AS1.11

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 29^th 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 1^st 1950 – January 1^st 2020, identifying other cyclogeneses with the same features, such as the event on November 4^th 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.

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.