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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?

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Co-organized by CL4/NH1/OS1
Convener: Gregor C. Leckebusch | Co-conveners: Joaquim G. Pinto, Uwe Ulbrich
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| Mon, 04 May, 16:15–18:00 (CEST)

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Chat time: Monday, 4 May 2020, 16:15–18:00

Chairperson: GC Leckebusch
D3268 |
EGU2020-19711
| Highlight
Ben Harvey, Peter Cook, Len Shaffrey, and Reinhard Schiemann

Understanding and predicting how extratropical cyclones might respond to climate change is essential for assessing future weather risks and informing climate change adaptation strategies. Climate model simulations provide a vital component of this assessment, with the caveat that their representation of the present-day climate is adequate. In this study the representation of the NH storm tracks and jet streams and their responses to climate change are evaluated across the three major phases of the Coupled Model Intercomparison Project: CMIP3 (2007), CMIP5 (2012), and CMIP6 (2019). The aim is to quantity how present-day biases in the NH storm tracks and jet streams have evolved with model developments, and to further our understanding of their responses to climate change.

The spatial pattern of the present-day biases in CMIP3, CMIP5, and CMIP6 are similar. However, the magnitude of the biases in the CMIP6 models is substantially lower in the DJF North Atlantic storm track and jet stream than in the CMIP3 and CMIP5 models. In summer, the biases in the JJA North Atlantic and North Pacific storm tracks are also much reduced in the CMIP6 models. Despite this, the spatial pattern of the climate change response in the NH storm tracks and jet streams are similar across the CMIP3, CMIP5, and CMIP6 ensembles. The SSP2-4.5 scenario responses in the CMIP6 models are substantially larger than in the corresponding RCP4.5 CMIP5 models, consistent with the larger climate sensitivities of the CMIP6 models compared to CMIP5.

D3269 |
EGU2020-3946
Daniel McCoy, Paul Field, Alejandro Bodas-Salcedo, Gregory Elsaesser, and Mark Zelinka

Global climate models (GCMs) differ greatly in their shortwave cloud feedback. One feature that is consistent across GCMs is a positive shortwave cloud feedback in the subtropics, and a negative shortwave cloud feedback across the midlatitudes. Confidence has grown in the mechanisms that lead to, and the strength of, the subtropical shortwave cloud feedback, but the midlatitude negative shortwave cloud feedback is not well-constrained or well-understood. It is critical to reduce uncertainty in midlatitude shortwave cloud feedback. A more positive midlatitude shortwave cloud feedback in the sixth coupled model intercomparison project (CMIP6) has been found to be one of the primary causes of the increased climate sensitivity of CMIP6 models relative to CMIP5. We show that changes in midlatitude cyclones in future climates are the primary cause of the negative shortwave cloud feedback and are thus key to understanding the high climate sensitivity in the most recent GCMs. Warming-induced changes in cloud liquid water path in midlatitude cyclones can almost entirely be explained by Clausius-Clapeyron increasing moisture convergence into cyclones. One concern with simulating midlatitude cyclones is the lack of predictive skill at low resolution. A more realistic relationship between moisture flux and cyclone liquid content is found at high horizontal resolution (∆x<25km), but the cloud feedback within cyclones can be explained by increased moisture convergence across low- and high-resolution models. Observations and models agree that the extratropical shortwave cloud feedback is moderated by precipitation processes in cyclones. This rules out a large contribution from ice-to-liquid transitions, as has been hypothesized in previous studies. Understanding and constraining these precipitation processes is crucial to constraining the response of midlatitude cyclones to warming and by extension climate sensitivity.

 

Predicted midlatitude cloud feedbacks based on convection-permitting model output (model output is shown in a). The moisture flux along the warm conveyor belt (WCB) of a cyclone plays a central role in determining cyclone cloud liquid water path (LWP) (b). Because WCB scales with water vapor path (WVP) and surface wind speed, WCB moisture flux increases following Clausius-Clapeyron and predicts a negative midlatitude cloud feedback.

 

D3270 |
EGU2020-5021
Chris Weijenborg and Thomas Spengler

The existence of cyclone clustering, the succession of multiple cyclones in a short amount of time, indicates that the baroclinicity feeding these storms undergoes episodic cycles. With the generally accepted paradigm of baroclinic instability for extratropical cyclones, one would anticipate that clustering coincides with increased baroclinicity, though simultaneously individual cyclones reduce baroclinicity to maintain their growth. This apparent contradiction motivates our hypothesis that some cyclones increase baroclinicity, which could be a pathway for cyclone clustering.

Using a new cyclone clustering diagnostic based on spatio-temporal distance between cyclone tracks, we analyse cyclone clustering for the period 1979 until 2016. We complement this analysis with a baroclinity diagnostic, the slope of isentropic surfaces. With the isentropic slope and its tendencies, the relative roles of diabatic and adiabatic effects associated with extra-tropical cyclones in maintaining baroclinicity are assessed. We first present a case study, for which a sequence of cyclones culminated in severe cyclones due to the fact that one of the storms significantly increased the background baroclinity along which the succeeding storms evolved. The life cycle of these storms is discussed in terms of how the storm changes and uses its environment to attain its intensity. We compare these findings to composites of clustered and non-clustered cyclones to quantify how consistent the proposed clustering-mechanism is.

D3271 |
EGU2020-3502
Hai Bui and Thomas Spengler

The sea surface temperature (SST) distribution can modulate the development of extratropical cyclones through sensible and latent heat fluxes. However, the direct and indirect effects of these surface fluxes, and thus the SST, are still not well understood. This study tackles this problem using idealised channel simulations of moist baroclinic development under the influence of surface fluxes. The model is initialised with a zonal wind field resembling the midlatitude jet and a different SST distribution for each experiment, where both the strength and position of the SST gradient are varied.

The surface latent heat flux plays a key role in enhancing the moist baroclinic development, while the sensible heat fluxes play a minor and dampening role. The additional moisture provided by the latent heat fluxes originates from about 1000 km ahead of the cyclone a day prior to the time of the most rapid deepening. When the SST in this region is higher than 15 degrees Celsius, the additional latent heat is conducive to explosive cyclone development. A high absolute SST with a weak SST gradient, however, can lead to a delay of the deepening stage, because of unorganised convection at early stages. In addition, the cyclone can maintain its intensity for a longer period with an SST above 20 degrees Celsius, because there is a continuous and extensive moisture supply from the surface. The cyclone in this case has characteristics of a hybrid cyclone, where the latent heat release near the cyclone’s centre plays a major role in the development.

D3272 |
EGU2020-17644
Thomas Röösli, Christoph Welker, and David Bresch

We compare the risk assessment for storm related building damage based on three different foundations: (1) insurance claims data, (2) modelled building damages based on a historic event set of wind gust data, and (3) modelled building damages based on a probabilistic extension of the historic event set. Windstorms cause large socio-economic damages in Europe. In the canton of Zurich (Switzerland) they are responsible for one third of the building damages caused by natural hazards.

The Wind Storm Information Service (WISC) of the Copernicus Climate Change Service provides open wind gust datasets for the insurance sector to understand and assess the risk of windstorms in Europe. This is the first open climatological data set covering a longer time range than the insurance claims data of most small insurance companies. Our science-practice collaboration is a case study to illustrate how climatological data can be used in risk assessments in the insurance sector and how this approach compares to risk assessments based on proprietary claims data. We describe and use a storm damage model that combines wind gust data with exposure and vulnerability information to compute an event set of modelled building damages. These modelled damages are used to calculate relevant risk metrics for the insurance industry like the annual expected damage (AED) as well as the damage of rare events, with a return period of up to 250 years.

The AED calculated based on the insurance claims data (i.e. the mean damage over the observation period of 35 years) is 2.34 million Swiss Francs (CHF). This is almost double the value of the AED computed based on the storm damage model and historic event set (CHF 1.36 million). The storm Lothar/Martin in December 1999 is the most damaging event in the insurance claims data (CHF 62.4 million) as well as the historic event set (modelled building damage of CHF 62.7 million).

Both the insurance claims data and the modelled building damages based on historic events are not well suited to derive information about rare events with return periods considerably exceeding the observation period. To provide some information about rare events, we propose a new probabilistic event set, by introducing various perturbations, resulting in 4’200 events. This probabilistic event set results in an AED of CHF 1.45 million and a damage amount of CHF 75 million for a return period of 250 years. The probabilistic event set allows for testing the sensitivity of the risk to e.g. portfolio changes and changes in the insurance condition for events of a higher intensity than the historic events.

Our analysis is implemented in the GVZ’s proprietary storm damage model as well as the open-source risk assessment platform CLIMADA (https://github.com/CLIMADA-project/climada_python). This guarantees scientific reproducibility and offers insurance companies the opportunity to apply this methodology to their own portfolio with a low entry threshold.

D3273 |
EGU2020-5885
Xiaolan Wang, Yang Feng, Rodney Chan, Gilbert P. Compo, Laura C. Slivinski, Bin Yu, Michael Wehner, and Xiao-Yi Yang

Preliminary results obtained from tracking cyclones in the ensemble-average and individual members of the NOAA-CIRES-DOE Twentieth Century Reanalysis version 3 (20CRv3) ensemble for the period 1836-2015 will be presented. Comparison with tracking in the 20CRv2c ensemble-average series will also be shown.

The results indicate that the 20CRv3 is an improvement in representing cyclone climate and variability compared to previous versions (20CRv2c or 20CRv2). However, as in previous versions, the 20CRv3 ensemble-average fields are too smooth to use for tracking cyclones and studying cyclone climate, especially for the period before 1960 for the NH and for the entire reanalysis period for the SH, and that there are still temporal inhomogeneity issues in the 20CRv3, especially in the SH and in the early period for the NH, due to the increases over time of observations available for assimilation. The improvements arise from the use of a higher model resolution and the assimilation of more observations. They include that the 20CRv3 ensemble shows cyclones of higher intensities and a higher number of deep cyclones (center pressure ≤ 960 hPa) in the Northern Hemisphere than the 20CRv2c counterpart. Historical trends of cyclone activity and their uncertainties will be discussed based on the results of tracking the individual members of the 20CRv3 ensemble, with the temporal inhomogeneity issues being taken into account.  

D3274 |
EGU2020-15778
Alexander Vessey, Kevin Hodges, Len Shaffrey, and Jonathan Day

Arctic sea ice has reduced significantly over recent decades and is projected to reduce further over this century. This has made the Arctic more accessible and increased opportunities for the expansion of business and industrial activities.  As a result, the exposure and risk of humans and infrastructure to extreme storms will increase in the Arctic.

Our understanding of the current risk from storms comes from analysing the past, for example, by using storm tracking algorithms to detect storms in reanalysis datasets.  However, there are multiple reanalysis datasets available from different institutions and there are multiple storm tracking methods.  Previous studies have found that there can be differences between reanalysis datasets and between storm tracking methods in the climatology of storms, particularly in mid-latitude regions rather than the Arctic.  In this study, we aimed to improve the understanding of Arctic storms by assessing their characteristics in multiple global reanalyses, the ECMWF-Interim Reanalysis (ERA-Interim), the 55-Year Japanese Reanalysis (JRA-55), the NASA-Modern Era Retrospective Analysis for Research and Applications Version 2 (MERRA-2), and the NCEP-Climate Forecast System Reanalysis (NCEP-CFSR), using the same storm tracking method based on 850 hPa relative vorticity and mean sea level pressure.

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

D3275 |
EGU2020-1278
Kenta Tamura and Tomonori Sato

Polar mesocyclones (PMCs) are mesoscale, maritime cyclones that occur around the high latitudes in the cold seasons. Over the northern Sea of Japan, PMC frequently occurs with cold air outbreaks from the east of the Eurasian Continent. In this study, effects of the mountains on the eastern end of the Eurasian Continent (Sikhote-Alin mountain range) on the PMCs genesis were examined by 36-years long-term numerical experiments. The sensitivity experiment, in which the Sikhote-Alin mountain range is removed, shows that the number of PMC genesis decreases and the duration between PMCs genesis and landfall becomes shorter compared with realistic experiment. These differences arise only in the southern part of the sea. This result suggests that the effect of the orographic forcing on PMC's behavior varies with the location of the PMCs genesis.

D3276 |
EGU2020-4431
Jaeyeon Lee, Jaeyoung Hwang, Seok-Woo Son, and John Gyakum

The extratropical cyclones (ETCs) over East Asia and their possible future changes are evaluated using the Coupled Model Intercomparison Project phase 5 (CMIP5) models. The East Asian ETCs are identified using an automated tracking algorithm applied to the 850-hPa relative vorticity field for both reference data (ERA-Interim reanalysis data) and model data. The CMIP5 models well capture the spatial distribution of East Asian ETC properties, although significant biases are present around the high-topography regions. Based on the individual model biases, Best 5 models are selected and used for examining the future changes of East Asian ETCs. In future climate, Best 5 shows declined cyclogenesis in the leeward side of the Tibetan Plateau, which is partly responsible for the decreased ETC frequency over the western North Pacific. The intensity of individual ETCs is also projected to decrease in a warm climate. These changes could be attributed to the combined effect of increased static stability and decreased vertical wind shear in East Asia, which means reduced local baroclinicity. It is also found that CMIP6 models have smaller bias than Best 5 CMIP5 models, indicating that the result documented in this study may change in quantity when newly-available CMIP6 models are utilized.

D3277 |
EGU2020-6659
Joaquim G. Pinto and Patrick Ludwig

Extratropical cyclones are a dominant feature of the mid-latitudes, as their passage is associated with strong winds, precipitation, and temperature changes. The statistics and characteristics of extratropical cyclones over the North Atlantic region exhibit some fundamental differences between Pre-Industrial (PI) and Last Glacial Maximum (LGM) climate conditions. Here, the statistics are analysed based on results of a tracking algorithm applied to global PI and LGM climate simulations. During the LGM, both the number and the intensity of detected cyclones was higher compared to PI. In particular, increased cyclone track activity is detected close to the Laurentide ice sheet and over central Europe. To determine changes in cyclone characteristics, the top 30 extreme storm events for PI and LGM have been simulated with a regional climate model and high resolution (12.5 km grid spacing) over the eastern North Atlantic and Western Europe. Results show that LGM extreme cyclones were characterised by weaker precipitation, enhanced frontal temperature gradients, and stronger wind speeds than PI analogues. These results are in line with the view of a colder and drier Europe, characterised by little vegetation and affected by frequent dust storms, leading to reallocation and build-up of thick loess deposits in Europe.

D3278 |
EGU2020-6829
Daniel Krieger, Oliver Krueger, Frauke Feser, Ralf Weisse, Birger Tinz, and Hans von Storch

Assessing past storm activity provides valuable knowledge for economic and ecological sectors, such as the renewable energy sector, insurances, or health and safety. However, long time series of wind speed measurements are often not available as they are usually hampered by inhomogeneities due to changes in the surroundings of a measurement site, station relocations, and changes in the instrumentation. On the contrary, air pressure measurements provide mostly homogeneous time series as the air pressure is usually unaffected by such factors.

Therefore, we perform statistical analyses on historical pressure data measured at several locations within the German Bight (southeastern North Sea) between 1897 and 2018. We calculate geostrophic wind speeds from triplets of mean sea level pressure observations that form triangles over the German Bight. We then investigate the evolution of German Bight storminess from 1897 to 2018 through analyzing upper quantiles of geostrophic wind speeds, which act as a proxy for past storm activity. The derivation of storm activity is achieved by enhancing the established triangle proxy method via combining and merging storminess time series from numerous partially overlapping triangles in an ensemble-like manner. The utilized approach allows for the construction of robust, long-term and subdaily German Bight storminess time series. Further, the method provides insights into the underlying uncertainty of the time series.

The results show that storm activity over the German Bight is subject to multidecadal variability. The latest decades are characterized by an increase in activity from the 1960s to the 1990s, followed by a decline lasting into the 2000s and below-average activity up until present. The results are backed through a comparison with reanalysis products from four datasets, which provide high-resolution wind and pressure data starting in 1979 and offshore wind speed measurements taken from the FINO-WIND project. This study also finds that German Bight storminess positively correlates with storminess in the North-East Atlantic in general. In certain years, however, notably different levels of storm activity in the two regions can be found, which likely result from shifted large-scale circulation patterns.

D3279 |
EGU2020-7199
Christian Passow, Uwe Ulbrich, and Henning Rust

Scientific work on European windstorms mainly focused on local damages, location (tracks), temporal evolution or the overall severity, often measured by severity indices of different definitions. Each of the aforementioned windstorm properties is directly related to important characteristics within the windstorm itself, such as wind speed, duration, spatial extent or internal variablity. Variation or changes within these characteritics are therefore defining aspects in the spatial and temproal evolution of windstorm or their overall severity in general. As a step towards a better understanding of such variations, we intend to classify windstorms based on their characteristics. For this purpose, we categorize individual storms based on their characteristics using a K-Means clustering procedure. As a result, we get a catalog of more than 400 storm tracks, each track having properties similar to the 20 most severe storm events in the European region. In an attempt to better understand driving mechanisms behind severe European windstorms, the catalog will be further examined to find key parameters that determine the cluster characteristics, such as large-scale situations or the sequencing of clusters.

D3280 |
EGU2020-8925
Juan José Gómez-Navarro, Enrique Pravia-Sarabia, and Juan Pedro Montávez

Medicanes are small-scale cyclones with tropical characteristics that take place in the Mediterranean basin, showing hazardous features such as intense wind gusts and precipitation. Our ability to predict their consequences is of great importance for those cases of medicanes reaching coastal inhabited areas. Succeeding in a precise prediction of their characteristics is heavily subject to getting insight in the fundamental factors that are involved in their genesis, strengthening and maintenance. Given their small nature compared to the synoptic scale, RCMs are specially suitable for the simulation of these storms. However, when using RCMs, there are a number of configurations that must be controlled to specify the way the different physical and chemical mechanisms are solved during the simulation.


In this work, we evaluate the role of three different factors affecting the outcome of WRF, namely the run-up time, the inclusion or not of the on-line simulation of aerosols and the use of spectral nudging. To that end, six different medicanes have been simulated combining different possibilities for the aforementioned factors, resulting in a set of above 360 simulations. Although in principle the on-line simulation of aerosols is expected to have the strongest impact in the simulation of medicanes, it turns out that the run-up time -time delay from the simulation start to the medicane maximum intensity moment- is far more decisive in their successful development than the former. The results are also sensible to the use of spectral nudging, and the three considered factors end up having a considerable impact. Indeed, whereas the majority of their combinations lead to an erratic reproduction of the observed medicanes, there exist some combinations that allow reasonable results, showing that these configurations are in fact interdependent, i.e., the change in the simulation outcome due to a different configuration for one of the factors is dependent on the configuration of the others. This complicates the assessment on the influence of one factor alone, but facilitates gaining insight on the factors that control the genesis and maintenance of medicanes.

D3281 |
EGU2020-10991
Lisa Degenhardt, Gregor Leckebusch, and Adam Scaife

Severe Atlantic winter storms are affecting densely populated regions of Europe (e.g. UK, France, Germany, etc.). Consequently, different parts of the society, financial industry (e.g., insurance) and last but not least the general public are interested in skilful forecasts for the upcoming storm season (usually December to March). To allow for a best possible use of steadily improved seasonal forecasts, the understanding which factors contribute to realise forecast skill is essential and will allow for an assessment whether to expect a forecast to be skilful or not.

This study analyses the predictability of the seasonal forecast model of the UK MetOffice, the GloSea5. Windstorm events are identified and tracked following Leckebusch et al. (2008) via the exceedance of the 98th percentile of the near surface wind speed.

Seasonal predictability of windstorm frequency in comparison to observations (based e.g., on ERA5 reanalysis) are calculated and different statistical methods (skill scores) are compared.

Large scale patterns (e.g., NAO, AO, EAWR, etc.) and dynamical factors (e.g., Eady Growth Rate) are analysed and their predictability is assessed in comparison to storm frequency forecast skill. This will lead to an idea how the forecast skill of windstorms is depending on the forecast skill of forcing factors conditional to the phase of large-scale variability modes. Thus, we deduce information, which factors are most important to generate seasonal forecast skill for severe extra-tropical windstorms.

The results can be used to get a better understanding of the resulting skill for the upcoming windstorm season.

D3282 |
EGU2020-11149
Michael Angus and Gregor Leckebusch
The inter-annual variability of the European windstorm season is dependent on a number of large-scale climate drivers and conditions, for example the North Atlantic Oscillation. For seasonal forecasts to provide valuable information to decision makers about the potential severity of the winter windstorm season, they must capture this relationship between large-scale climate drivers and seasonal windstorm frequency in advance. Here, we examine the performance of the latest state of the art ECMWF seasonal forecast product (SEAS5) in capturing this climate response. We apply a statistical model previously shown to well reproduce the explained behaviour of European windstorms from large-scale climate drivers (Walz et al. 2018) to SEAS5, and examine the choice of statistically significant drivers. The model applied is a stepwise Poisson regression approach to account for serial clustering within inter-annual variability of windstorms, the resultant of which categorizes each windstorm season as either active, neutral or inactive. In particular, we focus on the European region where the explained variance of the statistical model in observations is highest (Walz et al. 2018), the British Isles. In addition to comparing the performance of the model in SEAS5 and in observations, we examine which relationships are not recreated in the seasonal forecast successfully from a dynamical perspective, to provide further insight into the current ability of seasonal forecasts to represent European windstorm inter-annual variability.
 
Reference:
Walz, M. A., Befort, D. J., Kirchner‐Bossi, N. O., Ulbrich, U., & Leckebusch, G. C. (2018). Modelling serial clustering and inter‐annual variability of European winter windstorms based on large‐scale drivers. International Journal of Climatology38(7), 3044-3057.
D3283 |
EGU2020-13780
Vered Silverman, Shira Raveh-Rubin, and Jennifer Catto

Air-sea interaction in the midlatitudes is modulated by the passage of extratropical cyclones and their trailing fronts. Particularly strong ocean heat loss (both sensible and latent) is observed in the post-cold frontal region. In this region, airmasses within the dry intrusion (DI) airstream descend slantwise from the upper troposphere towards the cold trailing front. As the cyclone case-to-case variability is high, understanding the co-occurrence of DIs, cold trailing fronts and cyclones is important for understanding the variability of surface fluxes, especially in regions not usually associated with frequent frontal activity.

 

A climatological study quantifying the co-occurrence of fronts and DIs (Raveh-Rubin and Catto, 2019) found the presence of DIs to be associated with stronger surface heat fluxes. Here the climatological study is extended to account for the cyclone life-cycle by using feature-based identification and tracking in the ERA-Interim dataset, for the 1979-2018 winters. We focus on the relationship between extratropical cyclone characteristics, DIs and cold fronts, their co-evolution throughout the lifetime of a cyclone, and consequently their impact on air-sea interaction.

 

We show that 65-80% of the extratropical cyclones in the storm track region are matched with DIs, mainly during the early stages of the intensification period. Furthermore, cyclones associated with DIs are longer lived, induce up to 50% stronger precipitation in the frontal regions, and up to 60% stronger evaporation, especially in the DI region of influence, compared to non-DI cyclones. These transient events of strong evaporation induced by DIs account up to 40% of the observed climatology, demonstrating the significant role transient weather systems play in the air-sea interaction, at times through a fairly remote influence of the cyclones.

D3284 |
EGU2020-11316
Natália Machado Crespo, Rosmeri Porfírio da Rocha, and Eduardo Marcos de Jesus

Cyclones developing over and at the eastern coast of South America impact extreme events over the region. Understanding the present climate is crucial to assess future extremes tendencies, which are important for engineering constructions over the southeast Brazil basin. To evaluate these systems in climate change scenarios it is important to study their preferred region of formation and trajectories in the present climate. Therefore, in this study we tracked cyclones in a period from 1979 to 2018 (present climate) using different reanalyses dataset (CFSR, ERA-Interim and ERA5), pointing out the main cyclogenetic regions affecting South America and discussing the main differences between the different dataset. As a preliminary result, the cyclone tracking shows a higher number of systems in CFSR than in ERA-Interim, which would be explained by the finer resolution of CFSR.  Annually, this difference is about 6%, and seasonally, the difference is smaller in summer (3.5%) and similar (~7%) for the other seasons. The reanalyses identify basically the same four cyclogenetic regions, however, there are differences in the density center position. Other features as lifetime, intensity, traveled distance, and wind extremes associated with the cyclones will be also discussed.

D3285 |
EGU2020-16638
Jens Grieger, Mareike Schuster, Christopher Kadow, Andy Richling, and Uwe Ulbrich

This study analyzes the representation of the extratropical circulation over the North Atlantic (NA) region using the German decadal prediction system (MiKlip) of two different spatial resolutions. Four quantities are assessed, i.e. the storm track, blocking frequencies, cyclone frequencies, and windstorm frequencies. We investigate the effect of model initialization for the representation of the circulation in a lower resolution (LR, atm: T63L47, ocean: 1.5° L40) and higher resolution version (HR, atm: T127L95, ocean: 0.4° L40) of the decadal prediction system.

While LR shows common deficits in the climatological representation in both the initialized prediction system and the uninitialized historical projection, e.g. an overly zonal extratropical storm track and a deficit in blocking frequencies over the North Atlantic and Europe, the higher resolution version counteracts these biases. The initialized LR prediction system largely overestimates NA cyclone frequency, which is not the case for the uninitialized LR counterpart. This positive bias is mainly due to weak and short lived systems and is an effect of the initialization in the LR prediction system. Similar biases cannot be identified in the windstorm frequency which implies that the short lived cyclones are low of impact with respect to wind speed.

The initialization effect leading to an overestimation of weak and short lived cyclones cannot be found in the HR version. The overall better representation of the extratropical circulation in the HR version leads to an increased decadal prediction skill, which is measured in terms of anomaly correlation, with the increase in resolution for all four quantities. 

D3286 |
EGU2020-19358
Thomas Cropper and Stephanie Allen

Using the criterion of one Bergeron (24 hPa change over 24 h at 60°), we present the creation of a Eulerian explosive cyclogenesis climatology using hourly-temporal resolution data from the European Centre for Medium Range Weather Forecasting’s ERA5 reanalysis (1979-2018). This approach differs to the typically used Lagrangian methodologies adopted by many studies. The climatology created by this approach results in similar patterns to previous studies.

Assessments on the dataset are undertaken to analyse the influence of seasonality, teleconnections, climate change and individual events (the method picks up tropical cyclones as well as mid-latitude storms). The location experiencing the most consistent explosive cyclongenesis conditions (15% of the time during the Northern Hemisphere winter) is to the east of the Avalon Peninsula, Newfoundland. The preferred location of explosive cyclogenesis is shown to change in relation to patterns such as the El Niño Southern Oscillation and North Atlantic Oscillation. Potential applications of the dataset are suggested.

                                                

D3287 |
EGU2020-21874
Ana Gonçalves, Margarida L. R. Liberato, Alexandre M. Ramos, and Raquel Nieto

The occurrence of an increasing number of high impact storms over southwestern Europe (e.g. Klaus, 23-24 January 2009 and Xynthia, 27-28 February 2010; Liberato et al. 2011; 2013) has led to the meteorological services of France (Météo-France), Portugal (IPMA) and Spain (AEMET) to assign names to storms, since 1st December 2017. This new list of named storms has the main objective to better inform the general public and media while contributing to increasing public awareness to high impact storms and associated warnings and timely safety recommendations. The Institute of Meteorology of the Freie Universität Berlin has named all pressure systems in Central Europe since 1954; since 1998, lows are given male names and highs are given female names in odd years, and vice versa in even years. This new list built by the southwestern Europe meteorological services has the main difference of naming only high impact storms.

In this study an analysis of the extreme storms affecting the Iberian Peninsula during the 2017-2019 extended winters is performed. From nine named high impact storms during the 2017-2018 season, seven of them affected the Iberian Peninsula region; and from the thirteen high impact storms during 2018-2019 winter, ten of them affected the region. Firstly an assessment of the strong winds, heavy precipitation and socio-economic impacts is presented. Secondly, a characterization of the synoptic conditions and associated extratropical cyclones is performed. Finally, the events are ranked and classified into the groups previously defined on Karremann et al. (2016) and a variability assessment is made in order to understand how their magnitude and frequency of occurrence fits the identified multi-decadal variability.

Acknowledgements

The authors would like to acknowledge the financial support by Fundação para a Ciência e a Tecnologia, Portugal (FCT), through projects PTDC/CTA-MET/29233/2017 and UIDB/50019/2020 – IDL. A.M. Ramos is supported by Scientific Employment Stimulus 2017 from FCT (CEECIND/00027/2017).

References

Karremann et al. (2016) Atmos. Sci. Let., 17: 354-361 DOI: 10.1002/asl.665

Liberato et al. (2011) Weather, 66: 330-334 DOI: 10.1002/wea.755

Liberato et al. (2013) Nat. Hazards Earth Syst. Sci., 13: 2239-2251 DOI: 10.5194/nhess-13-2239-2013

 

D3288 |
EGU2020-12813
Yuan-Bing Zhao

Using a recently developed methodology, namely, the multiscale window transform (MWT), and the MWT-based theory of canonical transfer and localized multiscale energetics analysis, we investigate in an eddy-following way the nonlinear eddy-background flow interaction in the North Pacific storm track, based on the ERA40 reanalysis data from ECWMF. It is found that more than 50% of the storms occur on the northern flank of the jet stream, about 40% are around the jet center, and very few (less than 5%) happen on the southern flank. For storms near or to the north of the jet center, their interaction with the background flow is asymmetric in latitude. In higher latitudes, strong downscale canonical available potential energy transfer happens, especially in the middle troposphere, which reduces the background baroclinicity and decelerates the jet; in lower latitudes, upscale canonical kinetic energy transfer intensifies at the jet center, accelerating the jet and enhancing the middle-level baroclinicity. The resultant effect is that the jet strengthens but narrows, leading to an anomalous dipolar pattern in the fields of background wind and baroclinicity. For the storms on the southern side of the jet, the baroclinic canonical transfer is rather weak. On average, the local interaction begins from about 3 days before a storm arrives at the site of observation, achieves its maximum as the storm arrives, and then weakens.

D3289 |
EGU2020-3458
Meiji Honda, Satoru Kasuga, Jinro Ukita, Shozo Yamane, Hiroaki Kawase, and Akira Yamazaki

Cutoff lows are cyclones existing in the upper troposphere developing from precursory preexisting troughs. We introduce a new method to seamlessly detect cutoff lows and even preexisting troughs aiming to improve lead time of meso scale disturbances like tornadoes. The method is based on a geometric character; in this method, a slope defined as the tangential line from a minimum point of each height depression is measured on an isobaric surface. This slope evaluates an intensity and horizontal extension (radius) of each depression. Adopting a mathematical assumption, we successfully achieved to make an algorithm to separate the depression and the local background flow. To remove the background flow enables us to detect both cutoff lows and preexisting troughs seamlessly in reanalysis height fields. So, our method would allow the life cycle to be illustrated continuously from the birth of the cutoff low, that is, from the precursory preexisting trough, and is expected to contribute to the improvement of the lead time for predicting severe weathers. Some further application examples, including tornado accompanying cases, and even for blocking highs, would be shown.

D3290 |
EGU2020-4307
Joonsuk Kang and Seok-Woo Son

A method utilizing a prognostic potential vorticity (PV) inversion is designed and applied to quantify the processes that contribute to the explosive cyclone (EC) development over Northwestern Pacific and Atlantic in boreal winter. The ECs deepening in the two remarked regions are identified and tracked, by using the automated tracking method on ERA-Interim reanalysis data over the period of 1979–2017. The quantification process first involves time differentiation of linearized potential vorticity (PV), which results in a linear function of geopotential height tendency. It is then equated with the PV tendency equation that consists of mean and transient advection terms to represent dynamical processes that contribute to EC development. The quantification, finally, is performed through the inversion of PV tendency budgets, which yields corresponding geopotential height tendency. The results indicate that EC development is primarily caused by zonal advection of PV anomalies by mean flow (~65%) and diabatic production of PV (~40%), with some negative factors in both regions. The former contributes more for ECs deepening over Northwestern Atlantic (~71%) than Northwestern Pacific (~60%), whereas the latter contributes to a similar extent.

D3291 |
EGU2020-9235
Shenming Fu, Lizhi Jiang, and Jianhua Sun

The explosive cyclone (EC), which is the most destructive subcategory of the extratropical cyclone, has been a research center for decades. Many key features of this type of cyclone have been shown, however, as a three-dimensional system, their vertical extents and associated important characteristics still remain vague. This study attempts to fill this vacancy by focusing on ECs’ vertical extents related features in the Northern Hemisphere on the basis of the ERA-I reanalysis data during a 40-yr period. Some new findings are reached: (i) overall, the EC is a type of deep weather system, as more than 63% of them reach an upmost level above 300 hPa, whereas only less than 12% of them maintain below 500 hPa during their whole life spans. (ii) ECs’ vertical extents show remarkable latitude dependent features (maximum vertical extents appear in the zone of 55oN-65oN), and they also show obvious seasonal changes, with the minimum vertical extents appeared in January. (iii) ECs’ maximum vertical extents show a significant positive correlation with their minimum central pressure, whereas, their maximum vertical extents show no obvious relationship to the ECs’ maximum deepening rates and maximum 10-m winds. (iv) in general, ECs over the northern Pacific Ocean have larger intensity, longer life spans, and thicker vertical extents than those of the ECs over the northern Atlantic Ocean.