The widening of the Southern Hemisphere tropical meridional circulation has been attributed to various forcings from increased greenhouse gases, ozone depletion and natural variability. While climate models can reproduce some characteristics of this observed change, there is some uncertainty in the operating mechanisms and driving regions setting the edge of the tropical circulation. Here we examine the impacts of systematic model biases of the atmosphere-only Unified Model onto the simulation of the Southern Hemisphere tropical extent. We utilise nudging experiments with prescribed sea-surface temperatures, where potential temperature and horizontal winds are relaxed back to reanalysis for a 20-year period. Specifically, experiments with regionally-defined bias correction aide to determine the influence of remote model biases on the tropical width. The experiments are applied to different tropical width metrics previously identified to measure the boundary between the tropical to extratropical circulation. We uncover a more consistent improvement of the location of the Hadley cell edge by correcting Southern Hemisphere extratropical circulation biases, than tropical ones. The analysis is further expanded to the range of atmosphere-only model simulations of the Coupled Model Intercomparison Project Phase 6 (CMIP6). We explore the relationships between tropical and extratropical biases and the models’ representation of the Hadley cell.

How to cite: Freisen, P., Arblaster, J., Jakob, C., and Rodriguez, J.: Tropical and extratropical circulation biases and the Southern Hemisphere Hadley cell width, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10041, https://doi.org/10.5194/egusphere-egu23-10041, 2023.

European winter storms are a significant threat to communities, public infrastructure, and private and commercial properties. On seasonal timescales, potential predictability was evidenced in recent state-of-the-art seasonal hindcast suites e.g., the UK Met Office’s GloSea5. Related positive and potentially usable forecast skill for frequency and intensity measures were based on pre-season model initialisation around the beginning of November for the following core winter (DJF) season’s assessment.

This study expands on these findings by analysing extended lead times of seasonal forecast into autumn and late summer before the winter season. Here, in a systematic way, a multi-model ensemble of hindcasts is analysed to evaluate current models’ capability to forecast the seasonal activity for initialisations from September to November. First results indicate potential predictability precursors already from the September initialisations for storm frequencies. These results vary from model to model though. The presentation will discuss differences between models as well as lead times for both, storm frequency and intensity.

How to cite: Leckebusch, G. C., Degenhardt, L., Berrie, E., Ng, K. S., and Spreitzer, E.: Assessing the boundaries of seasonal forecast skill for European winter storms from different hindcast suites, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14695, https://doi.org/10.5194/egusphere-egu23-14695, 2023.

Windstorms pose continual risk to Europe. Among their associated hazards, strong near-surface winds can be particularly damaging, threatening infrastructure, life and billions of pounds in insured losses. Insurers (and reinsurers) therefore need to prepare for the potential cost of extreme windstorms. Storm severity indices (SSIs) have been developed to quantify the potential losses associated with windstorm winds. Here, the most extreme windstorms that could potentially occur in the current climate are estimated using seasonal forecast data together with a cyclone-tracking algorithm, and their potential losses quantified using an SSI. As maximum wind gusts, the typical input variable for SSIs, are not available in the seasonal forecast dataset, a method is developed to calculate SSIs using wind speed data and a bias correction used to convert to SSI values representative of those obtained when calculated using wind gusts. Nearly 700 extended winter seasons of forecast data are analysed, representing a much larger sample of potential windstorms compared to that available from reanalysis or observational products. The storm track is reasonably well represented in the seasonal forecast data: spatial features are similar to those in a reanalysis, but there exists a slight poleward bias and underestimation of number of storms per season (maximal underestimation of around 10%). Additionally, distributions of SSI values for several countries in Europe are similar in the forecast data and reanalysis. Together, these suggest that the seasonal forecast data is suitable for analysing windstorm statistics and informing on potential extreme storms. We give estimates of worst-case storms, and worst-case seasons, that are identified in the forecast data and compare to those seen in a reanalysis, highlighting the potential insurance loss implications.

How to cite: Maddison, J., Catto, J., Siegert, S., and Hansen, S.: Estimating worst-case European windstroms, and worst-case seasons, using seasonal forecasts., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14410, https://doi.org/10.5194/egusphere-egu23-14410, 2023.

European windstorms are a frequent and damaging natural hazard that can cause loss of human life and damage to property and infrastructure. As there is a high degree of uncertainty in climate projections, it is crucial to understand the physical risks and economic losses at regional and local scales associated with European Windstorms. In this study, we develop a simple model to estimate historical windstorm losses over the European region. The model uses winds from the ERA5 reanalysis and different exposure datasets based on countrywide total insured property values, gross domestic product, and historical population density.

We find that the estimated losses associated with major historical storms in North-western Europe and estimated average EU-wide losses are comparable to the reported estimates and those from propriety vendor models. However, estimated losses from windstorms in France and Germany are lower than reported. Differences in the estimated losses are attributed to the contrasts in the regional-level exposure within and between different exposure datasets. We also tested the sensitivity of regional-level vulnerabilities and find that accounting for regional-level vulnerability differences slightly improves the biases in countrywide losses. Further, we also find that the major contribution to the estimated losses comes from the United Kingdom, France, and Germany for most of the storm seasons, and thus it is important to correctly represent the exposure and vulnerabilities over these countries. The ability of the model to estimate reported losses is also limited by the representation of the winds in ERA5, which has limited skill in representing the hazard footprint, especially for specific storms such as the Great October Storm of 1987.

Keywords: Losses, Windstorms, Climate Change, Natural Hazards

How to cite: Kumar, D., Shaffrey, L., Dixon, R., Bloomfield, H., Bates, P., and Hillier, J.: High-resolution loss modeling for European Windstorms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12391, https://doi.org/10.5194/egusphere-egu23-12391, 2023.

Storm surges in the German Bight can have great destructive potential. This includes devastating floods, structural damage to infrastructure, and even loss of life. The most important driver of storm surge events in the German Bight is strong winds from north-westerly directions, often related to intense extra-tropical cyclones travelling from the North Atlantic into the North Sea region.

Making use of an objective, impact-oriented identification and tracking scheme, we analyse storm events related to storm surges in the German Bight. This particular version of the tracking algorithm includes the so-called Storm Surge Severity Index (SSSI) and is used as a complementary tool in operational forecasting by the German Federal Maritime and Hydrographic Agency (BSH). The SSSI takes wind speed and direction into account and intends to quantify storm surge risk in the German Bight. However, to date, the SSSI has never been systematically evaluated for past storm surge events. To fill this gap and to prove that the SSSI can be used as a proxy for storm surge risk, we analyse the relationship between SSSI values of past storm events and the associated water levels recorded in the German Bight using ERA5 atmospheric reanalysis data. Moreover, we analyse potentially storm surge-relevant storms in a multi-model ensemble of global climate model simulations to assess potential future changes in storm surge risk in the German Bight.

How to cite: Schaffer, L., Becker, N., Schenk, L., Hinrichs, C., Ditzinger, G., Schade, N. H., Befort, D. J., and Kruschke, T.: Objective assessment of storm surge risk in the German Bight – historical events and future climate change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7236, https://doi.org/10.5194/egusphere-egu23-7236, 2023.

In the mid-latitudes, synoptic-scale phenomena like high and low-pressure systems generate the variability of the regional-scale weather system. To identify the weather variability of extra-tropical region storm track activity has been analyzed based on observations since the mid-nineteenth century. After early-stage research that directly counted the movement of cyclones, the time filtering method based on grid analysis has been used for an isolated disturbance with periods of 2~7 days. This bandpass filtering method has the advantage of being able to examine the distribution and the variability of the storm track spatially in vertical and horizontal space.

In this study, we confirm the storm track activity in the East Asia region using the dynamical down-scale results from CORDEX (COordinated Regional climate Downscaling EXperiment) East Asia projects. We verify the reproducibility and confirm the temporal change in the storm track activity from various RCM data. In addition to the historical period, we examine the difference in storm track intensity over future climate change scenarios. Through this, we also discuss the role of added value from RCM.

This work was funded by the Korea Meteorological Administration Research and Development Program under Grant KMI2022-01210.

How to cite: Byun, U.-Y., Chang, E.-C., Kim, J., Cha, D., Ahn, J.-B., and Min, S.-K.: Extratropical storm track activity change in future climate change scenarios, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10466, https://doi.org/10.5194/egusphere-egu23-10466, 2023.

Windstorms (major winter storms) are one of the most important natural hazards in Europe. Despite the large observed socioeconomic losses, the impact of windstorms and its decadal variability is not yet fully understood. This study aims to assess the loss potentials associated with European windstorms and the variability in the wind speed climatology across Europe. We use the 12,500-years LAERTES-EU (LArge Ensemble of Regional climaTe modEl Simulations for EUrope) RCM ensemble to study the spatio-temporal distribution and variability of windstorms over Europe. LAERTES-EU is validated against reanalysis data (ERA5) and available ground-based station observations. The associated windstorm losses are estimated by computing statistics of extreme wind speeds and related indices. Different loss indices are validated using historical loss data from the insurance sector. The results reveal that the loss index (LI) is a good proxy for the estimation of potential losses associated with windstorms across Europe in winter. The derived statistics of extreme windstorms such as return periods (RP) show hardly any change in the severity and frequency of windstorms during the covered period 1900-2028, but a strong decadal variability is apparent.

How to cite: Sethunadh, J., Pinto, J. G., Ludwig, P., Feldmann, H., and Ehmele, F.: Decadal variability of extreme winds and potential storm losses in Europe using large RCM ensembles, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13914, https://doi.org/10.5194/egusphere-egu23-13914, 2023.

Extratropical cyclones produce extreme surface wind speeds and heavy rainfall which can individually and jointly influence impacts and potentially produce large aggregate impacts. Within this study, we assess the UKCP 12-member ensemble of local convection-permitting 2.2 km climate projections. We quantify the likelihood of cyclones producing large footprints of both extreme winds and rainfall over the UK in a control (1981-2000) and future (2061-2080, RCP8.5) climate simulation. Following this, we characterise the convective and frontal drivers of wet and windy conditions within cyclones, and identify the characteristics of cyclones, their tracks and interactions with the jet stream that contribute to the occurrence of large, combined footprints in the control and future simulations. The future simulations project an increased probability of extratropical cyclones producing extremely wet and windy conditions in the same storm, as well as an increase in the land area covered by such conditions. In both the control and future simulations, combined wet and windy extremes largely occur close to cold and warm fronts, likely due to the warm conveyor belt which produces heavy rainfall (due its ascent over the frontal boundaries) and high winds (when occurring within a region of tight pressure gradients). Cyclone composites reveal that the largest changes in joint extremes are closely located within the sector of cyclones where we expect to see the warm conveyor belt, suggesting their change arises partly through the response of this shared driver rather than being a simple consequence of increased rainfall due to thermodynamics. In further analysis, we identify favourable conditions and cyclone characteristics that lead to cyclones producing large rainfall and wind footprints over the UK.

How to cite: Manning, C., Kendon, E., Fowler, H. J., Catto, J. L., Chan, S. C., and Sansom, P.: Drivers of large footprints of extreme winds and rainfall and their projected future changes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13180, https://doi.org/10.5194/egusphere-egu23-13180, 2023.

The coastal region of southern and southeastern Brazil, which is part of the South Atlantic Ocean basin, is a genesis region for subtropical cyclones and, therefore, is susceptible to weather changes caused by these systems. The first named subtropical cyclone in the South Atlantic basin was Anita in 2010. Since then, some studies on subtropical cyclones have been carried out, but there are still several questions to be investigated. Thus, this study aims to: (a) describe the main physical mechanisms of genesis of the subtropical cyclones that were named in the South Atlantic Ocean between 2010 and 2021 and (b) identify the value of the Genesis Potential Index (GPI) between the pre-cyclogenesis and the phase in which these systems acquire subtropical characteristics. The rationale for analyzing the CPI is that we want to identify a possible pattern that helps in operational weather forecasting. The main database used in the study is the ERA5 reanalysis. Of the 14 cyclones studied, only two systems did not have cyclogenesis with subtropical characteristics, but acquired it 24 hours after cyclogenesis. The results indicate that 5 cyclones have a genesis associated with mid-level troughs in the atmosphere, and 9 with blocking patterns (cutoff low type). As most of the cyclones studied occur in an environment with blocking structure, this indicates that the condition of weak vertical wind shear is an important factor for subtropical cyclones. As the GPI does not show a standard value in the 14 cyclones studied, between pre-cyclogenesis and the moment when these systems become subtropical, as it varies from 0.35 in the Deni genesis to 22.71 in the Anita genesis, perhaps it is not possible to use it with a threshold in operational practices. The authors thank Programa de P&D regulado pela ANEEL e empresa Engie Brasil Energia e a Companhia Energética Estreito for the financial support.

How to cite: Ribeiro, J. G. M., da Paz, G. T., Reboita, M. S., Gozzo, L. F., Ferreira, G. W. D. S., and da Rocha, R. P.: Analysis of the Genesis Potential Index in Subtropical Cyclones off the Coast of Brazil, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-716, https://doi.org/10.5194/egusphere-egu23-716, 2023.

Extreme winds account for more than half of Norway’s insurance claims related to natural hazards [1]. Quantifying windstorm-damage relations is crucial to prepare for and mitigate the effects of future wind events. However, there has never been an attempt to quantify windstorm-damage relations at the municipality level in Norway. The work in hand employs four different damage functions at the municipality level of Norway. Along with the newly proposed modified Prahl damage function [2], an ensemble means of the damage estimates are tested for 356 municipalities in Norway. We evaluate the damage functions in terms of forecast accuracy. The spatial distribution of losses suggests severe damages along the west coast of Norway. Further inland in Norway, there are seldom any losses due to Norway’s unique topography and demography. The losses above the 99.7^th percentile in each municipality constitute 85% of total national loss, and we focus on this extreme loss class. A significant agreement between the observed and estimated losses at the municipality and national levels indicates that the damage functions are suited for forecasting storm-induced damages. The damage functions are also able to successfully reconstruct the spatial spread and pattern of losses caused by very extreme windstorms.

References

How to cite: Jaison, A., Sorteberg, A., Michel, C., and Breivik, Ø.: Storms and associated damages in Norway, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4675, https://doi.org/10.5194/egusphere-egu23-4675, 2023.

Cyclones are essential elements of the climate and of the water cycle in the Mediterranean. The most intense of them lead to natural disasters because of their violent winds and extreme rainfall, which can cause significant damage to the territories bordering the Mediterranean (coast and mountain ranges). Reliable forecasts of cyclones are therefore essential to better anticipate and prevent their societal impact. However, their predictability is often limited by their particularities: smaller cyclones with a shorter life cycle than in the North Atlantic, complex topography, interactions with the relatively warm sea and air masses laden with dust from the Sahara.

We investigate the predictability of Mediterranean cyclones in a systematic framework using an ensemble prediction system. A reference dataset was first obtained by tracking cyclones in the ERA5 reanalysis (1979-2021), using an algorithm developped for the North Atlantic and adapted for the Mediterranean region. We then investigated the predictability using ARPEGE ensemble reforecasts in a homogeneous configuration over 22 years (2000-2021).

We restricted the study on 500 cases, which were selected using a storm severity index based on wind gusts and adapted for the Mediterranean region. The cases were then divided in several categories following their dynamical context, their intensity and their geographical origin. The predictability of the reforecasts was finally quantified on each of those categories, using probabilistic scores on cyclone trajectories (along and cross track error) and on intensities (mean sea level pressure and storm severity index).

While past studies have been limited by the fact that regular updates of operational forecasting systems do not allow the predictability of cases to be compared with each other, the homogeneous configuration of the ARPEGE ensemble reforecasts makes it possible to systematically identify the limitation to the predictability of Mediterranean cyclones.

How to cite: Doiteau, B., Pantillon, F., Plu, M., Descamps, L., and Rieutord, T.: Investigating the predictability of Mediteranean cyclones and their severity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5690, https://doi.org/10.5194/egusphere-egu23-5690, 2023.

"All models are wrong. Some are wrong in a useful manner.” (adapted by the authors from George Box) In this presentation, we utilize an error in the surface flux formulation of the ICON-NWP numerical weather prediction model to elucidate how cloud-radiative heating affects the intensity of idealized extratropical cyclones.

We present idealized baroclinic life cycle simulations with two versions of the global atmosphere model ICON-NWP. Both versions simulate the same cyclone when run without radiative heating, but disagree when cloud-radiative heating is allowed to affect atmospheric temperature and the cyclone evolution. In version 2.1, taking into account cloud-radiative heating leads to a weaker cyclone, while in version 2.6 a stronger cyclone results. The simulations use a new modeling technique for which only cloud-radiative heating interacts with the cyclone and clear-sky radiative heating is omitted. The technique circumvents changes in the mean state due to clear-sky radiative heating that has complicated the interpretation of previous work.

A defining difference between the two model versions is the amount of simulated low-level clouds. Compared to version 2.6, version 2.1 simulates twice as many low-level clouds and a twice as strong cooling of the planetary boundary layer by cloud-radiative heating. While the increase in low-level clouds is tied to an error in the surface flux formulation in version 2.1 that was corrected in version 2.6, the error provides an opportunity to probe the impact of cloud-radiative heating in the boundary layer (below 2 km) versus the free-troposphere (above 2 km). Sensitivity studies show that negative cloud-radiative heating in the boundary layer from the tops of low-level clouds weakens the cyclone by making the atmosphere more stable. At the same time, they show that negative cloud-radiative heating near the tropopause from the tops of high-level clouds strengthens the cyclone by decreasing atmospheric stability. The changes in stability are particularly evident in regions of upward motion.

Overall, our results indicate that the vertical distribution of clouds and their radiative heating are an important factor for the dynamics of extratropical cyclones and that model differences in the simulation of low-level clouds can translate to model differences in cyclone intensity.

How to cite: Voigt, A., Keshtgar, B., and Butz, K.: Cloud-radiative heating shapes idealized extratropical cyclones by changing atmospheric stability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6570, https://doi.org/10.5194/egusphere-egu23-6570, 2023.

The Mediterranean is characterized by a high extratropical cyclone activity. These cyclones are an important source for water availability in the region, but at the same time they have the potential to cause extreme weather in the form of precipitation and wind extremes. The Mediterranean is heavily affected by the ongoing anthropogenic climate change, which is expected to have a profound effect on cyclones in this area. In this study, we investigate the effects of internal climate variability and anthropogenic climate change on the characteristics of Mediterranean cyclones. The analysis is based on two simulations from the Community Earth System Model 1.2 (CESM): a seamless simulation spanning 3500 years from 1500 BCE to 2012 CE and a simulation of future RCP8.5 scenario from 2013 to 2300 CE. The simulations have a 1.9°x2.5° horizontal resolution, and cyclones are identified using an established detection and tracking algorithm. Comparison with the ERA5 reanalysis for the period 1981–2010 shows that CESM is able to realistically represent cyclone frequency on a global scale, though it slightly underestimates cyclone activity in the Mediterranean. Our results indicate that cyclone activity in the Mediterranean varies on interdecadal to centennial time scales before 1850 CE. These variations are linked to positive and negative climate anomalies and fluctuations in strength of several modes of circulation, such as the North Atlantic Oscillation. The variations caused by internal variability are, however, of smaller magnitude than the effects of future climate change on the Mediterranean cyclones. In the RCP8.5 scenario, Mediterranean cyclones will become less frequent based on our simulation, and cyclone related precipitation will decrease in addition to that, which is contrary to what is being observed in other important storm track regions, such as the North Atlantic. We hypothesize that the changes in cyclone characteristics are more pronounced in the Western Mediterranean than in the Eastern Mediterranean. Overall, the study suggests that cyclone activity in the Mediterranean is projected to leave the bandwidth of variability of the last 3500 years near the end of the century.

How to cite: Doensen, O., Messmer, M., Kim, W. M., and Raible, C.: Effects of climate variability and change on cyclones in the Mediterranean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6629, https://doi.org/10.5194/egusphere-egu23-6629, 2023.

Extra-tropical cyclones are key components of the atmospheric general circulation due to their ability to transport large quantities of heat, moisture, and momentum. Cyclones are an important contributor to extreme weather as their passage is associated with strong winds, and large precipitation accumulations. Here we connect a cyclone compositing scheme with regionally derived distributions of precipitation to present a framework for classifying spatially dependent extremes relative to the cyclone centre. Using this framework, cyclone composites for both average (50th percentile) and extreme (90th and 98th percentile) precipitation are derived from ERA5 reanalysis output. Composites are then partitioned into different stages of the cyclone lifecycle to assess the spatial and temporal evolution of precipitation extremes. We find that most extreme precipitation occurs within the comma-cloud structure close to the cyclone centre, with the extreme precipitation occurrence and intensity occurring in that region. Extreme precipitation is also identified to be largest during the period of deepening before the maximum cyclone intensity is reached. These regions of the cyclone correspond to places where large fractions of precipitation are above the extreme threshold. Strong spatial correlation are also seen between the average and extreme precipitation during the deepening phase for the precipitation mean, occurrence and fraction. This correlation weakens as the cyclone evolves and as the threshold used to determine extreme precipitation increases.

How to cite: McErlich, C., McDonald, A., and Renwick, J.: An assessment of extreme precipitation within cyclone composites using ERA5, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10857, https://doi.org/10.5194/egusphere-egu23-10857, 2023.

Volcanic eruptions are well known to influence Earth's temperature, however, how eruptions influence the atmosphere's circulation pattern, especially on the scale of everyday weather is poorly understood. Changing Earth's temperature can affect temperature gradients which in turn could affect baroclinicity and hence high- and mid-latitude weather. Yet, to what extent volcanic eruptions do in fact exert  such an influence is not clear.

To answer this, we followed two independent lines of investigation: First, we query the Greenland ice-core proxy record for Indications of increased extra-tropical cyclone frequency that correlates with evidence for volcanism. This is done by comparing the storm proxy sea salt (a substance transported to the ice sheet by wind)  with the volcanological proxy sulfur. Secondly, we simulate eruptions with the MPI-ESM1.2 Earth System Model and use the TRACK algorithm to explore how extra-tropical cyclone frequency is affected in the model  experiments. Both approaches suggest that volcanic eruptions impact high- and mid-latitude weather by increasing the number of extra-tropical cyclones especially at higher latitudes. A detailed interrogation of the simulated eruption scenarios suggests that this increase in cyclone frequency is associated with features such as an increase in isentropic slopes and sea-ice extent most commonly found under  colder climate regimes and is the reverse of what one finds in more equable climates such as that projected for the future.

How to cite: Andreasen, L., Corner, J., Abbott, P., Sinclair, V., Riede, F., and Timmreck, C.: Volcanically induced increase in extra-tropical cyclone frequency, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12526, https://doi.org/10.5194/egusphere-egu23-12526, 2023.