The long-lasting hot and dry conditions in summer 2022 in Europe are an example of episodic persistence of surface weather on a sub-seasonal timescale that resulted in high impacts. Similarly, the series of atmospheric rivers that reached the US west coast in winter and spring 2023 are an example of recurrent circulation features which also resulted in persistent surface weather on a sub-seasonal timescale. We will first present a short overview and a classification of methodological (statistical and machine learning) approaches that capture and quantify these two flavours of persistence – stationarity and recurrence. We will illustrate the application of some methods to the European summer circulation and discuss their strengths and limitations.

We will then discuss the large-scale circulation features associated with persistent hot spells and recurrent heavy precipitation for selected areas in Europe using the ERA-5 data set.  The first example are three-week hot spells over both northern Europe and western Europe. They are associated with a change in the jet latitude upstream over the Atlantic prior to the events from a northward shifted to a strong zonal jet at the beginning of the events. The second example are recurrent heavy precipitation events in summer over the Western Atlantic. They occur in the left exit region of the jet and are associated with a positive cyclone frequency anomaly, a trough over the western Atlantic and significant positive IVT anomalies. Both examples illustrate an important role of the jet stream (and hence the circulation) over the North Atlantic for sub-seasonal persistence in summer in Europe prior to and during the events.        

How to cite: Martius, O., Tuel, A., Pappert, D., and Banderier, H.: Characterising sub-seasonal weather persistence -- an illustration on summer European circulation, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-51, https://doi.org/10.5194/ems2023-51, 2023.

Rossby Wave packets (RWPs) are atmospheric perturbations located at the upper troposphere of mid-latitudes which, sometimes, terminate in Rossby Wave Breaking (RWB) events. If RWB events reach certain spatial and temporal scale, they are considered to be synoptically identical to atmospheric blockings, which are linked to heatwaves and droughts. Thus, understanding RWB events linked to RWPs propagation and their link with atmospheric blocking development is key to enhance extreme weather events detection 10-30 days in advance. Hence, in this study we assess the occurrence of RWB events after the propagation of short-lived RWPs (RWPs with a lifespan below or equal to 8 days, or SLRWPs) or long-lived RWPs (RWPs with a lifespan above 8 days, or LLRWPs), whether SLRWPs or LLRWPs are linked to large-scale RWB events that could form an atmospheric blocking event, and, lastly, study the proportion of blocking situations that occur near RWB events. To do so, we applied a tracking algorithm to detect RWPs in the Southern Hemisphere during summertime between 1979-2020, developed a wave breaking algorithm to identify and follow RWB events, and searched for atmospheric blocking events with different intensities. Results show that LLRWPs and SLRWPs displayed large-scale RWB events around 40% of the time, which tend to last around 1-2 days, which is not sufficient to identify them as blocking situations. Nearly 17% of blockings show a RWB event nearby whenever they manifest, but barely 5% of blockings are linked to RWB caused by RWPs propagation, therefore, propagating RWPs do not seem to be strongly linked to atmopsheric blocking development. Lastly, the occurrence of large-scale RWB events associated with SLRWPs are influenced by El Niño-Southern Oscillation and the Southern Annular Mode.

How to cite: Perez, I., Barreiro, M., Ehstand, N., Hernández-García, E., and López, C.: Wave Breaking Events and their link to Rossby Wave Packets and Atmospheric Blockings during Southern Hemisphere Summer, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-56, https://doi.org/10.5194/ems2023-56, 2023.

Episodes of atmospheric blocking over Southeastern Pacific and South Atlantic oceans significantly affect the weather over South America. In Brazil, blocking events have been associated with heat waves and droughts or abnormally dry periods. Since there is no unique definition of atmospheric blocking, different objective methods for identifying and quantifying atmospheric blocking have been developed in recent decades, primarily targeting Northern Hemisphere blocks. For this reason, the observed frequency and characteristics of blocking depend on the method adopted, leading to changes in the resultant blocking climatology. In the Southern Hemisphere, studies addressing atmospheric blocking are still scarce. More specifically, there is no study assessing the reliability of the different identification methods in the Southern Hemisphere. To address this gap, this work examines blocking properties produced by five different objective detection methods over the Southeast Pacific and Southern Atlantic oceans, using ERA5 data. The five indexes used in this study rely on the 500 hPa geopotential field and were developed by modifying the approaches employed in earlier studies by Lejeñas (1984), D’Andrea et al. (1998), Schwierz et al. (2004), Davini et al. (2012), and Mendes et al. (2022). The study also examines the variations in the blocking climatologies that arise from using different objective detection methods. Additionally, in order to evaluate the accuracy of each objective method applied, the results are compared with observed blocking episodes. Moreover, atmospheric fields are analyzed to verify if a synoptic blocking pattern was present during the objectively identified blocking events. 

References:

D’Andrea, F. et al. Northern Hemisphere atmospheric blocking as simulated by 15 atmospheric general circulation models in the period 1979–1988. Climate Dyn., 14 (6), 383–407, 1998.

Davini, P. et al. Bidimensional diagnostics, variability, and trends of Northern Hemisphere blocking. Journal of Climate, v. 25, n. 19, p. 6496-6509, 2012.

Lejenãs, H. Characteristics Of Southern Hemisphere Blocking As Determined From A Time Series Of Observational Data. Q. J. Roy. Meteor. Soc., V.110, P.967-979, 1984.

Mendes, M.C.D. et al. Synoptic–dynamic indicators associated with blocking events over the Southeastern Pacific and South Atlantic oceans. Climate Dynamics, p. 1-17, 2022.

Schwierz C. et al. Perspicacious indicators of atmospheric blocking. Geophys Res, 2004.

How to cite: Ferreira, V., Maroneze, R. M., Toledo Bonfim, O., and Mortarini, L.: An evaluation of different objective detection methods of atmospheric blockings over Southeastern Pacific and South Atlantic oceans: impact on the blocking climatology, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-352, https://doi.org/10.5194/ems2023-352, 2023.

How to cite: Köhler, D. and Sinclair, V.: The impact of changing sea ice on the mid-latitude circulationand extra-tropical cyclones, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-136, https://doi.org/10.5194/ems2023-136, 2023.

Weather extremes in Europe and the Mediterranean can have a significant effect on societies across the region. These weather extremes include heatwaves, cold spells, windstorms, and intense rainfall events. In a climate that is continuously changing, their impact, both socio-economically and environmentally, can worsen with an increase in the frequency and intensity of these extremes. As recent studies have highlighted, some extremes such as heatwaves in the northern hemisphere are connected to atmospheric blocking, while heavy precipitation has been associated with intense cyclones occurring in the Mediterranean basin which are more commonly known as Mediterranean Cyclones (MCs). However, the connection between atmospheric blocking and MCs is still understudied, despite evidence suggesting their mutual importance for exacerbating and synchronizing surface extremes. The aim of the present study is the systematic investigation of how often MCs develop downstream of atmospheric blocks over the Euro-Atlantic region, and how the Atlantic storm track is modulated under such conditions. To this end we employ the combined ‘best tracks’ MCs dataset with objectively identified (using potential vorticity anomalies) blocking features in ERA5 for the 1980-2020 period. We find that in the presence of atmospheric blocks, MCs that develop downstream tend to be more intense and more persistent than other MCs. Moreover, MCs that develop under this scenario form particular subsets of MCs, with preferred seasonality and geographical distribution, compared to all MCs. The results have important implications for the predictability of MCs and their various impacts in the region on both weather and climate time scales.

How to cite: Loizou, P. and Raveh-Rubin, S.: Atmospheric blockings and downstream cyclones in the Euro-Mediterranean sector, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-39, https://doi.org/10.5194/ems2023-39, 2023.

Extra-Tropical Cyclones (ETC) cause most of the variability in weather in Europe and are associated with strong winds, heavy rainfall and storm surges which cause damage to infrastructure and can even lead to human fatalities. Although ETCs vary considerably from one system to another, the classification of ETCs based on their common features is a standard practice in meteorology. Classifications simplify the study of ETCs and thus improve understanding and predictability of them. Due to a large number of factors affecting the development and structure of ETCs, various classifications have been established. ETC classifications are based on e.g. conceptual or idealized models, forcing factors, dynamical and synoptic features, or impacts. In this study we construct a dataset of dynamical and impact-based measures of ETC intensity. The aim is to create a classification of ETCs by using this dataset as input in a cluster analysis.

ETCs and their intensity measures were studied using ERA5 reanalysis data from 1979 to 2022. Only the cold season (October--March) was considered in the analysis since that is when the strongest ETCs occur most often. ETC tracks were produced by using 850-hPa relative vorticity as input to the feature tracking software TRACK. To focus on the most relevant ETCs affecting Europe, only tracks in the North Atlantic and Europe were included and stationary and short-lived systems were excluded. Analysed intensity measures included 850-hPa relative vorticity, Mean Sea Level Pressure (MSLP), 850-hPa, 925-hPa and 10-m wind speed, 10-m wind gust, instantaneous Storm Severity Index (SSI), accumulated SSI and wind footprint. Sparse Principal Component Analysis (PCA) was performed to analyse the dependency of the intensity measures on one another and to select input features for the cluster analysis. The cluster analysis was performed using Gaussian Mixture Modelling (GMM).

Sparse PCA with four principal components indicated that all wind speed measures were grouped together as the first component and 850-hPa wind speed was grouped with 850-hPa relative vorticity as the third component. Wind footprint and MSLP were alone in the second and fourth components, respectively. The SSI measures were not represented, which means they did not explain variance in the dataset but formed a separate group of features. By including 850-hPa relative vorticity, MSLP, 850-hPa wind speed, wind footprint and instantaneous SSI, each group of features was represented in the cluster analysis. The number of clusters in the GMM was increased incrementally by one until each cluster was clearly separate from the others in at least one dimension. With seven clusters, weak ETCs, ETCs with high SSI, and ETCs with similar vorticity, MSLP and wind speed but unequal wind footprint and vice versa, were able to be separated into their own classes and their characteristics investigated.

How to cite: Cornér, J., Bouvier, C., and Sinclair, V.: Classification of extra-tropical cyclones with cluster analysis based on measures of intensity, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-315, https://doi.org/10.5194/ems2023-315, 2023.

Idealised simulations of extra-tropical cyclones (ETCs), often referred to as baroclinic life cycles, have previously been extensively used to understand ETC dynamics. More recently, baroclinic life cycle simulations have proved useful to investigate how ETCs may respond to climate change. However, these simulations are often difficult to set up and previous studies that have used baroclinic life cycle simulations to understand cyclone dynamics often include only a handful of different initial background states. Here we introduce a flexible set up for both dry and moist baroclinic life cycles that can be run with and without physics that has been implemented successfully into OpenIFS. A key strength of this set-up is that via the namelist a user can control many aspects of the background state: the height, width and strength of the jet, the average temperature, the environmental lapse rate, the surface relative humidity and the surface roughness. Using this set-up we have performed a huge ensemble (more than 5000 members) of baroclinic lifecycle simulations with the aid of climateprediction.net, a volunteer computing, climate modelling project coordinated by the University of Oxford. Using machine learning techniques we investigate how the intensity, as quantified by a range of metrics, depends on the prescribed background state. Furthermore, we attempt to physically understand the statistical relations between cyclone intensity and the background state by computing energy budgets and the energy efficiency of the simulations. These results concerning controls on ETC intensity will be presented and interpreted with regards to how ETC intensity may change in the future as the climate warms.

How to cite: Bouvier, C., van den Broek, D., Ekblom, M., and Sinclair, V.: Identifying controls of the intensity of extra-tropical cyclones using a massive ensemble of baroclinic life cycle simulations., EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-600, https://doi.org/10.5194/ems2023-600, 2023.

Even though tropical cyclones are well documented from the moment they materialise to the moment they evanesce, many physical processes governing them are not resolved by global circulation models (GCMs). Thus, the simulation and the projection of future changes in tropical cyclones characteristics remain challenging. In particular, due to the low spatial resolution of most GCMs, studying simulated tropical cyclones is difficult because they are hard to detect. Some cyclone detection schemes are now available but their results are subject to the spatial resolution of the simulations and  usually need numerous variables that are not always available in the models. In this study, the ability of dynamical system metrics to detect atmospheric situations related to cyclones in coarse-resolution data-sets, where maximum wind speed and minimum sea-level pressure may not be accurately represented, is explored.

Our analysis is based on 613 cyclones occurring in Tropical Atlantic over the 1980-2021 period and extracted from the IBTrACS database. Three dynamical system metrics, the local dimension d, local persistence θ and the co-recurrence ratio α, are computed from mean sea level pressure and the total column water of the ERA5 reanalysis dataset. They respectively characterise the number of degrees of freedom of the atmospheric flow, its stability and the  coupling between different atmospheric variables. In the preliminary results, atmospheric situations corresponding to cyclones stand out from other situations. This preliminary result shows potential for detection and attribution studies in climate change context.

How to cite: Vaittinada Ayar, P., Faranda, D., and Vrac, M.: Physics-informed detection of tropical cyclonesbased on dynamical systems metrics, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-411, https://doi.org/10.5194/ems2023-411, 2023.

Atmospheric blocking describes a quasi-stationary weather pattern in midlatitudes which is characterised by a disruption of the westerly flow. Within the blocking anticyclone, large-scale subsidence of air effects dissipation of clouds. Intensive solar radiation during summer leads to a positive radiation balance causing heatwaves which are intensified by adiabatic warming of subsiding air. In case of longer persistence of the anticyclone, dry episodes can extend to droughts which intensifies the heat due to reduced latent heat flux. In this study, we investigate the relationship between blocking and heatwaves in Germany in reanalyses and a subset of selected CMIP6-Simulations. Using the heatwave definition and heatwave magnitude index by Russo et al. (2014), we find that heatwaves are related to the occurrence of blocking in Central Europe. The probability of heatwave occurrence is round about the factor 10 higher with blocking than without blocking. To quantify the value of blocking as predictor for heatwaves, the Heidke Skill Score (HSS) is calculated. The HSS shows that blocking gives an additional value on predicting heatwaves. Strong heatwaves (with a magnitude > 1.5 following Russo) have even a higher relationship to blocking. However, the HSS gives lower values compared to the consideration of all heatwaves. This is related to the low number of days with strong heatwaves compared to the high numbers of blocking days leading to a high number of false alarms. Comparing the relationship of blocking and heatwaves between reanalyses and historical CMIP6-Simulations yields that the models can reproduce the results of the reanalyses in general for all heatwaves. For strong heatwaves, some models underestimate the influence of blocking. Furthermore, we show that no model is simulating such a strong heatwave as 2003 occurred in Central Europe.

How to cite: Lohmann, R. and Ahrens, B.: On the Relationship between Atmospheric Blocking and Heatwaves in Germany, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-202, https://doi.org/10.5194/ems2023-202, 2023.

Recent research has established a statistical link between North American cold spells (CS) and concurrent wet or windy extremes in Europe. Here, we investigate whether such a link can be related to changes in the characteristics of north Atlantic extratropical cyclones (hereafter cyclones). Despite large regions of anomalous baroclinicity during periods of North American CS, the number of cyclones across the north Atlantic as a whole is comparable to climatology. We however find that CS over different North American regions are associated with different large-scale atmospheric configurations, that enhance the jet stream in the vicinity of the CS but result in different upper level wind anomalies in the east Atlantic. These in turn modulate the regional distributions and characteristics of cyclones in the north Atlantic and Europe. For eastern Canada CS, the north Atlantic jet extends over northern Europe, with the Atlantic storm track as a whole experiencing more intense cyclones which then affect France, the British Isles and Northern Europe. For eastern United States CS, the jet is displaced equatorward and extends over southern Europe, with a significantly heightened number of cyclones affecting Iberia, France and Southern Europe. For central Canada CS, the jet is displaced north of climatology and only partly extends over Europe, resulting in an anomalously high cyclone density in the eastern Atlantic and a higher number of cyclones affecting Iberia only. For eastern Canada and eastern United States CS, we observe a significantly heightened occurrence of cyclones undergoing explosive deepening. Our results provide a dynamical explanation for previous statistical findings on the concurrence of north American CS and wet or windy extremes over Europe.

How to cite: Leeding, R., Riboldi, J., and Messori, G.: North American Cold Spells Modulate North Atlantic Extratropical Cyclones and Extreme Weather in Europe, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-215, https://doi.org/10.5194/ems2023-215, 2023.

Extremes in extratropical meridional energy transports in the atmosphere are associated with the dynamics of the atmosphere at multiple spatial scales, from planetary to synoptic. This is related to the nature of amplifying baroclinic waves, that are fundamentally intermittent and sporadic, significantly affecting the net seasonal transport across latitudes. In this work, we take advantage of a zonal wavenumber decomposition of meridional energy transports throughout the Northern Hemisphere mid-latitudinal channel to investigate how extreme transports affect the main modes of circulation, namely weather regimes identified through k-means clustering of 500 hPa geopotential height and sea-level pressure fields. We find that, in general, planetary-scale waves determine the strength and meridional position of the synoptic-scale baroclinic activity with their phase and amplitude. During winter, largest wavenumbers (k = 2–3) are key drivers of the meridional-energy-transport extremes, and planetary- and synoptic-scale transport extremes virtually never co-occur. During summer, extremes are associated with higher wavenumbers (k = 4–6), identified as synoptic-scale motions. We look further into the implications of such extreme transports by looking at heat waves and cold spells over continents in the Northern Hemisphere, as well as proxies of droughts, investigating the role of moisture transports and the moist component in static energy transport through the signature of changes into evapo-transpiration and related latent heat fluxes at the land surface. Characterizing the clusters of extreme transport events in space and time, the persistence and co-existence of heat/cold waves/spells and drought over specific regions is investigated in light of the dominant modes of atmospheric dynamics evidenced through the wavenumber decomposition.

How to cite: Lembo, V., Fabiano, F., Galfi, V. M., Graversen, R. G., and Messori, G.: The signature of extreme meridional eddy energy transports on modes of circulation, heatwaves and droughts in Northern Hemisphere mid-latitudes, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-579, https://doi.org/10.5194/ems2023-579, 2023.

Drought is a highly intricate hazard that can be challenging to define and detect. Its impact can be felt across numerous sectors and timeframes. The configuration of La Nina conditions is linked to drought conditions in Southern and Southeast Brazil, especially during late spring and summer months. Since late 2019, two successive La Nina events have resulted in various degrees of drought in the southern region of Brazil, causing significant effects on multiple sectors such as agriculture, water resources, and energy production. The 3-month Standardized Precipitation Index (SPI) timescale was utilized to define and characterize the drought conditions. In this study, a drought event starts when the analyzed indicator falls below a certain negative standard deviation value (e.g. SPI-3 < -1) for at least two consecutive months, and ends when the indicator rises above 0, following the methodology described in Spinoni et al. (2019). The aim of this work is to examine and compare the differences between the two drought episodes that occurred during the spring and summer of 2019/2020 and 2020/2021, respectively, and to understand the variations in their severity. Four different sources of monthly precipitation data were used: the ERA-5 global reanalysis dataset, CHIRPS and PERSIANN satellite datasets, and automated weather stations from INMET. A comparison is performed among these four datasets to identify if there is an impact in drought characteristics, such as area, duration and intensities. This study also explores the impact of large-scale circulation and sea surface temperature indices, namely El Nino 3.4, Indian Ocean Dipole (IOD), South Atlantic Subtropical Dipole mode (SASD), and South Atlantic Ocean Dipole mode (SAOD), on the two recent drought episodes.

How to cite: Toledo Bonfim, O., Ferreira, V., Maroneze, R., and Mortarini, L.: The 2019/2020 and 2020/2021 droughts events in Southern and Southeastern Brazil, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-457, https://doi.org/10.5194/ems2023-457, 2023.

Abstract: Local storms can often generate extreme rainfall of short duration that causes immediate and serious flooding in the local scale. The genesis and rapid development of such storms are a result of complex processes that occur in the dynamics of the storm. In this study are examined the Automatic Weather Station's rainfall data and reanalyses from NWP fields for Shkodra city to search the causes of such extreme rainfall in such a short time. The extreme precipitation generally is created as a result of high vertical instability of a air mass very rich in moisture. While the extreme rainfall of short duration at local scale is mainly related to the convective cells and local thermodynamic factors like the vertical profile of temperature and humidity, depending to the local atmospheric circulation. A retrospective analyses regarding the extreme rainfall of short duration at the local scale may provide a detailed picture of the extreme rainfall behaviour occuring in that area for short time intervals of 1, 3 , 6 up to 24 hours. The analyses of the events of short duration extreme rainfall at a local scale for a long term period may provide good results regarding the estimation of the future risk of the sudden local scale floods for the urban areas. Pairing the incidence of local flash floods with the triggers of these events can serve as an additional tool in forecasting local storms that produce short-term extreme rainfall and flash flooding. This study shows that the increased frequency of events with extreme rainfall and the increasing of the rainfall quantities for short time duration intervals is likely to become a prominent feature of the local storms that bring flash floods in the populated and urbanised areas.

Keywords: Extreme rainfall, local flood, vertical profile, convective rainfall

How to cite: Porja, T., Kafexhiu, E., and Peqini, K.: Extreme rainfall of short duration at local scale: case study of Shkodra, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-163, https://doi.org/10.5194/ems2023-163, 2023.

A derecho is a widespread, long-lived, straight-line windstorm that is associated with a fast-moving group of severe thunderstorms known as a mesoscale convective system.

During 18 August 2022, a highly intense and organized convective storm, classified as a derecho, developed over the western Mediterranean Sea affecting Corsica, northern Italy and Austria, with wind gusts up to 62 m/s and giant hail (~11 cm). There were 12 fatalities and 106 people injured. This event received much attention in the media for its extraordinary impact and the rareness over the Mediterranean Sea. The derecho developed over an extreme marine heatwave that persisted during the whole summer. Therefore, the hypothesis of a relationship between the extreme atmospheric event and the extreme marine heatwave rapidly arose, and thus, a possible link with anthropogenic climate change.

This convective event can be considered as extreme from the affected locations point of view (in terms of winds) but also is between one of the most powerful derechos ever recorded in the USA and Europe. Also, the event developed over an extreme marine heatwave that was mainly affecting the western Mediterranean Sea during summer 2022.

Here, by performing model simulations with both the NCAR Model for Prediction Across Scales and the Météo-France nonhydrostatic operational AROME model, we find a relationship between the marine heatwave, the actual anthropogenic climate change conditions, and the development of this extremely rare and severe convective event. We also find a future worrying increase in intensity, size and duration of such an event with future climate change conditions.

How to cite: González-Alemán, J. J., Insua-Costa, D., Bazile, E., González-Herrero, S., Miglietta, M. M., Groenemeijer, P., and Donat, M. G.: Relationship between anthropogenic climate change and the rare extreme derecho convective event over the Mediterranean Sea in summer 2022?, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-333, https://doi.org/10.5194/ems2023-333, 2023.

Winter storms remain a prediction challenge, from the synoptic-scale evolution of the associated mid-latitude cyclone to the formation, location, and intensity of mesoscale snowbands, to the transitions in precipitation type, to terrain-enhanced and lake-effect processes.  Analyzing the predictability of NWP model simulations gives insights to the growth of forecast errors and the underlying dynamical mechanisms for these extreme events.  The recent NASA IMPACTS field campaign, with two coordinated aircraft as well as ground observations across field operations in 2020, 2022, and 2023, provides an unprecedented dataset with which to evaluate winter storm simulations.   In order to analyze the practical predictability of east coast winter storms, operational models as well as convection-allowing ensemble WRF simulations that assimilate conventional and field campaign observations are assessed for several cases during the IMPACTS period.  These simulations are evaluated against conventional surface and radar observations as well as field campaign in-situ thermodynamic conditions and EXRAD radar onboard the ER2. A discussion of optimizing the performance of the data assimilation techniques, as well as evaluating the benefit of assimilating various types of observations is planned. Techniques such as ensemble neighborhood probability can illuminate areas that favor heavy precipitation such as mesoscale snowbands, and predictability horizon diagrams illustrate the timescales over which modeling systems converge on a solution. Finally, errors in precipitation amounts, locations, and types can be related to the representation of dynamical and physical processes in the models, including the thermodynamic environment and hydrometeor evolution in the microphysics scheme.  While the study focuses on examples from the eastern United States, techniques and insights are expected to have broader applicability to winter storms in other regions.

How to cite: Greybush, S., Seibert, J., Zhang, Y., and Kumjian, M.: Assessing Practical Predictability of Winter Storms using NWP and Ensemble Data Assimilation, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-565, https://doi.org/10.5194/ems2023-565, 2023.

In light of the increasing frequency of droughts and extreme fire weather events, in Europe and world wide, the need for accurate wildfire risk estimation is becoming more and more acute. Wildfire risk depends however on complex non-linear interactions between multiple factors such as fuel moisture content, winds, humidity, topography and others.
Traditional fire indices, currently used by weather services, are based on linear models and or empirical and statistical analyses. Consequently their performance is somewhat limited.  Here we propose a novel set of fire weather indices (FWIs), developed using machine learning (ML) – the MLFWI.  We find that the MLFWI significantly outperforms the traditional fire indices in predicting wildfire occurrence, achieving an AUC score of 0.99 compared to 0.62-0.80.
We also analyze the influence of the various factors and their interactions on the models, providing scientific insights and understanding of the mechanism by which the models work. Finally, we compare the performance of the MLFWIs to that of traditional indices in predicting the 100 largest wildfires in the dataset. We find that our models were able to predict the vast majority of these 100 extreme events.
In the talk we will present the Machine Learning methodology and examine the performance of the the MLFWI. The ultimate goal of this research is to allow implementation of  the MLFWI in actual wildfire warning systems. We propose to build upon this study to gradually replace the existing fire weather indices with ML-based indices, which have the potential of substantially improving fire weather alerts.

How to cite: Shmuel, A. and Heifetz, E.: Novel machine-learning-based fire weather indices, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-12, https://doi.org/10.5194/ems2023-12, 2023.

In a relatively coarse resolution atmospheric model, cumulus convective parameterization helps to solve the problem of insufficient precipitation simulation, and also to ameliorate the simulation of diurnal precipitation cycle, which typically displays early appearance of diurnal peak over land. In this study, the new simplified Arakawa-Schubert scheme (NSAS) with a closure of adjusting the cloud function to an observed climate sate, which is coupled in a global non-hydrostatic atmospheric model, i.e. the Yin-Yang-grid Unified Model for the Atmosphere (YUNMA), is used to investigate the impacts of convective scheme on diurnal cycle of precipitation. Two new diagnostic closures and a convective trigger function based on the tropospheric large-scale forcing are suggested, which are introduced into the NSAS scheme for comparison with the original scheme. Numerical results of the 0.25-degree model in 3-month batched real-case simulations reveal the simulated diurnal variation in close agreement with observation by using the revised NSAS convective scheme with a dynamical constraint on the initiation of convection and a tuned threshold of cloud function variation in the YUNMA model. By reducing the occurrence of convection during peak solar radiation hours, the revised scheme is demonstrated to be effective in delaying the appearance of early-afternoon rainfall peaks over most land areas and in accentuating the nocturnal peaks that are wrongly concealed by the stronger afternoon peaks. In addition, the revised scheme enhances the simulation capability of the precipitation probability density distribution, such as increasing the extremely low- and high-rainfall-rate events and decreasing small and moderate rainfall-rate events, which contributes to the precipitation bias reducing over mid-latitude and tropical lands.

How to cite: Zhao, Y., Peng, X., Li, X., and Chen, S.: Improving the diurnal cycle simulation of land precipitation using a revised NSAS convective scheme in a global non-hydrostatic model, EMS Annual Meeting 2023, Bratislava, Slovakia, 4–8 Sep 2023, EMS2023-358, https://doi.org/10.5194/ems2023-358, 2023.