AS1.11

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 CL3.2/NH1
Convener: Gregor C. Leckebusch | Co-conveners: Jennifer Catto, Joaquim G. Pinto, Uwe Ulbrich
Presentations
| Tue, 24 May, 15:10–18:30 (CEST)
 
Room M2

Presentations: Tue, 24 May | Room M2

Chairpersons: Gregor C. Leckebusch, Jennifer Catto, Uwe Ulbrich
15:10–15:20
|
EGU22-6508
|
ECS
|
solicited
|
On-site presentation
Chris Weijenborg and Thomas Spengler

Cyclone clustering, the succession of multiple extratropical cyclones during a short period of time, has a huge impact on European weather extremes. The idea that several cyclones follow a similar track already dates back to the concept of cyclone families of Bjerknes and Solberg. To investigate the dynamical causes of cyclone clustering, one needs to diagnose where cyclone clustering occurs and determine their characteristics. So far most diagnostics either focused on either local impact-based diagnostics or on a statistical analysis of storm recurrence. While the first cannot be applied globally, the latter is difficult to relate to individual events. We therefore present a new way to globally detect cyclone clustering that is closer to the original concept of Bjerknes and Solberg that extratropical cyclones follow similar tracks.

Using this new cyclone clustering diagnostic based on spatio-temporal distance between cyclone tracks, we analyse cyclone clustering globally in Era-Interim 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 role of diabatic and adiabatic effects associated with extra-tropical cyclones in maintaining baroclinicity are assessed. We find that cyclone clustering mainly occurs along the climatological storm tracks. In general, clustered cyclones are stronger than non-clustered cyclones. Moreover clustered cyclones are more often related to atmospheric rivers and stronger isentropic slope, indicating that diabatic effects might be an important mechanism in the formation of cyclone clustering. 

How to cite: Weijenborg, C. and Spengler, T.: Global climatalogy of cyclone clustering, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6508, https://doi.org/10.5194/egusphere-egu22-6508, 2022.

15:20–15:26
|
EGU22-3998
|
Virtual presentation
Helen Dacre and Joaquim Pinto

The weather conditions in the mid-latitudes are largely determined by the absence or presence of extratropical cyclones. Frequent passage of cyclones over the same location in quick succession (serial clustering) can lead to accumulated impacts such as flooding and wind damage. These impacts have motivated a wide variety of research studies into serial cyclone clustering.  However, the different definitions, metrics and datasets used in this research makes comparison of results difficult.  The aim of this study is to review the previous research and provide clear a framework for serial cyclone clustering into which past and future studies can be placed, allowing easier comparison of results irrespective of the research direction.

 

We find that several climatologies of serial cyclone clustering agree as to where clustering occurs preferentially, but these studies are largely limited to the North Atlantic. Future projections of cyclone clustering are highly uncertain.  This is largely due to sample uncertainty, caused by short timeseries, and poor representation of key processes such as Rossby wave breaking, caused by low spatial resolution. Research investigating the dynamical mechanisms determining when and why serial cyclone clustering occurs have shown that clustering is linked to the position of the jet stream and the occurrence of Rossby wave breaking.  Studies have investigated this link for different aggregation timescales. On daily timescales cyclone clustering is related to jet streaks and families of cyclones forming on the same frontal feature. On seasonal timescales active seasons are often associate with persistent large-scale flow patterns and successive Rossby wave breaking events. Current knowledge gaps and future research directions are identified.

How to cite: Dacre, H. and Pinto, J.: Where, when and why do extratropical cyclones cluster?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3998, https://doi.org/10.5194/egusphere-egu22-3998, 2022.

15:26–15:32
|
EGU22-2456
|
ECS
|
Virtual presentation
Elliott Sainsbury, Reinhard Schiemann, Kevin Hodges, Alexander Baker, Len Shaffrey, and Kieran Bhatia

Post-tropical cyclones (PTCs) are often associated with high winds and extreme precipitation over Europe. For example, ex-hurricanes Debbie (1961) and Ophelia (2017) were both responsible for national wind speed records in Ireland, and further east across Europe, ex-hurricane Debby (1982) caused significant wind damage over Finland. In previous work, we show that despite comprising only 1% of European impacting cyclones during hurricane season, almost 10% of those cyclones with storm force (>25ms-1) are PTCs, indicating that PTCs are disproportionately responsible for European windstorm risk.

By tracking and identifying observed TCs in two reanalyses, we explore the physical drivers for recurving TCs impacting Europe. Our methods of cyclone tracking and TC identification allow for a detailed analysis of the post-tropical stage of the TCs in the observational record, allowing us to separate the recurving TCs based on whether they impact Europe.

Using a composite analysis, we show that recurving TCs which impact Europe are significantly stronger at their lifetime maximum intensity, and for several days during and after extratropical transition. They are also 65% more likely to reintensify in the midlatitudes after completing extratropical transition. The Europe impacting recurving TCs interact more favourably with an upstream upper-level trough, which steers the TCs on a more poleward trajectory across a midlatitude jet streak. It is during the jet streak interaction that extratropical reintensification often occurs.

We show that TC lifetime maximum intensity and whether extratropical reintensification occurs both modulate the likelihood that a recurving TC will impact Europe as a PTC. Our results highlight the challenges of projecting PTC impacts over Europe in a future climate. Some climate model projections indicate a poleward shift in the jet, possibly indicating less opportunity for recurving TCs to interact with the jet and reintensify. However, sea surface temperatures are projected to warm, and lifetime maximum intensity may therefore increase. If the change in TC intensity outweighs any poleward shift in the jet, then a larger proportion of recurving TCs could reach Europe in the future.

How to cite: Sainsbury, E., Schiemann, R., Hodges, K., Baker, A., Shaffrey, L., and Bhatia, K.: Why do some Recurving Tropical Cyclones Impact Europe as Post-Tropical Cyclones?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2456, https://doi.org/10.5194/egusphere-egu22-2456, 2022.

15:32–15:38
|
EGU22-3589
|
ECS
Katharina Heitmann, Hanin Binder, Michael Sprenger, Heini Wernli, and Hanna Joos

The warm conveyor belt (WCB) transports moist air from low levels in the warm sector of an extra-tropical cyclone (ETC) as a coherently ascending airstream to the upper troposphere. WCBs are associated with an elongated cloud band and precipitation and were found to be responsible for 40-60% of the total precipitation in the midlatitude. Furthermore, the release of latent heat during cloud formation has the potential to modify potential vorticity (PV) below and above the level of maximum heating. Due to the modification of PV, WCBs can affect the synoptic-scale flow, e.g., by disturbing the jet stream on triggering Rossby waves in the upper troposphere, as well as the intensification of ETCs.

While the occurrence of WCBs has been studied from a climatological viewpoint before, the spatial distribution and temporal evolution of WCB characteristics and impacts, as well as the link between them, remain largely unknown. Therefore, we developed a novel method to quantify a set of WCB metrics that describe its characteristics (intensity, ascent rate, curvature, moisture content, position, and age relative to the cyclone evolution) and impacts (PV modification at low and upper levels, precipitation rate and volume). In addition, we considered the metric evolution along the whole lifecycle of the WCB. Applying this method in a case study, the WCB reached maximum intensity and ascent rate during the cyclone’s strongest intensification. In terms of impacts, maximum precipitation rates decreased over the lifetime of the WCB, while maximum PV values at lower levels increased. We then extended the analysis to the 40-year time span 1980 - 2020 covered by ECMWF’s most recent reanalysis ERA5, by calculating WCB trajectories globally for the entire period. Thereby, we were able to identify from a climatological viewpoint for the first time: (i) the global spatial distribution of WCB characteristics and impacts; (ii) the link between them; and (iii) their distinct lifecycle. This analysis showed that the characteristics and impacts of WCBs differ between different regions and seasons while the link between them remains largely constant. For instance, in the North Atlantic, we found two regions of enhanced WCB intensity which are also linked with enhanced precipitation volume. While the precipitation volume correlates strongly with the WCB intensity, the highest precipitation rates are associated with the most rapidly ascending WCBs. On a global scale, WCB-related low-level PV depends mainly on latitude, however, if restricted to a latitudinal band, inflow moisture becomes important.

How to cite: Heitmann, K., Binder, H., Sprenger, M., Wernli, H., and Joos, H.: WCB characteristics and impacts and how they are interrelated in ERA5, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3589, https://doi.org/10.5194/egusphere-egu22-3589, 2022.

15:38–15:44
|
EGU22-4144
|
ECS
|
On-site presentation
Behrooz Keshtgar, Aiko Voigt, Corinna Hoose, Michael Riemer, and Bernhard Mayer

Extratropical cyclones drive midlatitude weather, including extreme events, and determine midlatitude climate. Their dynamics and predictability are strongly shaped by cloud diabatic processes. While the cloud impact due to latent heating is well known and much studied, little is known about the impact of cloud radiative heating (CRH) on the dynamics and predictability of extratropical cyclones. Here, we address this question by means of baroclinic life cycle simulations performed at a convection-permitting resolution of 2.5 km with the ICON model. The simulations use a newly implemented channel setup with periodic boundary conditions in the zonal direction. Moreover, they apply a new modeling technique for which only CRH interacts with the cyclone, which circumvents changes in the mean state due to clear-sky radiative cooling. To understand the CRH impact on the upper-tropospheric circulation, we diagnose sources and the evolution of differences in potential vorticity (PV) between a simulation with and without CRH.

We find that CRH increases the intensity of the cyclone with the impact being more prominent at upper levels. The mechanism by which CRH affects the cyclone operates mostly via a modification of other diabatic processes, in particular an intensification of the latent heating associated with cloud microphysical processes. This changes PV tendencies, and these changes are then advected by the upper-tropospheric divergent flow to the tropopause region, where the large-scale rotational flow further changes the tropopause structure.

Our results indicate that although CRH is comparably small in magnitude, it can affect extratropical cyclones by changing cloud microphysical heating and subsequently the large-scale flow similar to a previously identified multi-stage upscale error growth mechanism. Our results further indicate that CRH can impact the predictability of the cyclones. This impact may be especially important in storm-resolving models, for which simplified radiative transfer calculations might bias CRH. 

How to cite: Keshtgar, B., Voigt, A., Hoose, C., Riemer, M., and Mayer, B.: Cloud radiative impact on the dynamics and predictability of an idealized extratropical cyclone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4144, https://doi.org/10.5194/egusphere-egu22-4144, 2022.

15:44–15:50
|
EGU22-4479
|
ECS
|
On-site presentation
Lea Eisenstein, Benedikt Schulz, Peter Knippertz, and Joaquim G. Pinto

Strong winds associated with extratropical cyclones are one of the most dangerous natural hazards in Europe. These high winds are mostly connected with four mesoscale dynamical features: the warm (conveyor belt) jet (WJ), the cold (conveyor belt) jet (CJ), (post) 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 listed above, based on a probabilistic random forest using each feature’s 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 errors specific to individual features, and to ultimately improve forecasts of high wind events through feature-dependent statistical post-processing. To achieve this, we strive to identify the features in irregularly spaced surface observations and in gridded analyses and forecasts in a consistent way.

To train the probabilistic random forest, we subjectively identify the four storm features – as well as high cold sector winds – in ten winterstorm cases between 2017 and 2020 in both hourly surface observations and high-resolution reanalyses of the German COSMO model over Europe, using an interactive data analysis and visualisation tool. Results show that mean sea-level pressure (tendency), potential temperature, precipitation amount and wind direction are most important for the distinction between the features. From the random forest we get probabilities of each feature occurring at the single stations, which can be interpolated into areal information using kriging. While the observational data are limited to surface measurements, the gridded data includes further useful parameters and the possibility to consider vertical structures.

The results show a good identification of CJ, CFC and WJ, while a distinction between SJ and CJ is difficult using surface observations alone, such that the two features are considered together at this stage. A climatology is currently being compiled for both surface observations and the reanalyses over a period of around 20 years using the respective trained probabilistic random forests and further for high-resolution COSMO ensemble forecasts, for which we want to analyse forecast errors and develop feature-dependent postprocessing procedures.

How to cite: Eisenstein, L., Schulz, B., Knippertz, P., and Pinto, J. G.: Extratropical high-wind feature identification using a probabilistic random forest, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4479, https://doi.org/10.5194/egusphere-egu22-4479, 2022.

15:50–15:56
|
EGU22-5231
|
ECS
Or Hadas, Joaquin Blanco, George Datseris, Sandrine Bony, Bjorn Stevens, Rodrigo Caballero, and Yohai Kaspi
Atmospheric albedo is one of the most influential properties of Earth's climate. Specifically, the midlatitude planetary albedo plays a vital role in shaping the Earth's albedo. Although, there is no one theory to connect midlatitude atmospheric albedo to the midlatitude climate. This study investigates the connection between baroclinic activity, which dominates the midlatitude climate, and cloud cover. We show that EKE and atmospheric albedo are highly correlated on the climatological level. Then, we show that, from a Lagrangian perspective, the positive correlation translates into a high correlation between cyclone and anticyclone strength and cloud cover at all levels. Observing the strength-cloud cover relation across various systems strengths, we see that this coupling is robust and saturates for intense cyclones. Using these insights, we reflect on two aspects of the Earth radiation budget: the Earth hemispheric symmetry in planetary albedo and future changes in Earth atmospheric albedo. Observing the relationship between the storms, mean cloudiness, strength, and spatial distribution, we find that the difference in eddy population between hemispheres can explain the difference in cloud-cover, which counter-balance the higher surface albedo at the NH. Finally, we use the relation between baroclinic activity and midlatitude cloudiness to understand the projected change in cloud patterns in a warmer climate. We show a high correlation between climatological baroclinic activity response and cloud response. We also suggest that the discrepancy between baroclinic activity and clouds response over the SH is due to the saturating nature of the strength-cloudiness curve.

How to cite: Hadas, O., Blanco, J., Datseris, G., Bony, S., Stevens, B., Caballero, R., and Kaspi, Y.: The role of baroclinic activity in shaping Earth's albedo in present and future climates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5231, https://doi.org/10.5194/egusphere-egu22-5231, 2022.

15:56–16:02
|
EGU22-5305
|
ECS
|
On-site presentation
Emanuele Gentile and Suzanne L. Gray

Located near the end of the North Atlantic storm track, the UK’s surrounding seas are characterised by a highly variable wind climate, making prediction of wind speeds challenging at all time scales. While wind speed trends over the UK’s land and seas have been the focus of several studies of the literature in the past 20 years, the question of what is the current systematic link between observed extreme wind speeds (and gusts) over these seas and distinct sub-synoptic features of midlatitude cyclones is, to date, unanswered.  To address this question, we have performed a 10-year climatological analysis of the observed extreme wind speeds and gusts, presenting the distribution of extremes and the prevailing wind direction, along with an analysis of their inter- and intra-annual variability. We find that between the 70 and 85% of the observed top 1% extreme wind and gust events recorded at each network site are within 1000 km of the centre of a cyclone (tracked in the ERA5 reanalysis), and that an even higher proportion of the top 0.1% of the wind and gust events is associated with a cyclone centre (between 80 and 100% depending on the site).  We then determine at each site whether the warm or cold conveyor belt flows are more likely to lead to extreme wind or gust events. Combining the observed extreme winds and gusts data with reanalysis significant wave heights, we further discuss the relationship between extreme winds and extreme ocean wave heights, and consider the relevance of the results to the safety and the smooth running of the operations of the wind energy and oil and gas industries in the UK’s surrounding seas. 

How to cite: Gentile, E. and Gray, S. L.: Midlatitude cyclone features associated with extreme winds and gusts in the seas surrounding the UK, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5305, https://doi.org/10.5194/egusphere-egu22-5305, 2022.

16:02–16:08
|
EGU22-5426
|
ECS
Noy Klaider and Shira Raveh-Rubin

Extreme cold weather events cause major damage to industry, agriculture and human health. While regional extremes are often associated with different large-scale atmospheric circulation anomalies, it is yet unclear which mechanisms and weather systems are relevant on a global scale, i.e., across regions. This study aims to identify the large-scale processes leading to extreme cold events from a global climatological perspective, and specifically quantify the non-local contribution of midlatitude weather systems using a Lagrangian approach. 

Here, we objectively identify anomalously cold extremes by applying local percentile-based thresholds of 2-m temperature in ERA5 reanalysis. We further track air parcel trajectories of dry, cold intrusions occurring in the wake of extratropical troughs and cyclones, previously shown to induce cold anomalies following cold frontal passages. We find a strong association between cold extremes and dry intrusions, reaching 45% of cold extremes in the midlatitudes, despite the intrusions’ natural occurrence frequency of only 12% in those areas. Using clustering methods, additional atmospheric precursors to cyclones producing cold extremes are highlighted. The identification of mechanisms governing the predictability of cold extremes, on a global scale, is key for societal preparedness.  

 

How to cite: Klaider, N. and Raveh-Rubin, S.: Extreme cold events: global climatology and relation to cyclones , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5426, https://doi.org/10.5194/egusphere-egu22-5426, 2022.

16:08–16:14
|
EGU22-5585
|
On-site presentation
Aiko Voigt, Klara Butz, and Behrooz Keshtgar

Extratropical cyclones are the main driver of everyday weather in the midlatitudes. These cyclones are known to be affected by latent heating and are a popular subject of research regarding possible changes in a warming climate. In contrast, the role of radiation - and especially the radiative impact of clouds - in shaping extratropical cyclones has hardly been investigated. To study how cloud-radiative heating of the atmosphere might impact cyclones, we present idealized baroclinic life cycle simulations with the global atmosphere model ICON-NWP in aquaplanet setup with prescribed sea surface temperatures. Several simulation setups are used to isolate not only the overall cloud-radiative impact but also the impacts of low-level clouds and high-level clouds. Moreover, the cloud-radiative impact is compared between two model versions, ICON 2.1 and ICON 2.6. While the model versions simulate similar cyclones when radiation is not taken into account, enabling cloud-radiation interaction leads to contradicting effects.In ICON 2.1 clouds lead to a weakening of the cyclone magnitude by 15%, whereas in ICON 2.6 they strengthen the cyclone by 7%. The different cloud impact results from a robust competition between the radiative impact of low-level clouds, which in both model versions weaken the cyclone, and high-level clouds, which in both model versions strengthen the cyclone. The difference in the overall cloud-radiative impact between the two model versions results from the fact that ICON 2.1 simulates much more low-level clouds than ICON 2.6. This shows that the vertical distribution of clouds and their radiative heating can be an important factor for the dynamics of extratropical cyclones. 

How to cite: Voigt, A., Butz, K., and Keshtgar, B.: Competing radiative impacts of low-level and high-level clouds on the strength of an idealized extratropical cyclone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5585, https://doi.org/10.5194/egusphere-egu22-5585, 2022.

16:14–16:20
|
EGU22-5592
|
On-site presentation
Johannes Lutzmann, Clemens Spensberger, and Thomas Spengler

The release of latent heat on the warm side of trailing cold fronts can leave elevated levels of baroclinicity. This can lead to one or multiple secondary cyclones forming in the wake of the parent cyclone, intensifying moisture advection and latent heating. Although this mechanism has been demonstrated in case studies, we still lack a consistent global mapping of the evolution of fronts and associated diabatic processes. We develop a novel algorithm to both detect fronts in global weather and climate datasets and track them in time. We utilise a watershed algorithm to identify individual fronts as volumes in the four-dimensional domain of space and time. We apply this algorithm to equivalent potential-temperature fields from the ERA5 reanalysis on three pressure levels in the lower to middle troposphere to compile a global climatology of frontal lifecycles. We then categorise these lifecycles with respect to their characteristics as well as dynamic and thermodynamic properties. Furthermore, the intensification mechanisms are explored, in particular with respect to latent heating.

How to cite: Lutzmann, J., Spensberger, C., and Spengler, T.: Frontal Life Cycles – Detection and Climatology, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5592, https://doi.org/10.5194/egusphere-egu22-5592, 2022.

16:20–16:26
|
EGU22-9324
|
Virtual presentation
Camille Li, Erica Madonna, Gabriel Hes, Clio Michel, and Peter Y.F. Siew

Extratropical cyclones are key players in the poleward transport of moisture and heat. This study investigates wintertime cyclone variability to better understand the large-scale controls on their frequency, path and impacts at higher latitudes. One of the main corridors for Arctic-bound cyclones is through the North Atlantic to the Barents Sea, a region that has experienced the greatest retreat of winter sea ice during the past decades. Large-scale atmospheric conditions are found to be decisive, with the strongest surface warming from cyclones originating south of 60N in the North Atlantic and steered northeastward by the upper-level flow. Atmospheric conditions also control cyclone variability in the Arctic proper: months with many cyclones are characterized by an absence of high-latitude blocking and enhanced local baroclinicity, due to the presence of strong upper-level winds and a southwest-northeast tilted jet stream more than changes in sea ice. Due to the large interannual variability in the number of Arctic-bound cyclones, no robust trends are observed over the last 40 years. Our results highlight the importance of accounting for internal variability of the large-scale atmospheric circulation in studies of long-term changes in extratropical cyclones.

How to cite: Li, C., Madonna, E., Hes, G., Michel, C., and Siew, P. Y. F.: Control of North Atlantic cyclone variability and impacts by the large-scale atmospheric flow, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9324, https://doi.org/10.5194/egusphere-egu22-9324, 2022.

16:26–16:32
|
EGU22-5816
Michelle Reboita, Rosmeri da Rocha, Natália Crespo, Luiz Gozzo, Maria Custódio, Vinicius Lucyrio, and Eduardo de Jesus

In June 2021, an unusual cyclone developed near the boundary of Uruguay and southern Brazil. It initially had extratropical characteristics, later acquired features of a Shapiro-Keyser extratropical cyclone and then underwent a subtropical transition. When the subtropical system reached Brazilian water (1200 UTC 29 June 2021), the local Navy named the cyclone “Raoni”. The aim of this study is to describe the main drivers that made the cyclone develop features of a Shapiro-Keyser extratropical cyclone. Cyclogenesis was registered at 1800 UTC 26 June, forced by a trough at mid-upper levels that crossed the Andes and caused surface pressure deepening. Less than 24-hours later, the cyclone evolved following the Shapiro-Keyser development model, presenting a frontal T-bone pattern and warm seclusion. Sensitivity numerical experiments carried out with two regional models (Regional Climate Model - RegCM and Weather Research Forecasting Model - WRF) driven by ERA5 reanalysis indicate that the suppression of the surface sensible and latent heat fluxes produces a weaker extratropical cyclone without warm seclusion. Hence, surface heat fluxes seem to be the main driver to the warm seclusion development.

How to cite: Reboita, M., da Rocha, R., Crespo, N., Gozzo, L., Custódio, M., Lucyrio, V., and de Jesus, E.: The role of surface heat fluxes on development of warm seclusion favouring subtropical cyclone Raoni transition over the Southwestern Atlantic Ocean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5816, https://doi.org/10.5194/egusphere-egu22-5816, 2022.

16:32–16:40
Coffee break
Chairpersons: Uwe Ulbrich, Joaquim G. Pinto, Gregor C. Leckebusch
17:00–17:06
|
EGU22-2599
|
On-site presentation
Andreas Trojand, Nico Becker, and Henning Rust

Severe winter storms are one of the most damaging natural hazards for European residential buildings. Previous studies mainly focused on the loss ratio (loss value/total insured sum) as a monetary value for damages (e.g. Prahl et al. 2012; Pardowitz et al. 2016). In this study the focus is on the claim ratio (number of insured claims/number of contracts), which is derived from a storm loss dataset provided by the German Insurance Association. In a first step, loss ratios and claim ratios in German administrative districts are compared to investigate differences and similarities between the two variables. While there is no significant change in the ratio between claim ratio and loss ratio with increasing wind speeds, a tendency for lower loss ratios in urban areas can be confirmed. In a second step, a generalized linear model for daily claim ratios is developed using daily maximum wind gust (ERA5) and different non-meteorological indicators for vulnerability and exposure as predictor variables. The non-meteorological predictors are derived from the Census 2011. They include information about the district-average construction years, the number of apartments per buildings and others to get a better understanding of these factors concerning the number of buildings affected by windstorms. The modeling procedure is divided into two steps. First, a logistic regression model is used to model the probabilty of storm damage occurence. Second, generalized linear models with different link functions are compared regarding their ability to predict claim ratios in case a storm damage occured. In a cross-validation setting a criteria for model selection is implemented and the models of both steps are verified. Both steps show an improvement over the climatological forecast.

How to cite: Trojand, A., Becker, N., and Rust, H.: Impacts of winter storms on residential building damage - Modeling claim ratio considering parameters of vulnerability and exposure , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2599, https://doi.org/10.5194/egusphere-egu22-2599, 2022.

17:06–17:12
|
EGU22-5904
|
ECS
|
On-site presentation
Kjersti Konstali, Asgeir Sorteberg, Clemens Spensberger, Chris Weijenborg, Johannes Lutzmann, and Thomas Spengler

Precipitation has increased globally in the mean during the past century and is expected to continue to increase with rising temperatures. In the mid- to high latitudes, extratropical cyclones, fronts, atmospheric rivers, and cold air outbreaks are associated with a substantial fraction of the total precipitation. As these weather features might respond differently to a changing climate, investigating precipitation changes in the context of weather systems provides further insight into the observed changes in precipitation. Therefore, we introduce a new method for attributing precipitation to weather features. The method allows us to decompose total precipitation into the respective contributions by extratropical cyclones, fronts, atmospheric rivers, cold air outbreaks, and their combinations.

We have classified precipitation between 1930-2010 in the ECMWF’s twentieth century reanalyses project, ERA-20C. Our method assigns 70% of the total precipitation poleward of 30° to the aforementioned categories, allowing us to assess the relative importance of these weather features for total precipitation and for precipitation extremes. We find that the combination of extratropical cyclones, fronts, and atmospheric rivers accounts for more than 50% of the total precipitation and for 90% of the extreme events in the northern hemisphere storm-track regions, despite these precipitation events occurring less than 20% of the time.

How to cite: Konstali, K., Sorteberg, A., Spensberger, C., Weijenborg, C., Lutzmann, J., and Spengler, T.: Wet – wetter – weather: Attributing Global Precipitation to weather features, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5904, https://doi.org/10.5194/egusphere-egu22-5904, 2022.

17:12–17:18
|
EGU22-3980
|
ECS
|
Virtual presentation
Qiaoling Ren, Reinhard Schiemann, Kevin I. Hodges, Xingwen Jiang, and Song Yang

The Tibetan Plateau (TP), as the highest and largest obstacle embedded in the westerly jet stream, can influence the development of synoptic eddies that are steered by the westerly jet stream. Since the synoptic eddies can significantly affect weather and climate over the plateau and further downstream, this study explores their behaviors at different altitudes (850, 500, and 250 hPa) around the TP using an objective feature tracking algorithm and 41-years of hourly data from the ERA5. All synoptic eddies that occur over the TP region (25-45°N, 60-110°E) for at least a part of their lifecycle are considered in this study.

Analysis shows that these eddies mainly enter the TP region from the western and northern boundaries or form locally. Regardless of altitude, more than half of the eddies coming from outside die out when they encounter the TP, suggesting a suppression effect of the TP on external eddies. About one in ten eddies will turn north and fewer turn south. Eddies do not generally directly pass the TP region from west to east, except for a few cases at the upper level (250 hPa). Additionally, some 500-hPa and 250-hPa eddies can reach East Asia travelling around the TP on its northern side, which tends to happen in transitional seasons, and few winter eddies can pass through on the southern side. The number of synoptic eddies moving in from outside increases with altitude, while the number of locally generated eddies is largest at the 500-hPa level, which is the surface height of the TP. These eddies tend to occur over the central and southeastern parts of the TP, indicating the orographic perturbation effect of the TP. Nearly half of the locally generated eddies die out over the TP region, and more than a third move to East Asia. These results pave the way for future dynamical investigation of the interactions between the TP and the synoptic eddies, and of the impacts associated with the different categories of eddies.

How to cite: Ren, Q., Schiemann, R., Hodges, K. I., Jiang, X., and Yang, S.: Behaviors of synoptic eddies around the Tibetan Plateau, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3980, https://doi.org/10.5194/egusphere-egu22-3980, 2022.

17:18–17:24
|
EGU22-491
|
ECS
|
Virtual presentation
Jaeyeon Lee, Jaeyoung Hwang, Seok-Woo Son, and John Gyakum

Future changes of extratropical cyclones (ETCs) over East Asia are investigated using the models participating in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). To quantify ETC frequency, intensity, and genesis changes in a warming climate, the objective tracking algorithm is applied to the CMIP5 models which provide 6-hourly wind data with no missing values in the high-terrain region. The historical simulations reasonably well capture the spatial distribution of ETC properties, except for noticeable biases in, and downstream of, the high-terrain regions. Such biases are particularly pronounced in the models with a coarse spatial resolution and a smooth topography which weakens lee cyclogenesis. The best five models, which show better performance for historical simulations than other models, are used to evaluate the possible changes of East Asian ETCs under the RCP8.5 scenario. These models project a reduced cyclogenesis in the leeward side of the Tibetan Plateau, and over East China Sea and western North Pacific in the late 21st century, resulting in a reduced ETC frequency from the east coast of China to the western North Pacific. The ETC intensity also shows a hint of weakening over the North Pacific. These ETC property changes are largely consistent with an enhanced static stability and a reduced vertical wind shear in a warming climate. This result indicates that the local baroclinicity, instead of increased moisture content, plays a critical role in determining the future changes of East Asian ETCs.

How to cite: Lee, J., Hwang, J., Son, S.-W., and Gyakum, J.: Future changes of East Asian cyclones in the CMIP5 models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-491, https://doi.org/10.5194/egusphere-egu22-491, 2022.

17:24–17:30
|
EGU22-7849
Do future projections in windstorm severity depend on climate model resolution?
(withdrawn)
Colin Manning, Elizabeth J. Kendon, and Hayley J. Fowler
17:30–17:36
|
EGU22-6949
|
ECS
|
On-site presentation
Daniel Krieger, Sebastian Brune, Patrick Pieper, Ralf Weisse, and Johanna Baehr

Can a decadal prediction system be used to generate skillful forecasts of small-scale climate extremes? For large-ensemble probabilistic predictions of German Bight storm activity (GBSA), the answer is yes. In this study, we show that the prediction skill of the Max-Planck-Institute Earth System Model (MPI-ESM) decadal hindcast system for GBSA is higher than the skill of persistence-based forecasts. We define GBSA every year via the most extreme three-hourly geostrophic wind speeds, which are derived from mean sea-level pressure (MSLP) data. Our 64-member ensemble of yearly decadal hindcast simulations spans the time period 1960-2018. For this period, we compare deterministically and probabilistically predicted MSLP anomalies and GBSA with a lead time of up to ten years against observations. The model shows limited deterministic skill for single forecast years, but significant positive skill for long averaging periods. For probabilistic predictions of high and low storm activity, the model is skillful over the entire forecast period, and outperforms persistence-based forecasts. For short lead years, the skill of the probabilistic prediction for high and low activity notably exceeds the deterministic skill.

How to cite: Krieger, D., Brune, S., Pieper, P., Weisse, R., and Baehr, J.: Skillful Decadal Prediction of German Bight Storm Activity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6949, https://doi.org/10.5194/egusphere-egu22-6949, 2022.

17:36–17:42
|
EGU22-7979
|
ECS
|
On-site presentation
Lisa Degenhardt, Adam Scaife, and Gregor Leckebusch

Seasonal forecasts of extratropical storms are of interest to the scientific community as well as insurers, government contingency planners and the general public.

In previous studies, seasonal forecasts of winter windstorm events over Europe from the Met Office GloSea5 model have shown significant skill especially over north-west Europe for windstorm frequency and were connected to large-scale patterns, i.e., the NAO. Recent investigations show links between windstorm intensities and the three dominant large-scale patterns over Europe (NAO, SCA and EA) which explain up to 80% of interannual windstorm variability.

This new investigation quantifies the role of additional, dynamical forcing factors that could influence windstorm predictions. The factor selection is based on known dynamical influences on cyclone development and is thus related to the existence to severe windstorms.  We analyse the Eady-Growth-Rate (EGR), 200hPa jet speed and location, a proxy for Rossby wave source (RWS), and one factor related to tropical precipitation. The seasonal forecast skill of the factors themselves shows positive and significant skill in regions they are expected to be most influential or dominant, like for the RWS around its dipole over the south-west of the North Atlantic or for the EGR east of North America.

The links between these dynamical forcing factors to windstorm impact-relevant regions in the model and reanalysis data will be presented and the explanatory power of these factors for the overall model skill is discussed.

How to cite: Degenhardt, L., Scaife, A., and Leckebusch, G.: Dynamical forcing factors of severe windstorms: their seasonal forecast skill and influence on seasonal windstorm predictions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7979, https://doi.org/10.5194/egusphere-egu22-7979, 2022.

17:42–17:48
|
EGU22-8358
|
ECS
|
Virtual presentation
Laura Owen, Jennifer Catto, David Stephenson, and Nick Dunstone

Extratropical cyclones and their associated extreme precipitation and winds can have a severe impact on society. These extremes can cause even greater risk when they occur at the same place and time. Studies have investigated stormy seasons and their associated precipitation and wind extremes using observational data. Although these results are limited when looking at the risk of very extreme events, since a large number of samples is needed to get robust estimates. Additionally, it is very difficult for estimates based on observations alone to help us understand the risk of future rare or unprecedented stormy seasons and associated events. Using the UNSEEN method (UNprecedented Simulated Extremes using ENsembles) this risk can be estimated from large ensembles of climate simulations. The Met Office's Global Seasonal forecast system version 5 (GloSea5) model ensembles are evaluated against ERA5 reanalysis data to find out how well they represent storm tracks along with their associated precipitation, wind and compound extremes over Europe. This model has not been evaluated in such a way before and this is needed before the model can be used to estimate the likelihood of unprecedented stormy seasons and associated extremes using the UNSEEN method. We find that although GloSea5 underestimates the numbers of storms over Europe, particularly over the Mediterranean, seasons are found with larger numbers of storms than seen historically. Cyclone composites of precipitation, wind and compound extremes are also compared between ERA5 and GloSea5 ensembles. GloSea5 estimates the spatial pattern and frequency of wind, precipitation and compound extremes around cyclones averaged over their whole lifecycle well. The spatial pattern of extremes around cyclones at maximum intensity is also estimated well but the frequency is underestimated. Given this GloSea5 can be used to investigate the spatial pattern of larger extremes as well as extremes from the most intense storms.

How to cite: Owen, L., Catto, J., Stephenson, D., and Dunstone, N.: Unprecedented stormy seasons and their associated precipitation and wind extremes over Europe, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8358, https://doi.org/10.5194/egusphere-egu22-8358, 2022.

17:48–17:54
|
EGU22-11515
|
On-site presentation
Antonio Ricchi, Giovanni Liguori, Leone Cavicchia, Mario Marcello Miglietta, Davide Bonaldo, Sandro Carniel, and Rossella Ferretti

Over the Mediterranean basin we can occasionally observe intense cyclones showing tropical characteristics and known as Mediterranean Tropical-Like Cyclones (TLC) or Medicanes (short for “Mediterranean Hurricanes”). Previous studies focusing on past TLCs events have found that SST anomalies play a fundamental role in modulating the intense air-sea exchange of latent and sensible heat fluxes, hence controlling both development and evolution of TLCs. However, given the connection between ocean mixed layer, ocean heat content and temperature, it is important to explore also the role of the mixed layer depth (MLD). In this study we investigated the role of both SST and MLD on genesis and evolution of a recent record-breaking TLC. Specifically, we focus on TCL “IANOS”, a cyclone that originated over the southern Ionian Sea around 14 Sept 2020, moved over the Central Ionian Sea from south-west to North-East, and made landfall around 19 Sept 2020 over Greece mainland coast. It developed over a basin where a positive SST anomaly up to 4 °C was detected, which coincided with the sea area where it reached the maximum intensity. We conducted a series of experiments using an atmospheric model (WRF - Weather Research and Forecasting system) driven by underlying SST (standalone configuration) with daily update or coupled to a simple mixed-layer ocean model (SLAB ocean), with SST calculated at every time step using the SLAB ocean for a given value of the MLD. WRF was implemented with 3 km grid spacing, forced with GFS-GDAL analysis (0.25°x0.25° horizontal resolution), while SST or MLD initialization, for standalone or coupled runs, respectively, are provided by the MFS-CMEMs Copernicus dataset at 4 km of horizontal resolution. For the studied TLC, the mean MLD is modified by increasing or decreasing its depth by 10 m, 30 m, 50 m; the preliminary results show that the MLD influences not only the intensity of the cyclone but also the structure of the precipitation field both in terms of magnitude and location. At first  the MLD thickness was characterized  for the days in which the cyclone developed using ocean modeling data. Then we identified possible past and future climatological scenarios of MLD thickness. Starting from these data, we simulated the impact of the MLD, and consequently of the Ocean Heat Content, on the TLC. The preliminary results show that the MLD influences not only the intensity of the cyclone but also the structure of the precipitation field both in terms of magnitude and location. The results deserve further investigation in particular in the context of climate change scenarios.

How to cite: Ricchi, A., Liguori, G., Cavicchia, L., Miglietta, M. M., Bonaldo, D., Carniel, S., and Ferretti, R.: On the influence of Ocean Mixed Layer and Sea Surface Temperature Anomaly in the genesis and evolution of the Mediterranean Tropical-Like cyclones “IANOS”., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11515, https://doi.org/10.5194/egusphere-egu22-11515, 2022.

17:54–18:00
|
EGU22-2626
|
ECS
|
On-site presentation
Wahiba Lfarh, Florian Pantillon, and Jean-Pierre Chaboureau

Windstorms associated with extratropical cyclones belong to the most destructive natural disasters in the mid-latitudes, potentially causing tens of fatalities and hundreds of millions euros in damages yearly. The impact of windstorms is caused by gusts mainly, which arise from the downward transport of strong winds to the surface. The processes leading to the transport of wind gusts are still poorly understood, because they cannot be studied directly due to their short duration and local extent that are too small scale for both observing networks and numerical weather prediction systems.

The opportunity to address this issue arose when the windstorm Adrian (also known as Vaia) occurred over the north-western Mediterranean on 29 October 2018. Although cyclones are usually less intense over the Mediterranean than over the Atlantic, gusts exceeding 180km/h causing several material damages were recorded in Corsica and make Adrian an ideal case study to analyze the transport of strong winds in numerical simulations.

First, we perform a mesoscale analysis of windstorm Adrian, based on simulations on a 1 km grid with Meso-NH. Even at short range <12h, simulations exhibit high sensitivity to the initial conditions and can delay the cyclone by several hours. In a reference simulation, we show that the strongest surface winds occur below the occluded front, and they are due to the cold conveyor-belt (CCB). From the reference simulation, a Large Eddy Simulation (LES) with a horizontal resolution of 200m is performed over a large domain to capture both the mesoscale dynamics and the fine scale characteristics.

Focusing on the LES, we identify two types of strong wind structures: local cells and elongated structures with surface wind speed > 40m/s and duration < 10min. In the strong wind region, boundary layer convection is organised in rolls oriented along the wind direction, with vertical extension and spacing < 1km. It is found only in the convective and unstable boundary layer characterised by moderate surface sensible heat fluxes and vertical wind shear. This suggests that convective rolls are responsible for transporting strong winds to the surface. To ensure that, passive tracers initiated in the CCB region are computed to illustrate the way strong winds are transferred downward. Subsequently, a detailed study of the turbulent fluxes at the air-sea interface is carried out to evaluate their role in the transport of winds in the atmospheric boundary layer. It shows the influence of the various processes considered in the parameterisations of surface fluxes on the presence and intensification of the convective rolls.

The results show, using the LES, that the downward transport of strong winds in the cold conveyor-belt of Adrian is caused by small-scale convective rolls.

How to cite: Lfarh, W., Pantillon, F., and Chaboureau, J.-P.: The downward transport of strong winds by convective rolls in a Large Eddy Simulation of Mediterranean cyclone Adrian, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2626, https://doi.org/10.5194/egusphere-egu22-2626, 2022.

18:00–18:06
|
EGU22-11763
|
ECS
|
Virtual presentation
Cosimo Enrico Carniel, Rossella Ferretti, Antonio Ricchi, and Dino Zardi

The Mediterranean Sea is a semi-enclosed, fairly temperate, mid-latitude marine basin, strongly influenced by the North-Atlantic atmospheric circulations. A wide variety of cyclogenesis mechanisms are known to develop within this basin, including baroclinic waves coming from the Atlantic, Mediterranean cyclogenesis originating from the cut-off of baroclinic waves, Tropical-Like Cyclones (TLC) and explosive-cyclogenesis (EC). Depending on the cyclone type, the frequency of appearance can vary, ranging from tens per month to 1.5 per year, as in the TLC case. ECs are among the rarest and probably most intense and destructive cyclogenesis events that can develop within the Mediterranean basin; they usually originate at high latitudes, during wintertime, and mainly over the sea, preferring areas with high Sea Surface Temperature (SST) gradients. These events are determined by 12 different parameters, among which the main one is the quick drop of pressure, close to 1hPa/hr for 24 hours, within the eye of the cyclone. ECs formation is an extremely complicated process, and in the Mediterranean basin it is probably driven by air intrusions from the stratosphere and by the presence of Atmospheric Rivers. Starting from the analysis of the EC event called “Vaia Storm”, occurred in the Central Mediterranean Basin on October 29th 2018, and using ERA5 dataset, we firstly conducted a physical and dynamical analysis of the event, by pointing out some recurring characteristics previously highlighted in other works, on both local and synoptic scale. Secondly, we analyzed the results given by the reanalysis model ERA5 regarding the period January 1st 1950 – January 1st 2020, identifying other cyclogeneses with the same features, such as the event on November 4th 1966. On the basis of these information, the return period of the EC events was defined, as well as its statistical distribution and seasonality and correlation with NAO and EA indexes (both strongly negative). Further analysis are currently undertaken to determine correlations with SCAND index and possible SST anomalies in the Central Mediterranean Basin.

How to cite: Carniel, C. E., Ferretti, R., Ricchi, A., and Zardi, D.: On the statistical analysis of explosive-cyclogenesis over the Mediterranean Sea using ERA5 dataset , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11763, https://doi.org/10.5194/egusphere-egu22-11763, 2022.

18:06–18:12
|
EGU22-12373
|
ECS
|
Virtual presentation
Matthew Priestley and Jennifer Catto

How extratropical cyclones will respond to changes in future climate forcing is often uncertain. Changes in the overall number of cyclones and precipitation rates is well understood, however, there is less consensus on how the frequency of extreme cyclones and the near-surface winds will respond to a warmer climate. Using an ensemble of models from CMIP6 across a range of climate scenarios we aim to reduce the previous uncertainty and have investigated how extreme cyclones will change using a composite analysis method across a variety of intensity metrics.

 

We find an increase in the frequency of extreme cyclones in the Northern Hemisphere winter, with the reverse being found in the summer. For the cyclone winds in the lower troposphere we examine both the maximum wind speed and the area of wind speeds above a high intensity threshold. Results show that despite there being little change in the maximum wind speed by the end of the century, the portion of the cyclone with wind speeds above a high intensity threshold may be at least 15% higher in the NH winter. This increase is partly driven by changes in the cyclone propagation speed, although dynamical changes within the cyclones leads to further increases in wind speeds for extreme cyclones compared to those of average intensity. These results have significant implications for risk modellers and the loss potential of high impact wind storms.

How to cite: Priestley, M. and Catto, J.: The response of extreme extratropical cyclone wind fields to climate change, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12373, https://doi.org/10.5194/egusphere-egu22-12373, 2022.

18:12–18:30