pure frequency, pure intensity and a combined measure of intensity and frequency.

Additionally to the model skill investigation, a regression analysis based on the three dominant teleconnection patterns over Europe (NAO, SCA and EA) was performed in order to gain better understanding in the connection of storms and these modes. This regression predicts the three storm parameters out of the given indices and explains up to 40-50% of variance. A statistical-model based approach of the storm parameters using three large-scale modes is showing improvements in skill compared to previous studies with NAO as only predictor. But the forecast model output shows still the best storm predictions.

Further studies will investigate the dynamical and physical reasons of the skill and their connections between the windstorm parameters, the dominant large-scale modes, and other atmospheric parameters.

How to cite: Degenhardt, L., Leckebusch, G. C., and Scaife, A. A.: Seasonal forecast skill of windstorm frequency and intensity over Europe and their dynamical and physical reasons, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12169, https://doi.org/10.5194/egusphere-egu21-12169, 2021.

Ilona Láng and Antti Mäkelä

Strong wind related to extratropical cyclones causes severe socioeconomic impacts every year in Europe. Especially in highly forested countries, such as Finland, the civil protection, insurance companies and energy sector are strongly affected by windstorms. Falling trees cause damage to the properties and transmission lines, interrupt the traffic and in the worst case can even cause fatalities. With better preparedness measures, such as highly developed early warning systems (EWSs), windstorm impacts can be reduced significantly.

For better preparedness and mitigation of storm impacts, it is essential to understand the windstorm and environmental features which contribute to the damages. Wind speed and gusts alone do not always explain why the windstorm is or is not causing disturbances in the society. To increase the understanding of the processes that lead to windstorm impacts, it is crucial to use additional data alongside the traditional meteorological data sources. There is high potential in combining wind impact data (e.g. electricity interruption records or emergency calls) with meteorological parameters to develop tools, for instance as a part of EWSs for crisis decision making. Such tools can help the civil protection or energy companies to prepare for the windstorm with sufficient human resources and other precautionary measures, which ultimately reduces impacts and increases the resilience of the society. Additionally, impact database can benefit the forecasters in their daily work with weather warnings or researchers with easier access to impact data. 

Impact database development has been done for instance on a national scale in SILVA project (2020-2021, Finnish National Emergency Supply Agency and Finnish Meteorological Institute) and on a pan-European scale in LODE project (2018-2021, the European Commission – DG ECHO). In this work we aim to share the lessons we learned in the impact data collection and processing, and the possibilities to connect the socioeconomic impacts with windstorms. We highlight especially how the quality and comprehensiveness of the impact data are the key factors in the development of wind impact tools. For example, to be able to identify significant trends in windstorm impacts, a sufficient temporal coverage and data homogeneity of the datasets are essential. The centralisation of the data collection is an additional important aspect: a centralised impact database maintained by one research organisation can be a solution to store and combine different types of impact data and connect it with the relevant meteorological or environmental data (e.g. forest or land use data).

How to cite: Láng, I. and Mäkelä, A.: Lessons learned from working with windstorm-related impact data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8288, https://doi.org/10.5194/egusphere-egu21-8288, 2021.

Tristan Perotin

Winter windstorms are one of the major natural hazards affecting Europe, potentially causing large damages. The study of windstorm risks is therefore particularly important for the insurance industry. Physical natural catastrophe models for the insurance industry appeared in the 1980s and enable a fine analysis of the risk by taking into account all of its components (hazard, vulnerability and exposure). One main aspect of this catastrophe modeling is the production and validation of extreme hazard scenarios. As observational weather data is very sparse before the 1980s, estimates of extreme windstorm risks are usually based on climate models, despite the limited resolution of these models. Even though this limitation can be partially corrected by statistical or dynamical downscaling and calibration techniques, new generations of climate models can bring new understanding of windstorm risks.

In that context, PRIMAVERA, a European Union Horizon2020 project, made available a windstorm event set based on 21 tier 1 (1950-2014) highresSST-present simulations of the High Resolution Model Intercomparison Project (HighResMIP) component of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). The events were identified with a storm tracking algorithm, footprints were defined for each event as maximum gusts over a 72 hour period, and the footprints were re-gridded to the ERA5 grid and calibrated with a quantile mapping correction method. The native resolution of these simulations ranges from 150km (typical resolution of the CMIP5 models) to 25km.

We have studied the applicability of the PRIMAVERA European windstorm event set for the modeling of European windstorm risks for the insurance sector. Preliminary results show that losses simulated from the event set appear to be consistent with historical data for all of the included simulations. The event set enables a better representation of attritional events and storm clustering than other existing event sets. An alternative calibration technique for extreme gusts and potential future developments of the event set will be proposed.

How to cite: Perotin, T.: Simulation of wind damages associated with the PRIMAVERA European windstorm event set, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8181, https://doi.org/10.5194/egusphere-egu21-8181, 2021.

Carolina Gramcianinov et al.

This work aims to assess the future projected changes in the cyclones originated in the South Atlantic, focusing on their genesis and intensifying mechanisms. The TRACK program was used to identify and track cyclones based on the relative vorticity from winds at 850 hPa. Spatial distribution maps of the atmospheric environment at the time of genesis were built using information sampled from individual features, e.g., mean upper-level jet speed, low-level moisture transport. First, we evaluated the HadGEM2-ES ability to reproduce the main characteristics of the South Atlantic cyclones and access their future projected changes using the RCP8.5 scenario. Then, we performed a dynamical downscaling using the WRF model to improve the resolution of the climate model in the historical (ExpHad-HIST) and RCP8.5 (ExpHad-RCP85) scenarios. Our results showed that HadGEM2-ES were able to reproduce the South Atlantic storm track pattern and its four main cyclogenesis regions: (1) Southern Brazilian coast (SE-BR, 30ºS); (2) Northern Argentina, Uruguay, and Southern Brazil (LA PLATA, 35ºS); (3) central coast of Argentina (ARG, 40ºS-55º) and; (4) Southeastern South Atlantic (SE-SAO, 55ºS and 10ºW). However, HadGEM-ES presented less intense cyclones and a negative density bias on the subtropical storm track, as a consequence of an underestimated genesis in the LA PLATA and SE-BR regions. The ExpHad-HIST provided a better representation of these two genesis regions, where the effects of an improved orography, mesoscale processes and strong and more organized low-level jet seem to reduce the static stability and support cyclone development. HadGEM2-ES RCP8.5 future projection showed a decrease of 10% in the number of cyclones over South Atlantic and a poleward shift of the main storm track, linked to the larger reduction of systems in mid than high latitudes. This increase in the cyclone activity at 30ºS led to the high track density in the South Atlantic subtropical storm track, both in the summer and winter. The ExpHad-RCP85 also showed a poleward shift of the main storm track, but mainly in the summer. The reduction and southward displacement of the cyclone occurrences can be addressed to the increase in the static stability at mid-latitudes. However, the increase in the moisture content at low levels seems to balance the effect of the static stability as long as there is an increase in the genesis in the equatorward genesis regions. In fact, the ExpHad-RCP85 simulated growth in the genesis in the northern edge of SE-BR (20ºS, 50ºW) and ARG (45ºS) regions, in the summer, and the LA PLATA region in the winter - being the last change also observed in HadGEM2-ES RCP8.5. The large increase in the low-level moisture and a strengthening of the equatorward flank of the upper-level jet could justify more genesis at these locations, competing with the increase in static stability. Moreover, the large content of low-level moisture available in the future simulation may also be connected to the observed intensification of the cyclones over the Uruguayan and Brazilian coast.

How to cite: Gramcianinov, C., de Camargo, R., and Silva Dias, P.: Driving mechanisms of South Atlantic storm track changes in an extreme climate scenario according to HadGEM2-ES and WRF downscaling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13128, https://doi.org/10.5194/egusphere-egu21-13128, 2021.

Alexander Vessey et al.

The Arctic has undergone significant change over the past few decades, and there has been great reductions in Arctic sea ice extent. The Arctic ocean has become more accessible, and this has allowed for more human activity in the Arctic.  The risk of storms impacting human activities in the Arctic has consequently increased, and as sea ice extent continues to decline in the near-future, the risk of storms impacting human activities in the Arctic is likely to increase further.  In this study, the present climatology of Arctic storms is evaluated between modern reanalysis datasets, and the future climatology of Arctic storms is also evaluated in climate model simulations.

There are multiple reanalysis datasets available from different institutions, which each give an approximation of past atmospheric conditions over the last few decades.  In addition, there are multiple storm tracking methods, which may impact the climatology of Arctic storms that is identified in a reanalysis datasets.  In this study, we aimed to improve the understanding of Arctic storms by assessing their characteristics in multiple global reanalyses, the ECMWF-Interim Reanalysis (ERA-Interim), the 55-Year Japanese Reanalysis (JRA-55), the NASA-Modern Era Retrospective Analysis for Research and Applications Version 2 (MERRA-2), and the NCEP-Climate Forecast System Reanalysis (NCEP-CFSR), using the same storm tracking method based on 850 hPa relative vorticity and mean sea level pressure.  In addition, the response of Arctic storms to climate change has been evaluated in the UPSCALE climate simulations, and the affect of horizontal resolution on the representation of future Arctic storminess has been assessed.

The results show that there are no significant trends in Arctic storm characteristics between 1980-2017, even though the Arctic has undergone rapid change.  Although some similar Arctic storm characteristics are found between the reanalysis datasets, there are generally higher differences between the reanalyses in winter (DJF) than in summer (JJA).  In addition, substantial differences can arise between using the same storm tracking method based on 850 hPa relative vorticity or mean sea level pressure, which adds to the uncertainty associated with current Arctic storm characteristics.

The results also show that Arctic storms will change significantly in a future climate, particularly in their spatial distribution.  Differences have been found between the future simulations of Arctic storms between an ensemble of high resolution climate models (25km) and low resolution climate models (130km), which adds uncertainty to how Arctic storms may change in a future climate.  The possible reasons for why the representation of future climate Arctic storms may be different in climate models of differing horizontal resolution has been explored.

How to cite: Vessey, A., Hodges, K., Shaffrey, L., and Day, J.: Quantifying Arctic Storm Risk in a Changing Climate, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16077, https://doi.org/10.5194/egusphere-egu21-16077, 2021.

Nicole Albern et al.

During boreal winter (December to February, DJF), the North Atlantic jet stream and storm track are expected to extend eastward over Europe in response to climate change. This will affect future weather and climate over Europe, for example by steering storms which are associated with strong winds and heavy precipitation towards Europe. The jet stream and storm track responses over Europe are robust across coupled climate models of phases 3, 5, and 6 of the Coupled Model Intercomparison Project (CMIP; Harvey et al., 2020, JGR-A, https://doi.org/10.1029/2020JD032701). We show that the jet stream response is further robust across CMIP5 models of varying complexity ranging from coupled climate models to atmosphere-only General Circulation Models (GCMs) with prescribed sea-surface temperatures (SSTs) and sea-ice cover. In contrast to the jet stream response over Europe, the jet stream response over the North Atlantic is not robust in the coupled climate models and the atmosphere-only GCMs.

In addition to the CMIP5 simulations, we investigate Amip-like simulations with the atmospheric components of ICON-NWP, and the CMIP5 models MPI-ESM-LR and IPSL-CM5A-LR that apply the cloud-locking method to break the cloud-radiation-circulation coupling and to diagnose the contribution of cloud-radiative changes on the jet stream response to climate change. In the simulations, SSTs are prescribed to isolate the impact of cloud-radiative changes via the atmospheric pathway, i.e., via changes in atmospheric cloud-radiative heating, and global warming is mimicked by a uniform 4K SST increase (cf. Albern et al., 2019, JAMES, https://doi.org/10.1029/2018MS001592 and Voigt et al., 2019, J. Climate, https://doi.org/10.1175/JCLI-D-18-0810.1). In all three models, cloud-radiative changes contribute significantly and robustly to the eastward extension of the North Atlantic jet stream towards Europe. At the same time, cloud-radiative changes contribute to the model uncertainty over the North Atlantic. In addition to the jet stream response, we investigate the impact of cloud-radiative changes on the storm track response in ICON-NWP and discuss similarities and differences between the jet stream and storm track responses over the North Atlantic-European region.

In ICON-NWP, the impact of cloud-radiative changes on the jet stream response is dominated by tropical cloud-radiative changes while midlatitude and polar cloud-radiative changes have a minor impact. A further division of the tropics into four smaller tropical regions that cover the western tropical Pacific, the eastern tropical Pacific, the tropical Atlantic, and the Indian Ocean shows that cloud-radiative changes over the western tropical Pacific, eastern tropical Pacific, and Indian Ocean all contribute about equally to the eastward extension of the North Atlantic jet stream towards Europe because these regions exhibit substantial upper-tropospheric cloud-radiative heating in response to climate change. At the same time, cloud-radiative changes over the tropical Atlantic hardly contribute to the jet response over Europe because changes in atmospheric cloud-radiative heating under climate change are small in this region. As for the impact of global cloud-radiative changes, we also discuss the impact of the regional cloud-radiative changes on the storm track response over the North Atlantic-European region to climate change.

How to cite: Albern, N., Voigt, A., and Pinto, J. G.: Tropical cloud-radiative changes contribute to robust DJF jet exit strengthening over Europe under global warming, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3026, https://doi.org/10.5194/egusphere-egu21-3026, 2021.

Jennifer Catto et al.

Extratropical cyclones that impact Europe are significant natural hazards that can cause severe damages and lead to large socio-economic losses. There are large uncertainties associated with changes in storm number, and storm intensity, in future climate projections. Here we use a Lagrangian tracking algorithm applied to reanalysis data and to historical and two future scenario simulations of a number of CMIP6 models to investigate future changes in characteristics of windstorms over Europe. As well as storm frequencies and peak wind speeds, we also quantify changes in two versions of a storm severity index (SSI), one of which is population weighted. These metrics are calculated using the footprints of cyclones as they pass over the European continent.

The models show differing abilities to represent the historical SSI compared to ERA5. Future changes in SSI are somewhat uncertain, but tend to show an increase in severities over central and northwest Europe, and a decrease over lower latitudes and the Mediterranean, with responses tending to be larger for the more extreme climate change scenario. These changes are associated with the changes in the extreme winds over land.

By considering the parameters of population density and wind intensity threshold we could explore the relevance of future socio-economic and adaptive changes on the windstorm impacts. For the population-weighted SSI, smaller increases are found in the future cases where population densities do not change and/or adaptation to increases in extreme wind speed climatologies occur.

How to cite: Catto, J., Little, A., and Priestley, M.: Future changes in European windstorm severities and impacts , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15321, https://doi.org/10.5194/egusphere-egu21-15321, 2021.

Edgar Dolores Tesillos et al.

Strong low-level winds are among the most impactful effects of extratropical cyclones on society.  The wind intensity and the spatial distribution of wind maxima may change in a warming climate, however, the dynamics involved are not clear. Here, structural and dynamical changes of North Atlantic cyclones in a warmer climate close to the end of the current century are investigated with storm-relative composites based on Community Earth System Model Large Ensemble (CESM-LE) simulations. Furthermore, a piecewise potential vorticity inversion is applied, to attribute such changes in low-level winds to changes in PV anomalies at different levels.

We identify an extended wind footprint southeast of the cyclone centre, where the wind speed tends to intensify in a warmer climate. Both an amplified low-level PV anomaly driven by enhanced diabatic heating and a dipole change in upper-level PV anomalies contribute to this wind intensification. On the contrary, wind changes associated with lower- and upper-level PV anomalies mostly compensate each other upstream of the cyclone center. Wind changes at upper levels are dominated by changes in upper-level PV anomalies and the background flow. All together, our results indicate that a complex interation of enhanced diabatic heating and altered upper-tropospheric wave dynamics shape future changes in near-surface winds in North Atlantic cyclones.

How to cite: Dolores Tesillos, E., Pfahl, S., and Teubler, F.: Future changes in North Atlantic winter cyclones in CESM-LENS: cyclone intensity and horizontal wind speed, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8926, https://doi.org/10.5194/egusphere-egu21-8926, 2021.

Matthew Priestley and Jennifer Catto

Extratropical cyclones have the potential to cause large damages across the mid-latitudes. Future climate change is projected to have a large impact on the location of the storm tracks, and the frequency of these cyclones, however the sign and magnitude of these responses has been uncertain for regions near the end of the storm tracks in previous coupled and idealized modelling studies.


Through the use of a Lagrangian cyclone tracking method we quantify changes in the storm tracks for both summer and winter seasons in both hemispheres for four future climate scenarios using a number of CMIP6 models. A cyclone compositing technique is employed to identify changes in cyclone circulation for the strongest cyclones in the lower, middle, and upper troposphere. We identify an intensification of the cyclone circulation in all seasons, apart from NH summer, where a weakening is detected. Cyclone size is also projected to increase, with a widening of the pressure and wind fields.


These results have significant implications from a socio-economic perspective. Despite a projected decrease in cyclone numbers, an increase in severity may lead to more drastic windstorms and larger impacts across heavily populated regions of the mid-latitudes.

How to cite: Priestley, M. and Catto, J.: Changes in cyclone circulation and storm tracks under different future climate scenarios, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15423, https://doi.org/10.5194/egusphere-egu21-15423, 2021.

Mareike Schuster and Uwe Ulbrich

Windstorms are considered the most devastating natural peril in many regions around the globe. For insurance associations in Europe for example, the damages generated by windstorms make up to about 90% of the claims in the category of natural hazards. The interannual variability of windstorms can be quite strong and thus research has recently focused on this topic.

However, storm risk and its changes under anthropogenically induced climate change are so far rather little discussed in literature. Thus, there are still large uncertainties about how climate change will affect the extratropical circulation. CMIP5 models showed at times opposing signals regarding number and strength of windstorm generating cyclones and storm tracks. In more detail, the latest IPCC AR5 states that substantial uncertainty and low confidence remains in projecting changes in NH storm tracks, especially for the North Atlantic basin.

With the lately released CMIP6 simulations, providing model output of increased spatial and temporal resolution, there is potential for new insights and enhanced confidence regarding future trends of storminess.

In our study, we assess characteristics and trends of windstorm diagnostics in an ensemble of the latest CMIP6 climate scenario simulations, with a focus to the North Atlantic basin and winterstorms affecting Europe.

In the CMIP6 model ensemble the trends of winter windstorm frequencies appear to be overall weaker in an anthropogenically changed climate than in the preceding CMIP5 scenarios; yet, first results indicate that they are somewhat more consistent amongst models. All CMIP6 models exhibit a windstorm frequency increase locally confined over the Arctic, while in the mid and high latitudes a wide-ranging decrease of windstorm activity is simulated. In our study we will also assess what this entails for characteristics like life time, intensity and size.

How to cite: Schuster, M. and Ulbrich, U.: Trends and characteristics of winter storms in CMIP6, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15487, https://doi.org/10.5194/egusphere-egu21-15487, 2021.

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