How to cite: Gascón, E., Maier-Gerber, M., Vannière, B., Sützl, B., Magnusson, L., and Sandu, I.: Enhancing Extreme Weather Forecasts: Diagnostic Strategies in ECMWF's Destination Earth Initiative  , EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-273, https://doi.org/10.5194/ems2024-273, 2024.

The Destination Earth (DestinE) is a flagship initiative of the European Commission to develop a highly accurate digital model of the Earth on a global scale. This model will monitor, simulate and predict the interaction between natural phenomena and human activities. DestinE will unlock the potential of digital modelling of the Earth system at a level that represents a real breakthrough in terms of accuracy, local detail, access-to-information speed and interactivity.

The initial focus will be on the effects and prediction of climate change and extreme weather events, their socio-economic impact and possible adaptation and mitigation strategies. One of the subcomponents of DestinE is the On-Demand Extremes digital twin (DestinE_330), which will build a configurable framework for impact assessment and prediction of weather-induced extremes at sub-kilometre scales.

Within the framework of this initiative and from diagnostics point of view, here we evaluate initial simulations of both the global (~4 km) and the regional weather-induced (~500 km) DestinE prototype components for the representation of high-impact and extreme convective weather over the Mediterranean region, with two specific problems which are highly sensible to increasing resolution and which anthropogenic climate change could worsen: A very high-impact static convective storm formed in front of Valencia (Spain) and a rare tropical-like convectively-driven cyclone (Medicane Ianos) over the Ionian Sea.

The storm in Valencia, probably a mesoscale convective system, produced heavy precipitation (>100 mm in 2 hours) and reported the largest accumulation for one day in May for Valencia. The socioeconomic impact was also severe. This event had very low predictability in high-resolution convection-allowing models, being a challenge from a numerical modelling point of view. None of the national operational models nor global models over the region showed signals of convective activity with such features in the east of Spain.

Ianos was a extremely rare Mediterranean tropical-like cyclone, behaving as a hurricane, that impacted the eastern Mediterranean on 17 and 18 September 2020, especially Greece, leaving severe damage with heavy rains (peak of >600 mm) and strong winds (peak of 195 km/h) and fatalities. Operational forecasts of this event were not highly valuable, thus it a another highly recommended case study.

How to cite: González-Alemán, J. J., Gascon, E., Martin, D., Vannière, B., Maier-Gerber, M., Jimenez, A., Calvo-Sancho, C., Viana, S., and Calvo, J.: Diagnostics-based evaluation of Mediterranean extreme convective storms on high-resolution simulations under Destination Earth initiative, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-380, https://doi.org/10.5194/ems2024-380, 2024.

Hail events are common in Catalonia (NE of the Iberian Peninsula). The first two giant hailstones (with diameters equal to or exceeding 10 cm) occurred in this region in 2022 and 2023. These occurrences align with the hypothesis that extreme severe weather events have increased with rises in air and sea surface temperatures. This behavior coincides with the pattern observed in most of regions around the World.

The research presented here focuses on the thunderstorm characteristics of both cases as observed by the weather radar network of the Servei Meteorologic de Catalunya. Regarding the volumetric structure, the giant hail-bearing storms exhibited dimensions unprecedented in Catalonia, as it is observed from the analysis comparing with a set of more than 200 different cases.

We have compared these two events with the rest of the hailstorms set in terms of affected area, duration, dimensions, or life cycle evolution. The study was conducted using a high-resolution 3D grid of 1 km per 1 km per 0.5 km. This grid enabled the identification of the most relevant elements associated with the reflectivity fields in each case.

Furthermore, we sought general parameters through initial automatic identification and characterization algorithms, allowing us to determine reflectivity distribution along the vertical, which has been compared also with lightning data. Additionally, we conducted more in-depth analyses for certain life cycle instants with particular elements, such as a change in the nature of the storm, the inclination, or the observation of concrete elements such as the three-body scatter spike, the boun weak echo region, or the hook echo.

How to cite: Rigo, T. and Farnell, C.: A radar analysis of two giant hailstones in Catalonia, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-76, https://doi.org/10.5194/ems2024-76, 2024.

During the last two years, Catalonia has experienced the most severe hail episodes of the previous three decades. The first one was on August 30th, 2022, and it affected the northeast of the country, resulting in significant urban damage, injuries, and one fatality—a two-year-old girl. The second episode impacted the region's southern part, causing extensive damage, particularly in urban areas. In both instances, giant hail was recorded, with a maximum diameter of 12 cm and 10 cm, respectively.

In response to these extraordinary episodes, we conducted exhaustive fieldwork in the core of the affected areas to understand the path and behavior of the thunderstorm over the terrain. Additionally, we collected hailstones preserved in the freezers by observers living in the affected village. In total, we have collected 20 samples during the first event and 12 samples in the second one.

We calculated most samples' sizes of the larger, mid, and minor axes, weight, and sphericity index. Furthermore, we underwent analysis through two methods. One of these methods involved employing an innovative technique in this field: Computed Tomography Scanning (CT scanning). This technique enables us to examine the internal structure of hailstones in three dimensions, thereby observing the diverse layers within them and their varying densities. Such analysis offers valuable insights into the processes these stones undergo within thunderstorms, which are influenced by temperature and humidity in various atmospheric layers. The second analysis involved a chemical analysis conducted in the laboratory, which unveiled a clear correlation between the main components and their proximity to the sea.

How to cite: Farnell Barqué, C., Rigo Ribas, T., Martín Vide, J., and Úbeda Cartaña, X.: Internal Structure of Giant Hail in a Catastrophic Event in Catalonia (NE Iberian Peninsula)., EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-77, https://doi.org/10.5194/ems2024-77, 2024.

Extreme precipitation events are both changing in frequency of occurrence and intensity. Among them, we can identify the Mesoscale Convective Systems (MCSs), which have typical dimensions of the order of ~100 km and consist of large clusters of powerful thunderstorms that produce intense rainfall, large hailstones and, occasionally, dangerous tornadoes. Italy is being affected more and more frequently by such systems.

Here, we carried out sensitivity tests using a limited-area setup at 2km horizontal resolution of the ICON numerical model, combined to a double nesting at 1km and 500m to simulate a strong quasi-linear convective system that triggered a huge flooding in central Italy on 15-16 September 2022, causing 13 deaths and 2 billion euros worth of damage. The convergence of warm, humid southwesterly winds (that produced an atmospheric river with peak values of 1000 kg*m^-1*s^-1) with northwestern atmospheric currents triggered intense convection over the central Apennine regions leading to a destructive V-shaped self-regenerating thunderstorm, despite the absence of very cold air at higher levels. The numerical models underestimated such event and did not reproduce well the spatial extent and intensity of the system. These discrepancies are related to model errors usually due to uncertainties in (i) estimation of moisture in the several layers of the atmosphere, (ii) parameterization schemes for cloud microphysics, (iii) orography representation and (iv) turbulence diffusion. It follows that convection-permitting models are crucial to better solve these phenomena. Thus, we did some sensitivity tests operating on the above-mentioned parameterizations to identify the main physical processes that are the most likely to impact the forecasts’ accuracy.

Results showed that the activation of the Smagorinsky 3D turbulence scheme, compared to the COSMO 1D scheme, noticeably improves the forecast accuracy in terms of  precipitation amounts and spatial distribution. Moreover, we employed an ensemble-based approach to enhance predictability, with a focus on extreme percentiles within the ensemble tail of specific variables such as total precipitation. This method offers more valuable insights compared to the traditional analysis of ensemble means, especially when  it comes to rare and unusual weather patterns that lead to extreme precipitation events.

How to cite: Siena, M., Ruggieri, P., Marsigli, C., Gastaldo, T., and Di Sabatino, S.: Improving predictability of Mesoscale Extreme Precipitation events with Convection-permitting models, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-772, https://doi.org/10.5194/ems2024-772, 2024.

The variations in the track and intensity of a tropical cyclone (TC) are closely correlated with the fine-scale evolution of its structure. The fine-scale track, intensity, and structural evolution of TC can be comprehensively analyzed by combining multi-source observations. Based on observations from space-borne synthetic aperture radars (SARs), the Advanced Microwave Scanning Radiometer-2 (AMSR2), the Special Sensor Microwave Imager/Sounder (SSMIS), weather radars, in-situ buoys and island stations, an analysis was conducted on the fine-scale evolutionary characteristics of high-impact Super Typhoon Doksuri (2023). The results show that Doksuri (2023) experienced secondary eyewall formation (SEF), concentric eyewall maintenance (CEM), and eyewall replacement cycle (ERC) processes when entering the South China Sea and prior to landfall. These processes can be further delineated into three subsequent stages. In the first stage, the SEF phase, the secondary (outer) eyewall formed, exhibiting features that were non-concentric with the inner eyewall. Concurrently, the track of Doksuri (2023) displayed notable oscillations in both its forward translational direction and speed, accompanied by the emergence of two radial maxima centers of wind speed. Subsequently, during the second stage, the CEM phase, the geometric centers of the inner and outer eyewalls of Doksuri (2023) coincided, initiating a rapid intensification process characterized by an accelerated forward translational speed. Both the inner and outer eyewalls further contracted during this phase. In the third stage, the ERC phase, the asymmetry of the inner eyewall increased, and the outer eyewall gradually contracted while the inner eyewall dissipated until the replacement was completed prior to landfall. Accordingly, Doksuri (2023) experienced rapid weakening. With special attention been paid to the possible connection between the evolution of the typhoon structures and the changes in its track and intensity, it is found that the substantial track oscillations were closely associated with the asymmetric development of TC inner and outer eyewalls during the SEF, and, the rapid intensity changes were correlated with the CEM and ERC processes. These findings have the potential to enhance our understanding of the physical mechanisms governing the intricate structures of TCs at fine scales, bolstering the forecast accuracy of TC tracks and intensities.

How to cite: Wang, Q., Zhao, D., Duan, Y., Wang, H., Sun, Z., and Xu, Y.: Fine-scale evolutionary characteristics of Super Typhoon Doksuri (2023) revealed by multi-source observations, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-43, https://doi.org/10.5194/ems2024-43, 2024.

The paper presents a model intercomparison study to improve the prediction and understanding of Mediterranean cyclone dynamics. It is based on a collective effort with five mesoscale models to look for a robust response among ten numerical frameworks used in the community involved in the networking activity of the EU COST Action "MedCyclones". The obtained multi-model, multi-physics ensemble is applied to the high-impact medicane Ianos of September 2020 with focus on the cyclogenesis phase, which was poorly forecast by numerical weather prediction systems. Models systematically perform better when initialised from operational IFS analysis data compared to the widely used ERA5 reanalysis. Reducing horizontal grid spacing from 10 km with parameterised convection to convection-permitting 2 km further improves the cyclone track and intensity. This highlights the critical role of deep convection during the early development stage. Higher resolution enhances convective activity, which improves the phasing of the cyclone with an upper-level jet and its subsequent intensification and evolution. This upscale impact of convection matches a conceptual model of upscale error growth in the midlatitudes, while it emphasises the crucial interplay between convective and baroclinic processes during medicane cyclogenesis. The ten numerical frameworks show robust agreement but also reveal model specifics that should be taken into consideration, such as the need for a parameterization of deep convection even at 2 km horizontal grid spacing in some models. While they require generalisation to other cases of Mediterranean cyclones, the results provide guidance for the next generation of global convection-permitting models in weather and climate.

How to cite: Pantillon, F., Davolio, S., Avolio, E., Calvo-Sancho, C., Carrió, D. S., Dafis, S., Flaounas, E., Gentile, E. S., Gonzalez-Aleman, J. J., Gray, S., Miglietta, M. M., Patlakas, P., Pytharoulis, I., Ricard, D., Ricchi, A., and Sanchez, C.: The crucial representation of deep convection for the cyclogenesis of medicane Ianos, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-84, https://doi.org/10.5194/ems2024-84, 2024.

In recent years, the impact of cyclones with tropical characteristics in areas of the Mediterranean Sea, also called Medicanes (MEDIterranean hurriCANES; Emanuel, 2005), has caused extensive damage, particularly in coastal areas of Greece, Italy and North Africa. These cyclones have a relatively low frequency in Mediterranean areas, although they are associated with extreme weather phenomena (heavy rainfall, strong winds, and landslides). Therefore, the main goal of this work is to create an index of meteorological conditions that can determine favourable environment for the development of cyclones with tropical characteristics in the Mediterranean basin. For this purpose, we have selected some of the most intense Medicanes occurring during the last years. Using ERA5 data we study variables such as Convective available potential energy (CAPE), coupling index (CI; Bosart & Lackmann, 1995), Wind shear, Geopotential in 500 hPa and Sea surface temperature (SST).The findings of this study reveal that the development of cyclones with tropical characteristics in the Mediterranean is strongly linked to the formation of an intense trough, which is exposed to strong convective processes (high CAPE and CI values), low wind shear at high levels and a Mediterranean Sea temperature above 15ºC. The combination of all these  conditions at the same time is quite uncommon, so this could explain the infrequent and extreme nature of this type of cyclones.

References

Emanuel, K. (2005). Genesis and maintenance of" Mediterranean hurricanes". Advances in Geosciences, 2, 217-220.

Bosart, L. F., & Lackmann, G. M. (1995). Postlandfall tropical cyclone reintensification in a weakly baroclinic environment: A case study of
Hurricane David (September 1979). Monthly Weather Review, 123(11), 3268–3291. https://doi.org/10.1175/1520-0493(1995)123<3268:
ptcria>2.0.co;2

How to cite: Gutiérrez-Fernández, J., Alvarez-Castro, C., Scoccimarro, E., Cavicchia, L., and Gualdi, S.: Investigating the Development Conditions for Cyclones with Tropical Features in the Mediterranean Basin. , EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-1055, https://doi.org/10.5194/ems2024-1055, 2024.

Eastern Mediterranean Cyclones (EMCs) are a major contributor to extreme weather in this region, including precipitation, strong winds, cold extremes or dust storms, significantly impacting the population and natural environment. Thus, understanding the relationship between EMC variability and associated impacts is key to understanding their predictability and forecasting these hazards. 

Previous approaches for cyclone classification over the Eastern Mediterranean region involved the cyclones’ location, intensity, or structure. Such approaches do not consider the involved processes and, therefore, result in limited physical interpretations of cyclone variability and associated impacts and predictability. 

Various processes come together to govern the genesis and development of EMC, and these processes have distinct signatures on the potential vorticity (PV) distribution. Here, we classify EMCs based on their associated upper-tropospheric PV structures into 6 clusters and analyse the impacts of resulting clusters.

We find that each cluster has its own signature of precipitation pattern. For each of the seasons, there are dominant clusters that bring extreme precipitation. In particular, two clusters of anti-cyclonic Rossby wave breaking PV patterns dominate the eastern Mediterranean's annual precipitation. Evidently, a strong ridge upstream of the PV trough has a greater impact on precipitation and temperature extremes than the PV pattern with a weak ridge upstream. Moreover, cyclones with low PV values, which correspond to heat-lows in the transition seasons, have an increasing trend in their occurrence frequency. 

This classification approach enhances our understanding of the link between cyclone variability and their surface impacts in the region through processes reflected in upper-level PV distributions. These findings could benefit strategies for managing the societal and environmental impacts of EMCs at weather and climate timescales.

How to cite: Gens, T. S. and Raveh-Rubin, S.: A climatological perspective on cyclones and precipitation in the Eastern Mediterranean using potential vorticity-based classification, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-134, https://doi.org/10.5194/ems2024-134, 2024.

Heavy wet snowfall events are responsible for several and harmful winter blackouts on Italian power networks due to the formation of cylindrical snow sleeves on overhead line conductors and shield wires. Despite field observations and modelling studies in the last decades, accurately forecasting these events remains challenging because of the peculiar local meteorological conditions. Although the total number of snowfall events decreased over the alpine region during the last 20 years, last winter season (2023-2024) witnessed several significant wet snowfall events, characterized by mixed convective-advective precipitation, causing several power disruptions, and underscoring the persistent threat posed by this atmospheric phenomenon.

The current research activity carried out in RSE involves monitoring systems of sleeve formations at the Wet-snow Ice Laboratory Detection (WILD) station in the south-western Alps, a historical reconstruction of wet snow loads through the MEteorological Reanalysis Italian Dataset (MERIDA), and an operational forecast system Wet-snow Overload aLert and Forecasting (WOLF) for the identification of weather conditions favorable to the formation of snow sleeves on the overhead conductors.

WOLF consists of: a) a version of the Weather Research and Forecasting (WRF) model at 4 km spatial resolution properly configured to optimize the description of temperature and precipitation, variables primarily involved in the identification of wet snow and in the modelling of snow sleeves accretion b) an identification method for wet snow conditions, either considering a “thermal window” or the “snow ratio” of precipitation, and c) a sleeve accretion model (Makkonen model for wet-snow, ISO 12494:2017).

This study presents some exemplary case studies from the 2023-2024 winter season to highlight current challenges of wet snowfall prediction. High resolution simulations (1 km) were carried out for an accurate description of the mountainous area of south-western Alps where the WILD station is located. Different model drivers (IFS and GFS), model resolutions and wet-snow identification methods were evaluated against meteorological and snow mass measurements collected at the WILD station.

Preliminary results showed that the snow load forecast accuracy is primarily related to the choice of the model driver and, secondarily, to the wet snow identification method. The analyzed case studies are particularly challenging because of mixed convective-advective characteristic and because of complex orography. Additional case studies should be investigated to further assess the relative skill of meteorological drivers and of wet-snow identification methods for the snow load forecast in the WOLF system and in high resolution simulation focusing on the WILD station.

How to cite: Vitali, B., Bonanno, R., and Lacavalla, M.: Monitoring and Forecasting wet-snowfall and snow sleeves accretion on overhead power lines in the Italian western Alps , EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-520, https://doi.org/10.5194/ems2024-520, 2024.

Agricultural production in Central South America (CSA) is highly sensitive to frost events. This study aims to provide a detailed characterisation and quantification of the physical processes leading to frost conditions in CSA between 1979 and 2022, using gridded observational data. To accomplish this objective, we focus on three innovative elements: a thorough examination of regional frost properties, a novel multi-parametric method for the description of upper-level jets, and the quantification of the underlying mechanisms through decision trees. We selected this methodology due to its high interpretability, particularly advantageous for users within the agricultural sector, owing to its visually intuitive representation.

After regionalisation and statistical description of the frosts, we have been able to identify five homogeneous regions in CSA. Regarding the synoptic mechanisms that promote the occurrence of extensive frosts, we have found that the physical processes are similar across all regions. These processes are triggered by the passage of a cold front and the establishment of a migratory post-frontal anticyclone, which is reinforced by the subsidence generated on the southern flank of the entrance region of the Subtropical Jet (STJ). We have computed six parameters to characterise the STJ and seven for the Polar Front Jet (PFJ). They consider the latitude, intensity, height, tilting, longitudinal extent, and branches of the jets. By employing decision trees, we have effectively discerned a subset of jet parameters that are closely linked to frost events. The most relevant of these are the latitude of the STJ, the longitudinal extent of the Atlantic branch of the STJ, and the tilt of the Atlantic branch. In general, when the STJ is positioned north of 31°S, the likelihood of frost in the CSA rise. However, in more northern areas, the probability of frost occurrence further increases with a slight northward shift in its location. In addition, extensions of the Atlantic STJ ranging from 25 to 55°, coupled with a negative tilt, are two characteristics often linked to frost occurrences. These features are especially accentuated in events occurring in the northernmost regions. The application of decision trees has allowed us to summarise this information for every region of CSA in a user-friendly way, helpful for decision-makers in the agricultural sector.

Acknowledge: The SAFETE project, which has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 847635

How to cite: Collazo, S. and García-Herrera, R.: Getting to the core of winter frosts in central South America through decision trees, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-277, https://doi.org/10.5194/ems2024-277, 2024.

Downslope windstorm is a synoptically forced wind forming on the lee side of mountains. In Hungary this phenomenon is frequently observed at Lake Balaton, where the Hungarian Meteorological Service operates a lake storm warning system. North to Lake Balaton lies the Bakony Mountains with highest peak of around 700 m. The axis of this hilly area is perpendicular to the most frequent, northwest wind direction. This pattern allows the frequent formation of downslope windstorms. Although these downslope winds are significantly weaker than that can form on the lee side of the higher mountains, accurate forecast of them is crucial for the operation of the lake warning system. Our experience suggests that the lake due to its lower surface roughness and its effect on surface temperature can significantly impact the large scale wind locally.

In this study the WRF (Weather Research and Forecasting) numerical model was used, and the model results were validated by data provided by dense wind measurements network. Our results reveal: (i) even the relatively low Bakony Mountains can generate downslope windstorms and the wind speed is amplified by the lake effects over Lake Balaton; (ii)  the backside of a cyclon with a north, northwest, nearly geostrophic wind in the Bakony-Balaton region is the most preferable synoptic condition for the formation of the severe downslope wind;  (iii)  the presence of lee trough, stable layer over the mountain, self-induced critical layer in the wind cross sections can also promote the formation the severe downslope wind; (iv) in the case of weaker downslope wind events the turbulence has considerable role in the formation of strong wind gusts; (v) the wind field observed by the surface network and the cloud structure can be also used to justify impact of the mountain waves on the formation of downslope wind.

The comparison of the observation data and the model results of NWP (WRF) model shows that the forecast of strong downslope wind events is reliable.  However, in the case of weaker downslope winds the WRF frequently overestimates the peak of the gusts. The results of this study can be taken into consideration for the improvement of the reliability of the lake storm warning system.

How to cite: Kurcsics, M., Geresdi, I., and Horváth, Á.: Synoptically-forced downslope wind induced by low mountain over lake surface, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-251, https://doi.org/10.5194/ems2024-251, 2024.

To understand the mechanisms behind precipitation extremes, one can determine the origin of this precipitation, i.e. its moisture sources. The temporal and spatial distribution of these sources provide insights into the synoptic situation of the extreme event, and the importance of land-atmosphere interactions and moisture recycling. To determine moisture sources, moisture tracking models are used and forced with gridded atmospheric data to track water vapour in the atmosphere backward in time to its origin. Moisture tracking models have become an increasingly popular tool in scientific studies in recent years, but diversify in their underlying assumptions. Validation of tracking models is difficult due to the scarcity of isotope measurements as a benchmark. Further, structured intercomparisons among different models are lacking.

Here, we present our efforts to coordinate a moisture tracking intercomparison study. Therefore, we reached out to many members of the moisture tracking community, and asked them to run their own moisture tracking model for three selected extreme precipitation events that occurred in 2022. Those three selected events cover different precipitation mechanisms: a monsoon event in Pakistan, a convective precipitation case in Australia, and an atmospheric river driven precipitation case over Scotland. We aim to compare the sources for the three cases among the different models during a one-week workshop in May 2024 in Leiden, The Netherlands. To this date, this intercomparison study covers about eight different moisture tracking models, allowing us to address and quantify the uncertainty in the moisture sources. At the EMS Annual meeting, we will present our approach and preliminary results of this intercomparison. This coordinated model intercomparison facilitates the explanation and quantification of uncertainty, acting as a point of reference for future work and literature on moisture tracking.

How to cite: Benedict, I., Weijenborg, C., van der Ent, R., Keune, J., Koren, G., and Kalverla, P.: A moisture tracking intercomparison study - Addressing the uncertainty in modelling the origins of precipitation, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-1040, https://doi.org/10.5194/ems2024-1040, 2024.

Atmospheric rivers (ARs) are filaments of enhanced moisture in the atmosphere, typically located in subtropical and mid-latitudes oceanic regions. These structures are of extreme importance for the climate of the western coasts, as they provide a large fraction of the annual precipitation recorded in these areas. In addition, ARs can eventually cause extreme rainfall when the enormous amounts of water vapor they carry are abruptly forced upwards. As a result, much research has been done on ARs in recent years, and one of their most studied and debated properties is the origin of the moisture in them. Although some studies have identified sources using different moisture transport models for specific AR cases, it remains unclear whether tropical or extratropical contributions are generally more prevalent, and even the AR definition in the Glossary of Meteorology reflects this lack of consensus.

To fill this gap, a climatology of moisture sources for precipitation in ARs is needed. There are a variety of moisture transport models that can be employed to address this issue. However, they tend to produce considerably different results, and the lack of observations hampers any validation. Here we use the WRF with Water Vapor Tracers (WRF-WVTs) model as a proxy for reality to validate a Lagrangian moisture transport technique based on the FLEXPART model for selected cases. This allows us to minimize the discrepancies with the WRF-WVTs model, assumed to be the most accurate moisture tracking method, while efficiently simulating air particle trajectories within a wider range of ARs with a Lagrangian tool.

Preliminary results reveal a wide diversity of moisture sources, including both oceanic and continental regions, with substantial variability in their contributions across different AR cases. Importantly, our findings also indicate a less relevant role of tropical moisture than previously known. Ultimately, this highlights the complexity of the moisture uptake process in ARs.

How to cite: Crespo-Otero, A., Insua-Costa, D., and Míguez-Macho, G.: Unravelling moisture sources for precipitation in atmospheric rivers, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-1019, https://doi.org/10.5194/ems2024-1019, 2024.

The Montsià county is the southernmost region of Catalonia (NE Spain), located between the final stretches of the Ebro River and the Sénia River. It is crossed by a relatively high mountain north-south oriented range (Serra del Montsià, 764 m.a.s.l). This range runs very close and parallel to the coastline. Numerous steep torrents originate in these mountains and traverse densely populated areas (particularly in the summer months). This affects mainly tourist and summer resorts, producing a heightened vulnerability to flooding. The municipality of Alcanar is a clear example of this situation. Between 1980 and 2020, Montsià County recorded 42 flood events, with 21 causing significant damage in Alcanar. These flooding events are typically triggered by very convective and high-intensity local and sudden rainfall, making accurate predictions challenging. The recent flood events of October 2018 and September 2021 and 2023 are good examples of such situations.

In this framework, our research seeks to fulfill two main objectives. The first one is to perform the meteorological analysis of the former three events  (2018, 2021 and 2023). This part has consisted of the characterization of the synoptic environment and the role of the mesoscale in the lightning and precipitation patterns. Data from radar, lightning detection systems, and rain gauges have helped to reach this point. The events display high variability in several factors such as the electrical activity, cloud vertical development, the affected regions, and the precipitation field. Hence, they represent a complex analysis that might lead to better understand processes that could affect most of the Mediterranean basin, as well as other regions around the globe. The second goal has been the characterization of the frequency and possible temporal trends of heavy rainfall events using daily and sub-daily precipitation data. 

How to cite: Rigo, T., Llasat, C., Marcos, R., and Pascual, R.: The vulnerability of the small mediterranean village of Alcanar to floods: hasincreased the impact or the number of cases in the last years?, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-560, https://doi.org/10.5194/ems2024-560, 2024.

An extreme Mei-yu rainfall event occurred on 2 June 2017 along the northern coast of Taiwan, where a peak amount of 635 mm fell in about 10 hours from 0100-1100 LST. Using ensemble-based sensivity analysis, an earlier study (Wang et al., 2021) showed that the areal-averaged rainfall in the wider region surrounding northern Taiwan in this event was influenced by frontal moving speed, frontal intensity (contrast), and location and timing of frontal disturbance in experiments using grid sizes of 2.5-5 km, but peak amounts in such runs were no more than 360 mm, suggesting low predictivity for its occurrence. Using 1-km grid size, predictions in a later study (Wang et al., 2023), on the other hand, showed that a peak amount of up to 541 mm can be achieved with a finer grid, but the detailed rainfall pattern and maximum amount are strongly dictated by the presence of a frontal low-pressure disturbance that affected the location and persistence of the surface convergence zone, thus again suggesting high nonlinearity and low predictability. However, in some initial times about 2.5-3 days before the event, the evolutions were more favorable for larger and more concentrated rainfall.

In this study, the above extreme rainfall event is further examined, using ensemble forecasts both at 1 km and larger grid sizes, including the experiments carried out in the Taiwan Area Heavy-rainfall Prediction Experiment (TAHPEX). Special attention is given on the predictability of this extreme event and the strategy to produce useful results more effectively with longer lead times for early warning and preparation.

How to cite: Wang, C.-C., Huang, P.-Y., and Huang, S.-Y.: Cloud-Resolving Ensemble Prediction of the Extreme Rainfall Event on 2 June 2017 in Northern Taiwan, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-467, https://doi.org/10.5194/ems2024-467, 2024.

An extreme rainfall event with maximum 12-h accumulated rainfall of 464.8 mm associated with a quasi-stationary mesoscale convective system occurred over the coastal region of South China (a monsoon coastal area) in June 2017. An observational analysis shows that early convective storms were initiated near the mountains and moved northeastwards. A convergence line between cold northerly winds and warm southerly winds in the lower levels was formed, which favored the development of the quasi-stationary system. Cold northerly winds were associated with land breeze, downslope winds as well as previous rainfall during earlier period, and cold pool outflows during later period. High rain rate was mainly contributed by abundant small- and medium-sized raindrops. Cloud-resolving simulations were performed to examine the roles of terrain, land-sea surface roughness contrast, and cold pool outflows in the formation of heavy rainfall. Results demonstrate that orographic lifting and lee side convergence near the coastal hills helped initiate convection. Land-sea surface roughness contrast and mountains in the middle of Yangjiang and Jiangmen affected the formation of convergence line during earlier period, and cold pool outflows sustained it during later period. Mountains in the middle of Yangjiang hindered the movement of the convective system, without which the associated heavy rainfall would shift farther north. Mt. Tianlu with westward concave morphology was vital for the formation of local convergence and the concentration of heavy rainfall. This study suggests the importance of representing processes associated with complex underlying surface in models for the prediction of heavy rainfall in coastal areas.

How to cite: Li, H., Huang, Y., Hu, S., Wu, N., Liu, X., and Xiao, H.: Roles of terrain, surface roughness, and cold pool outflows in an extreme rainfall event over the monsoon coast, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-553, https://doi.org/10.5194/ems2024-553, 2024.

This study presents a comprehensive diagnostic analysis of a southward deviation in the forecasted track of a southwest vortex (SWV) by the China Meteorological Administration's Global Assimilation and Prediction System (CMA-GFS). The SWV is a significant weather system in China, often responsible for severe weather events such as rainstorms. The study aims to identify the sources of forecast errors and propose improvements for more accurate tracking of SWVs. A severe convective weather process occurring on 11–13 April 2022 is selected for case study, where the CMA-GFS forecast exhibits a noticeable southward deviation in the SWV track, leading to incorrect rainfall area predictions. The analysis traces the deviation to the steering flow at 500 hPa, finding that the SWV shift is directly attributed to the deviation caused by the weak forecast of the upper-level trough. The underestimation of the initial vorticity advection by CMA-GFS is identified as the dominant factor contributing to the weak development of the trough. This underestimation is linked to the weak geostrophic wind resulting from the weak initial meridional and zonal gradients of the midlevel height in front of the trough. The study further examines the assimilation process of CMA-GFS, revealing that it has a negative effect on the SWV track forecast. The assimilation process is found to weaken the π field at mid-model level, leading to a weak midlevel height gradient in front of the trough. A verified numerical experiment using a more reasonable initial field significantly modifies the southward shift of the SWV, underscoring the importance of accurate initial analysis for precise SWV track forecasting.The research also explores the impact of initial field deviation in key regions on late-period forecasts, emphasizing the need for systematic diagnosis and improvement of SWV track forecasts. The study suggests that the initial height field, particularly at midlevels, can significantly influence the SWV track in the late forecast period. The use of ensemble prediction with initial condition perturbation strengthens the conclusions drawn from the diagnosis.

The important impact of initial field deviation in key regions on the forecast in the late period is highlighted and a systematic diagnosis process for identifying and addressing forecast issues on SWV track is proposed in this study. This research could provide a comprehensive approach for diagnosing the forecast deviation associated with SWV track.

How to cite: Wang, L., Chen, Q., Jiang, N., Hu, J., and Xu, G.: A diagnostic study on a southward southwest vortex track forecasted by CMA-GFS: the role of initial field, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-186, https://doi.org/10.5194/ems2024-186, 2024.

Cloud-resolving models coupled to fire propagation models is a powerful tool to understand fire-atmosphere environment. Portugal is one of the countries in Southern Europe with most burned area and numerous ignitions. In 2017, Portugal was affected by several mega-fires with burned areas larger than 10 000 hectares. Some of these fires presented the development of convective clouds, namely pyro-cumulus (pyroCu) and pyro-cumulonimbus (pyroCb). These phenomena can significantly influence the evolution of fire fronts by altering surface winds, increasing burned areas, and accelerating spread rates.  This study aims to study pyro-convection during two mega-fires occurred in 2017. In this study, the Quiaios (October 15) and Pedrogão Grande (June 17) mega-fires are chosen as case studies. It was made 3 numerical simulations with the MesoNH atmospheric model coupled with the ForeFire fire propagation model. The experiments were configured into three nested domains with horizontal resolution of 2000m, 400m and 80m. Two simulations were conducted in one-way mode, with the fire front evolution directly imposed by ForeFire and based on a pre-defined map obtained from official reports. The third experiment, conducted only for Pedrogão Grande, utilized the two-way mode (i.e., fully coupled), allowing for the simulation of the impact of fire-induced atmospheric motions on the fire’s own propagation. The results show the development of pyroCu in Quiaios mega-fire and a pyroCb cloud in the Pedrogão Grande event. In the PyroCb case, the simulations showed the development of intense updrafts, that contributed to the vertical transport of water vapor up to the upper troposphere, allowing cloud formation, which was represented by several hydrometeors’ species inside the plume. The extreme fire environment was also verified in the simulation due to downbursts originating from the pyroCb cloud. This study has provided important insights into the numerical modelling of pyro-convective clouds using Meso-NH/ForeFire simulations. This study was funded by FCT-Foundation for Science and Technology, I.P. under the PyroC.pt project (Ref. PCIF/MPG/0175/2019).

How to cite: Campos, C., T. Couto, F., Filippi, J.-B., Baggio, R., and Salgado, R.: Modelling fire-induced meteorological phenomena using the MesoNH/ForeFire model, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-477, https://doi.org/10.5194/ems2024-477, 2024.

Meteorological conditions are an important factor contributing to extreme fire environment. Southern Portugal has some susceptible regions to fire with at least 4 mega fires occurring in the last two decades. Aiming to investigate in detail the atmospheric environment of large fires, several numerical simulations were performed using the Meso-NH non-hydrostatic research model. Some simulations were configured using a simple domain with a horizontal resolution of 2.5 km, while others were performed using a grid nesting technique with an inner domain of 500 m resolution. The vertical grid was configured with 50 stretched levels following the terrain. Initial and boundary conditions were provided by the ECMWF analyses and updated every 6 h. The simulations have shown the important role played by regional orography in creating favourable fire weather conditions. For instance, the interaction of the airflow with orographic effects created conditions for the rapid spread of the fire fronts in some cases, namely downward motions on the leeward side of the mountains. These effects were associated with strong wind gusts and turbulent motion at surface. Moreover, the numerical simulations indicated air temperatures near the surface above 30⁰C, and relative humidity below 30% influencing the fine fuel availability in the mega-fire cases. The study highlighted the advantages of using atmospheric modelling to better understand the significant wildfires in Southern Portugal, especially in a region marked by its fire susceptibility and vulnerability. This study was funded by national funds through FCT-Foundation for Science and Technology, I.P. under the PyroC.pt project (Ref. PCIF/MPG/0175/2019).

How to cite: Purificação, C., Henkes, A., Kartsios, S., and Couto, F. T.: Application of atmospheric modelling to understand wildfires environment in Southern Portugal, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-579, https://doi.org/10.5194/ems2024-579, 2024.

Understanding the Role of Soil Moisture-Atmosphere Interactions during European Summer Heat Wave Using Regional Climate Modeling

Abstract:

 European summer heat waves pose significant challenges to society, ecosystems, and infrastructure. Understanding the intricate interplay between soil moisture and the atmosphere during these events is crucial for accurate prediction and effective mitigation strategies. This topic delves into the complexities of this relationship, utilizing regional climate modeling to explore the mechanisms driving heat wave dynamics over Europe.The developments of extreme heatwave often linked to the emergence of local high pressure, lead to decrease cloud cover and more shortwave solar radiation at the surface as well as adiabatic subsidence that leads to anomalous high temperature. In this paper, state-of-the-art regional climate model, RegCM5 has been dynamical downscaling over EURO-CORDEX domain (12km horizontal resolution) with 3 hourly initial and boundary condition of ERA-5 to replicate the dynamical features of extreme heatwave event that occurred during 2019, 2020 and 2021 over Europe.This study investigates the sensitivity experiments and coherence in the combination of different physical parameterization with specific focus on role of soil moisture in modulating synoptic pattern and spatio temporal evaluation of extreme heat wave event in the model simulation.  
The coupling of drought and heatwaves can intensify individual hazards by linking soil moisture with atmospheric conditions (Shukla et al., 2015), accelerating projected impacts on ecosystems, socioeconomics, and human health. This combined effect results in increased evaporation, causing surface soil moisture to decline, which in turn creates a positive feedback loop. This alteration affects the surface energy balance by changing the distribution of sensible and latent heat fluxes, resulting in above-normal surface air temperatures. Additionally, this study aims to elucidate the synoptic pattern and spatiotemporal evaluation of extreme heatwave events, with a particular focus on the feedback mechanism between soil moisture and the atmosphere. This mechanism acts as a secondary positive feedback by amplifying anticyclonic conditions (Thompson et al., 2022), which promote a strong high-pressure ridge over the Iberian Peninsula, France, and neighboring regions.

Keywords: Regional climate model, European Heatwave, Soil moisture-atmosphere feedback

How to cite: Verma, S., Crespo, N. M., Belda, M., Halenka, T., and Huszar, P.: Understanding the Role of Soil Moisture-Atmosphere Interactions during European Summer Heat Wave Using Regional Climate Modeling, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-934, https://doi.org/10.5194/ems2024-934, 2024.

Climate change is increasing the intensity, frequency, and persistence of droughts, as observed by the recent record-breaking events in Europe. Two factors contribute to this intensification: dynamic changes —changes in the likelihood of weather patterns— and thermodynamic changes. While the former are uncertain in future climate projections, the latter are characterized by a high signal-to-noise ratio, as there is a robust and ubiquitous rise in land-surface temperatures.

To better analyze the contributions of these factors, we utilize the so-called "event-based storyline approach". This involves nudging our global CMIP6 coupled climate model (AWI-CM1) towards the observed (according to ERA5) large-scale free-troposphere winds using various climate background conditions and initial states. In contrast, the model reproduces the circulation in the atmospheric boundary layer and the rest of the variables. This enables us to simulate the same weather conditions (including jet streams, ridges and blockings) in different climates: preindustrial, present, and 2 °C, 3 °C, and 4 ºC warmer worlds. This method creates an efficient way of showing experts and non-experts the consequences of climate change by describing the storyline of specific extreme events in different plausible climates using moderate computational resources.

Focusing on Europe, our simulations successfully replicate recent hot and dry extreme events, like the 2019 or 2022 European heatwaves and the record-breaking 2018 or 2022 drought. Our experiments reveal that these extremes have intensified from preindustrial to present climates (which answers the conditional attribution question), mainly in southern Europe, with no significant changes in Western and Northern Europe. However, we project that this exacerbation will expand northward in future warmer climates, leading to even more severe drought in Central Europe and the Mediterranean by the end of the century. These changes are not only associated with an increase in atmospheric evaporative demand in a warmer atmosphere but, in some cases, with significant decreases in the accumulated precipitation.

However, the response to climate change depends on the event, with events with similar present-day values showing a different climate change response. With our methodology, we can explore the physical mechanisms, including weather regimes, changes in vertical stability, or humidity profiles, that help explain the event-dependent response to climate change.

How to cite: Sánchez Benítez, A., Ionita, M., Athanase, M., Jung, T., Ma, Q., and Goessling, H.: Event-dependent changes of recent European droughts in a warming climate using event-based storyline simulations., EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-902, https://doi.org/10.5194/ems2024-902, 2024.

In the context of escalating temperatures under climate change conditions, drought events are growing globally, particularly affecting Mediterranean Europe, a recognized climate change 'hotspot'.  Consequently, the vulnerability of ecosystems and human populations is increasingly pronounced in areas such as the Iberian Peninsula (IP).

This study delves into the repercussions of 1.5ᵒ and 2ᵒC Global Warming Levels (GWL) by the end of the 21st century on drought occurrences and population exposure in the IP. Utilizing EURO-CORDEX experiments, incorporating 13 simulations as a weighted multi-variable multi-model ensemble, spanning historical periods, 1.5ᵒ and 2ᵒC GWL years, and projections for the end of the century, this research employs the Standardized Precipitation Index (SPI) and the Standardized Precipitation-Evapotranspiration Index (SPEI) to assess droughts. Three greenhouse gas emission scenarios are considered alongside demographic projections up to 2100 for Portugal and Spain (RCP2.6, RCP4.5 and RCP8.5).

Under the RCP8.5 scenario, a significant increase in moderate, severe, and extreme droughts is anticipated, with SPI and SPEI projections indicating a substantial rise in drought occurrences due to additional warming. Population exposure to extreme droughts is projected to be higher under the 2ᵒC scenario, emphasizing the impact of rising temperatures in the IP. By the end of the century, population exposure to extreme droughts could surge by varying percentages, particularly under RCPs 4.5 and 8.5. The study highlights a projected surge in population exposure to droughts across the IP, primarily driven by climate change. These findings stress the critical need for regional authorities, policymakers, and society to prioritize adaptation planning and enhance understanding of vulnerabilities to tackle challenges posed by dry extreme events.

This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020), UIDP/50019/2020 (https://doi.org/10.54499/UIDP/50019/2020) and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020). This work was performed under the scope of project https://doi.org/10.54499/2022.09185.PTDC (DHEFEUS) and supported by national funds through FCT. DL and AR acknowledge FCT I.P./MCTES (Fundação para a Ciência e a Tecnologia) for the FCT, 2022.03183.CEECIND/CP1715/CT0004 (https://doi.org/10.54499/2022.03183.CEECIND/CP1715/CT0004) and (https://doi.org/10.54499/2022.01167.CEECIND/CP1722/CT0006 (Complex), respectively.

How to cite: Russo, A., Bento, V., Ribeiro, A., C.A. Lima, D., A. Careto, J., M.M. Soares, P., Libonati, R., M. Trigo, R., and M. Gouveia, C.: Changes in population exposure to drought in the Iberian Peninsula under 1.5 and 2º GWLs, EMS Annual Meeting 2024, Barcelona, Spain, 1–6 Sep 2024, EMS2024-1095, https://doi.org/10.5194/ems2024-1095, 2024.