How to cite: Tahvonen, S., Köhler, D., Räisänen, P., and Sinclair, V.: Impact of changing sea surface temperatures and sea ice cover on Rossby wave breaking in the Northern Hemisphere, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-21, https://doi.org/10.5194/ems2025-21, 2025.

Droughts are complex and far-reaching natural hazards, with cascading impacts that span ecosystems, agriculture, water security, and socio-economic stability. Unlike sudden-onset disasters, droughts develop across multiple spatial and temporal scales, often persisting for months to years, and their full consequences may only emerge long after the onset. This complex spatiotemporal dynamic hinders both early warning systems and risk management efforts, particularly as climate change alters the frequency, intensity, and spatial distribution of drought events. A critical uncertainty lies in how drought hazard and risk may evolve under extreme climate scenarios, such as in response to potential disruptions of the Atlantic Meridional Overturning Circulation (AMOC). Given the AMOC’s established influence on Northern Hemisphere precipitation patterns, its weakening under anthropogenic forcing could affect future drought dynamics. To address this, we analyse four pairs of model experiments. Each pair compares the weakening of AMOC against their respective control simulation, which has a stronger AMOC. Three pairs of experiments were conducted with the EC-EARTH3.3 climate model, where the AMOC is artificially weakened by introducing freshwater anomalies into the North Atlantic high-latitude ocean, under fixed radiative forcing respectively at pre-industrial, 2025, and 2050 (SSP5-8.5) conditions. The fourth pair of experiments was conducted with the NASA GISS ModelE climate model, in which the AMOC strength stochastically bifurcates (i.e., between 'on' and 'off' states) under identical SSP2-4.5 extended scenario and without artificial freshwater forcing. We evaluate changes in drought persistence and intensity by using an innovative approach based on Meteorological Drought Tracking built on the Standardized Precipitation Index (SPI).

How to cite: Volpi, D., Acosta-Navarro, J. C., Bellucci, A., Caporaso, L., Corti, S., Fioravanti, G., Hrast Essenfelder, A., Meccia, V. L., Simolo, C., Toreti, A., and Zampieri, M.: Assessing drought risk in the Northern Hemisphere under weakened AMOC, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-168, https://doi.org/10.5194/ems2025-168, 2025.

Droughts are among the most impactful climate-related hazards in Europe, with severe consequences for agriculture, ecosystems, water resources, and energy production. Despite their importance, the meteorological precursors and drivers of droughts remain difficult to anticipate due to their complex link with atmospheric circulation variability. This study investigates how large-scale North-Atlantic weather regimes (WR) influence the spatial and temporal characteristics of meteorological droughts across Europe, throughout the entire annual cycle.

Using ERA5 reanalysis data from 1960 to 2023 and the Standardized Precipitation Index (SPI) at a three-month timescale, we examine the statistical relationships between WR frequency and the occurence of drought events across different areas of Europe. These areas result from a regionalization strategy consisting in aggregating adjacent grid cells according to the concurrence of dry spells. We compute year-round WR from anomalies of geopotential height at 500 hPa. Each WR is found to be associated with a mean precipitation anomaly pattern across Europe that exhibits a relatively weak seasonality.

Our results reveal that, for each considered region, the 90-day periods preceding the peak of drought events are characterized by a significantly anomalous occurrence of some WRs, with respect to their climatological frequency. We can thus recontruct a precipitation signal by using these frequency anomalies as weights to compute a weighted mean of the precipitation pattern maps associated to the relevant WR. The reconstructed precipitation signal is consistent with the precipitation anomaly pattern from drought composites, albeit with a weaker amplitude. We then characterize the situations for which WR frequency anomaly substantially contributes to the developement of meteorological droughts. Our results also highlight regional contrasts, suggesting that drought early-warning systems could benefit from tailored WR-based indicators.

Overall, these findings underscore the relevance of WRs as a physically interpretable framework to monitor and anticipate drought risk at sub-seasonal to seasonal timescales. Integrating regime diagnostics into operational monitoring could enhance the preparedness and responsiveness to drought episodes under both current and future climate conditions.

How to cite: Savary, O., Ardilouze, C., and Cattiaux, J.: Linking European droughts to large-scale atmospheric circulation, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-82, https://doi.org/10.5194/ems2025-82, 2025.

 We consider effects of two-way cloud-aerosol interactions on  cumulus clouds (Cu) in the maritime boundary layer and on  properties of mesoscale convective system (MCS) accompanying by a squall line formation. The simulations are performed using spectral bin microphysics (SBM). The fields of Cu clouds are simulated using the model SAM in LES with an 10 m grid resolution. The mixed phase MCS was simulated using  the WRF model with 1.5 km resolution and calculating aerosol, drop, snow, graupel and hail size distributions. The  SAM/SBM includes an aerosol budget that tracks aerosols as they are activated into cloud droplets, evolve within growing droplets through diffusional growth and drop collisions. The WRF simulate mixed phase clouds. Acordingly, the budget of aerosols in ice is taken into account. In the WRF/SBM aerosols are transported into ice hydrometeors via immersion freezing. Aerosol mass (size) increases in drops due to drop-drop collisions and in ice particles during drop-ice (riming) and ice-ice collisions. Finally, aerosols are either released back into the surrounding environment through droplet evaporation and ice sublimation or fall to the surface within precipitating particles. The results show that Cu as well as deep convective clouds effectively transport aerosols to the upper atmosphere. It was found that convective clouds and cloud systems  are effective generators of giant CCN even in pulluted cases. The released aerosols significantly raise background aerosol concentrations, altering the mean size distribution over a large area surrounding cumulus clouds and the MCS, respectively. Some of the released aerosols re-enter the cloud through the  lateral boundaries, leading to changes in cloud microphysics.   These changes  lead to increased liquid water content, as well the mass contents of graupel and hail.  Strengthening cloud dynamics intensify convection and precipitation.

.

How to cite: Khain, A., Lynn, B., Arieli, Y., Gavze, E., and Koren, I.: Effect of mutual cloud-aerosol interaction on boundary layer cumulus clouds and a mesoscale convective system, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-118, https://doi.org/10.5194/ems2025-118, 2025.

Clear-Air Turbulence (CAT) is a major aviation hazard that occurs in cloudless regions, primarily near jet streams at upper tropospheric levels. It is responsible for the majority of weather-related aircraft incidents, posing risks to both passenger safety and airline operations. As the climate warms, jet streams are projected to intensify, potentially leading to more frequent and severe CAT events. This study assesses historical and projected changes in CAT frequency over the Northern Hemisphere using a combination of atmospheric reanalysis datasets and climate model simulations from coupled model experiments.

We apply several well-established CAT diagnostics to evaluate the sensitivity of the results to different representations of turbulence. The reanalysis data for the period 1980–2021 reveal significant positive trends in CAT frequency, particularly over North Africa, East Asia, and the Middle East. Signal-to-noise analysis indicates that these trends are likely attributable to anthropogenic climate forcing. In contrast, over the North Atlantic and North Pacific, internal climate variability dominates, obscuring the response to external forcing.

Projections from climate models show that CAT will continue to intensify throughout the 21st century, with a high level of model agreement across multiple emission scenarios and diagnostic methods. The most pronounced increases are projected over East Asia, reinforcing this region as a future hotspot for turbulence-related flight risks. However, over the North Atlantic, uncertainty remains high due to inter-model discrepancies and diagnostic sensitivity.

These findings highlight the emergence of CAT as a climate-driven risk that is already detectable in recent decades and projected to worsen in the future. Improved turbulence forecasting and adaptation strategies will be essential to mitigate the increasing threat CAT poses to global aviation safety in a warming world.

How to cite: Foudad, M.: Clear-Air Turbulence in a Changing Climate: Past Trends, Future Projections, and Aviation Impacts, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-263, https://doi.org/10.5194/ems2025-263, 2025.

Accurately predicting blocked weather regimes in the Euro-Atlantic region remains challenging despite their relevance for extreme weather events. Previous studies have suggested a connection between the misrepresentation of blocking events in numerical weather prediction models and sea surface temperature (SST) biases in the Gulf Stream region. However, the pathway linking SST in the Gulf Stream region to the downstream upper-level flow remains not fully understood.

Here, we employ numerical sensitivity experiments with varying SST conditions for an atmospheric blocking event associated with a winter heat wave to enhance our physical understanding of the relationship between Gulf Stream SST and downstream atmospheric blocking. The blocking event was preceded and accompanied by several extratropical cyclones, whose associated rapidly ascending air streams, so-called warm conveyor belts (WCBs), played a key role in the development and amplification of the downstream upper-level ridge.

Our sensitivity experiments, which include idealized and weakened SST gradients in the Gulf Stream region, show that the SST gradient influences moisture availability in the WCB inflow region through modified air-sea interactions and, consequently, WCB ascent. For example, in our case study a stronger SST gradient leads to increased specific humidity in the lower troposphere, resulting in more pronounced WCB ascent, increased cross-isentropic ascent as well as increased WCB outflow heights, which subsequently strengthens the downstream ridge. We conclude that variations in SST representation can affect the large-scale atmospheric flow via the WCB airstream as mechanistic link between lower and upper troposphere. In particular, low-level moisture availability - influenced by SST characteristics - affects subsequent WCB ascent and outflow characteristics, which in turn modulate the downstream upper-level ridge. Moreover, available low-level moisture consistently modifies WCB characteristics throughout the inflow, ascent, and outflow phases.

How to cite: Christ, S., Wenta, M., Grams, C. M., and Oertel, A.: The influence of sea surface temperature characteristics on downstream atmospheric blocking from a numerical model perspective, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-209, https://doi.org/10.5194/ems2025-209, 2025.

Despite extensive research on processes leading to surface heat extremes, so far only a few studies have investigated the vertical temperature profile during heat extremes.
This study aims to fill this gap by providing a global analysis of the vertical temperature profile during heat extremes using ERA5 data during the period from 1951 to 2021. We systematically characterize the vertical temperature anomaly profiles, i.e., the deviation from the climatological profiles, during heat extremes, which were defined as the maximum hourly 2-m temperature of the year (so-called TXx events). At every grid point, the temperature anomaly profiles were determined for all 71 TXx events in the considered period, and these profiles were normalized first with the climatological temperature variance at the respective level and second with the normalized surface temperature anomaly. A median profile for each grid point was derived from these normalized temperature anomaly profiles. Then, these median temperature anomaly profiles across the globe were clustered with a k-means approach, which reveals a set of distinct vertical profiles and their regions of occurrence. In tropical regions, extreme heat events have positive temperature anomalies confined to the lower troposphere, whereas positive temperature anomalies associated with heat extremes in extratropical regions tend to extend throughout the troposphere.
In particular, heat extremes in continental mid-latitude regions feature a common median normalized temperature anomaly profile that extends throughout the troposphere. Normalized temperature anomaly profiles during recent record-breaking heatwaves have the same shape as the climatological heat extreme cluster, which indicates that temperature anomaly profiles during the most extreme heatwaves have a similar vertical structure as during average TXx events. Our approach also allows investigating the temporal evolution of these clusters, which provides insights into the three-dimensional processes driving heat extremes, e.g., whether heat extremes typically develop bottom-up or top-down. By advancing our understanding of their vertical structure, this study yields novel information about the processes leading to surface heat extremes and determining their intensity.

How to cite: Hotz, B., Noyelle, R., and Wernli, H.: Global analysis of the vertical temperature anomaly structure of surface heat extremes, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-216, https://doi.org/10.5194/ems2025-216, 2025.

Dry intrusions (DIs) are a peculiar dynamical feature. They are synoptic scale descending air streams, located behind cold fronts of midlatitude cyclones. They are strongly linked to extreme weather events such as cold spells, extreme winds, dust storms and extreme wildfires. In these extreme events, strong negative temperature anomalies are induced by the arrival of DIs, somewhat surprising considering the strong subsidence of the air.

In this work we focus on the dynamical processes leading to the formation of the temperature anomalies. To this end, a global Lagrangian database of DIs and a temperature anomaly decomposition along the trajectories is used to highlight the contribution of adiabatic, advective and diabatic processes. Although DIs with cold temperature anomalies are frequently discussed due to their association with extreme events, our results show that both positive and negative anomalies are found at the end of the DI descent.  Globally, DIs with a positive temperature anomaly are as frequent as DIs with negative temperature anomalies.

Most DIs start from a negative temperature anomaly and the air is adiabatically heated as it descends. The heating is counterbalanced by adiabatic cold horizontal advection. The diabatic contribution is also negative along the DI descent. However, in the last 24 hours of the descent, the diabatic contribution increases substantially only in the negative temperature anomaly trajectories. Change in diabatic cooling occurs as the DI air reaches the boundary layer and is associated with an increase in humidity in the airmass. This diabatic contribution is the main difference between the positive and negative anomalies seen at the end of the DI descent.

We present a comprehensive look on the dynamical processes that affect the formation of temperature anomalies near the surface from descending air streams. We include global results to first understand DI climatology, as well as individual case studies demonstrating the uniqueness of each case.

How to cite: Magaritz Ronen, L. and Raveh Rubin, S.: A global analysis of dry intrusion temperature anomalies, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-315, https://doi.org/10.5194/ems2025-315, 2025.

The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken by the end of this century across all future scenarios considered by the IPCC Sixth Assessment Report. Consequently, the climate system will likely be influenced not only by continued global warming but also by the effects of a reduced AMOC. In this study, we assess the impact of AMOC weakening on extreme cold events in winter and extreme warm events in summer over Europe, using targeted sensitivity experiments with the EC-Earth3 climate model. Starting from a fully coupled ocean-atmosphere simulation with an artificially weakened AMOC, we conducted a series of atmosphere-only integrations with prescribed sea surface temperatures and sea-ice cover to isolate the atmospheric response to both moderate and strong reductions in AMOC strength.

Our results show that during boreal winter, a weakened AMOC induces average cooling over Europe and intensified cold extremes. However, the cooling at the Northern Hemisphere’s high latitudes intensifies the near-surface temperature meridional gradient at high northern latitudes. The enhanced meridional temperature gradient strengthens the jet stream, which in turn reduces the frequency of atmospheric blocking over the North Atlantic and northwestern Europe. Since wintertime blocking is typically associated with prolonged cold spells, this mechanism leads to a paradoxical reduction in such events despite the overall cooling.

In boreal summer, the weakening AMOC causes widespread cooling across the Northern Hemisphere, especially over the North Atlantic. While most of Europe experiences a decrease in extreme warm events, Eastern Europe and western Russia emerge as exceptions, with an increased frequency of heatwaves. As the AMOC weakens, maximum cooling during boreal summer occurs over the North Atlantic, reducing the temperature gradient at higher latitudes. Consequently, the jet stream weakens which facilitates the development of atmospheric blocking patterns. These blocking, through mechanisms such as subsidence warming and increased shortwave radiation under clear skies, contribute to more frequent heatwaves in the region.

Our findings underscore the pivotal role of large-scale ocean circulation in shaping regional climate extremes. As the AMOC is expected to weaken in the coming decades, understanding its interaction with atmospheric dynamics is essential for improving projections of future climate risks, particularly the compound effects of global warming and ocean circulation changes on European weather extremes.

How to cite: Meccia, V. L., Simolo, C., Bellomo, K., and Corti, S.: Extreme temperature events in Europe under a reduced AMOC, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-74, https://doi.org/10.5194/ems2025-74, 2025.

The increasing frequency of extremely hot events poses significant societal and scientific challenges due to their adverse impacts on human and natural systems, compounded by their unpredictable nature. Climate models are essential tools for investigating the amplification mechanisms of extremes and anticipating their changes under continued greenhouse gas warming, conveying vital information for decision-making. Future projections, however, remain limited by inherent uncertainties stemming from the range in the emission scenarios, inter-model structural differences, and internal climate variability. Furthermore, despite progress, models still struggle to accurately capture observed regional trends, particularly in extreme events.
Observational constraint theories, which link past and future behavior of physical observables, provide an effective paradigm to address model deficiencies and reduce uncertainties, though they often rely on empirical, region-specific relationships. Here, we show that future changes in the probability of hot extremes, and their uneven spread across global land areas, critically depend on the historical mean properties of thermal distributions and their evolution. Among these, historical variability plays a central role in shaping the rate of increase in hot extremes, either amplifying or suppressing the effects of regional background warming. Thus, the accuracy of historical simulations can substantially impact projected hot-event probabilities. Based on this, we develop an analytical approach to seamlessly combine global-scale observations with model outcomes, aiming for more reliable projections.
Despite uncertainties in both simulation and observational data, our results suggest that hot extremes may grow faster than models imply across much of the land surface. In regions like the Euro-Mediterranean and Southeast Asia, observation-constrained growth rates of hot extremes could nearly double bare model projections, even under moderate levels of warming. Crucially, exceeding the 2 °C global warming threshold could push highly vulnerable areas, such as the Amazon and tropical lands, into uncharted climate conditions where extremes become routine. These findings provide a more realistic foundation for assessing future risks and highlight the urgent need for strengthened adaptation and mitigation efforts to prevent rapid, potentially irreversible climate shifts.

How to cite: Simolo, C. and Corti, S.: Growth rates of hot extremes: insights from observation-constrained model projections, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-351, https://doi.org/10.5194/ems2025-351, 2025.

Extreme daily temperatures, whether hot days in summer or cold days in winter, have important societal impacts.  Such impacts can in part be mitigated with advance warning. However, current seasonal forecasts do not typically give information on the likelihood of daily extremes within the coming season, instead focussing on the seasonal mean climate. The first step in improving forecasts of the likelihood of extreme days is to understand the relationship between the seasonal mean and daily extremes. We examine reanalysis fields to show that for many regions of the Northern Hemisphere winter, the seasonal mean temperature and coldest day are more strongly correlated than random subsampling alone would suggest. We investigate the spatial variability of the seasonal mean/ extreme daily temperature correlation.

We show that in winter regions of high correlation overlap regions where known dynamical drivers of winter climate act on both the mean and extremes of the daily mean temperature distribution. In summer, however, the mean-extreme relationship is typically weaker than in winter and much less driven by large scale dynamical drivers.

Given this strong mean-extreme relationship in reanalysis, we examine whether the temperature of the most extreme day within a season can be inferred using a prediction of the seasonal mean climate from the Met Office Decadal Prediction System. We find significant skill over many ocean regions but more limited skill over land regions. However, in regions where skill is significant, forecasts primarily predict the long-term warming trend, rather than interannual variability. Our work offers hope for progress in future, since the dependence of the relationship between mean and extreme on underlying dynamical drivers in reanalysis suggests that should seasonal mean predictions improve, predictability of extremes would also improve in many regions.

How to cite: Maidens, A., Thornton, H., Bett-Williams, P., and Smith, D.: Can we forecast the most extreme daily temperature within a season using the forecast seasonal mean temperature?, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-466, https://doi.org/10.5194/ems2025-466, 2025.

Extra-tropical cyclones (ETCs) are a key part of the mid-latitude circulation and can lead to hazardous weather such as heavy precipitation and strong winds. However, how the intensity of extra-tropical cyclones will change in the future remains somewhat uncertain. This is partially due to the “tug-of-war” between the predicted increase in the upper-level temperature gradient, which would theoretically increase ETC intensity, and the predicted decrease in the lower-level temperature gradient which would decrease the intensity of ETCs. Additionally how diabatic processes will influence the intensity of ETCs in the future also remains uncertain. To quantify the relative contribution of these different factors, we adopt an idealised modelling approach. We have performed a large (6500 members) ensemble of baroclinic life cycle simulations with OpenIFS, which is a version of ECMWF’s Integrated Forecast System. Each ensemble member has a different initial state which is generated by varying 7 parameters: surface temperature; surface relative humidity; jet width, height and strength; lapse rate; and surface roughness. We objectively track the ETCs which develop in each simulation and for each ETC we compute 13 intensity metrics, such as maximum vorticity, wind footprint, a storm severity index and accumulated precipitation. A Random Forest Regressor (RFR) is then use to separately predict each of the 13 intensity measures using 5 features describing the initial state as input. One of the properties of RFR is its ability to rank its input features during the training. As a result, this embedded feature selection allow us to quantify the strength of relationship - called feature importance – and thus identify the aspects of the initial state which have the strongest influence on the resulting ETCs intensity. With the exception of the storm severity index, the RFR is able to predict the intensity measures with a coefficient of determination between 0.65 and 0.92. Wind-based measures are better predicted than all of the precipitation intensity measures. When all ensemble members are considered, we find that the low-level temperature gradient is the most important feature explaining ETC intensity, closely followed by the upper-level temperature gradient. However, when only the most extreme ETCs are considered, the upper-level temperature gradient becomes the dominant feature. These results, along with a discussion of their implications, will be presented.

How to cite: Sinclair, V., Bouvier, C., and Cornér, J.: Identifying controls on extra-tropical cyclone intensity using baroclinic wave simulations and machine learning, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-594, https://doi.org/10.5194/ems2025-594, 2025.

Extratropical cyclones (ETCs) produce most of the day-to-day weather variability in Europe and can cause potentially damaging winds and precipitation. Therefore, it is of special societal interest to understand the atmospheric conditions which lead to impactful ETCs. In this work, we build on well-established theories of ETC intensification by using statistical and machine learning methods to quantify relationships between atmospheric conditions at time of ETC genesis, here called precursors, such as baroclinicity and potential vorticity, and ETC intensity measures, such as relative vorticity and wind speed. To capture full variability in ETC intensity, we analyse multiple measures to quantify the intensity from both dynamical and impact-relevant perspectives. The aim of the study is to identify the precursors which control the intensity of ETCs and quantify how changes in their pattern and magnitude relate to responses in the intensity. A further aim is to investigate if impactful ETCs are associated with specific patterns in their precursors.

ERA5 reanalysis data from 1979 to 2022 between October and March was used to track ETCs in the North Atlantic and Europe and to analyse five intensity measures for each ETC. The response of these measures, namely 850-hPa relative vorticity, 850-hPa wind speed, wind footprint, precipitation, and a storm severity index, to changes in ETC precursors analysed from ERA5 was quantified using ensemble sensitivity analysis (ESA). Sensitivity patterns were calculated for an ensemble of all tracked ETCs, and the precursors which were found to have the most control on ETC intensity were identified. These precursor fields were then classified with a self-organizing map (SOM), and correspondence between the precursor classes and ETC intensity was investigated to identify precursor states potentially leading to impactful ETCs.

The ESA indicated that large values in wind-based intensity measures were associated mostly with a stronger jet stream, especially downstream of the ETC and tilting northeastward, as well as a larger meridional temperature gradient, with an emphasis on warming on the equatorward side at upper levels and cooling on the poleward side at lower levels. Precipitation response was controlled by one-sided changes in temperature and moisture, with warming and more moisture especially in the warm sector of an ETC being associated with more precipitation. Potential vorticity anomalies at both upper and lower levels were found to have little control on ETC intensity. The SOM classification was able to detect precursor states which were associated with impactful ETCs at a statistically significant level, and these states were consistent with signals from the ESA. These results can be used to study the occurrence of intense ETCs in future climate projections and to better forecast potentially impactful ETCs.

How to cite: Cornér, J., Bouvier, C., and Sinclair, V. A.: Genesis Conditions and Extratropical Cyclone Intensity in the North Atlantic and Europe: A Statistical and Machine Learning Analysis, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-22, https://doi.org/10.5194/ems2025-22, 2025.

The Mediterranean region is highly sensitive to climate change, largely due to the variable response of its hydrological cycle. Mediterranean cyclones are the major weather system governing the variability of precipitation (P) and ocean evaporation (E). Mediterranean cyclones themselves are a heterogeneous group of systems in terms of their development mechanisms and surface impact, ranging from winter baroclinic lows to transition-season convective systems and spring heat lows over land, governed by different upper-level forcing and interaction with the lower troposphere and near surface. It is, however, unclear how P and E respond to different types of cyclones, and if any links emerge between observed trends in Mediterranean cyclones, P and E.

Here we integrate P and E under the influence of differently-driven cyclones by combining climatological ERA5 datasets (1980-2020) of P and E, composite cyclone tracks and a recent potential-vorticity based cyclone classification. Our results confirm the prime importance of Mediterranean cyclones for yearly means of P and E. We find alternating dominance of different cyclone types in different regions and seasons, with the strongest precipitation and ocean evaporation attributed to the winter cyclones, and the least contribution to both quantities from spring and summer systems. Interestingly, climatological P-E budget varies by cyclone type, and display variable, and at time, opposing observed trends. The trends are tightly linked to Mediterranean cyclones and are largely a result of changing cyclone frequency and/or P and E totals per cyclone. Overall, if observed trends continue, we expect P to decrease and E to increase due to decreasing P and increasing E per some winter cyclones and increasing frequencies of non-precipitating heat lows.

How to cite: Raveh-Rubin, S., Givon, Y., Keller, D., and Drobinski, P.: The climatological contribution of Mediterranean cyclones to precipitation and ocean evaporation, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-614, https://doi.org/10.5194/ems2025-614, 2025.

The Greater Alpine Region (GAR) stands out as a hotspot for hydroclimatic extremes, where the interplay between complex orography and atmospheric circulation enhances the occurrence of intense precipitation events. As climate change progresses, understanding how these extremes are represented in climate models — and how they may evolve — is crucial for anticipating future hydrological risks in mountainous areas and downstream lowlands, as well as for managing water resources. This work explores daily precipitation extremes using a multi-model, multi-dataset approach. Alongside ERA5, we incorporate high-resolution gridded datasets to benchmark simulations from 29 CMIP6 global models and 18 EURO-CORDEX regional models. Extreme precipitation is characterized through the suite of ten indices defined by the ETCCDI, applied consistently across datasets. Our evaluation focuses on both the statistical properties of precipitation extremes and their spatial patterns across the GAR, with particular emphasis on how model resolution and orographic complexity influence skill. Regional simulations with higher resolution are further analyzed over the Piedmont domain, where terrain-driven variability is especially pronounced. We apply a clustering technique to identify coherent model behaviors and select those that best reproduce observed extremes. The analysis reveals a subset of models that outperforms the broader ensemble, especially in capturing spatial signals shaped by orography. In addition, a comparison is made between regional results in the Piedmont domain and projections derived from a trend-based scaling of ERA5, allowing us to assess how model projections perform in relation to observed extreme precipitation. This refined selection provides a more credible basis for projecting future changes in extreme precipitation over the Alpine region.

How to cite: Arnone, E., Cravero, M., Saglietto, G., Cortesi, N., Kushwaha, V. K., Ferguglia, O., Palazzi, E., Cremonini, R., Brussolo, E., and Terzago, S.: A multi-model view of future extreme precipitation in the Greater Alpine Region, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-621, https://doi.org/10.5194/ems2025-621, 2025.

Based on the station rain gauge and ERA5 reanalysis data, the seasonal and diurnal variations of rainfall over South China have been analyzed and unique climate features of the rainfall center along the northern coast of the Beibu Gulf have been revealed. The average accumulated rainfall amount in Dongxing along the northern coast of the Beibu Gulf exceeds 2300 mm/a from 1981–2020, which is the largest annual rainfall amount in mainland China (under the density of the national-level station network) and the area with the highest summer water vapor content over China. Moreover, unlike the dominant rainfall amount during the pre-rainy season (autumn) in the eastern part of South China (over Hainan Island), rainfall along the northern coast of the Beibu Gulf reaches its highest level in June–August. The high morning peak at 8:00 Beijing time (BJT) is out phase with the significant unimodal afternoon peak over central Hainan Island and bimodal peaks during the early morning and afternoon in the eastern part of South China. In summer, the strong low-level southerly winds over the Beibu Gulf and local topography are crucial to the formation of this rainfall center. The strong southerly winds, blowing towards the coastlines, induce intense convergence at 900-1000 hPa in the coastal areas of South China. Under the influence of the Shiwa Mountain Range near the northern coast of the Beibu Gulf, the greatest divergence occurs here in the lower troposphere (700-900 hPa) at the same latitude in South China. The interaction between the divergence and convergence in the lower troposphere, combined with the abundant moisture, greatly contributes to the formation of the rainfall center. The ascending motion and water vapor transportation are even enhanced in the morning, favoring the diurnal rainfall peak.

How to cite: Yuan, W.: Unique climate features of the rainfall center along the northern coast of the Beibu Gulf, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-693, https://doi.org/10.5194/ems2025-693, 2025.

Cut-off lows are, and will be in the future, one of the main threats related to severe weather in the Iberian Peninsula, especially in the Mediterranean arc. Cut-off lows are often accompanied by heavy precipitations in a short time promoting flash-floods, as well as hail, strong convectively wind gusts and/or tornadoes.   

On the week of October 27^th – November 4^th, 2024, a cut-off low affected the Iberian Peninsula with extreme socio-economic impacts in several Spanish regions and, especially, in the Valencia area. The severe weather phenomena on the surface have differed depending on the region: large hail (5-7 cm), several tornadoes, strong wind gusts and, above all, extreme precipitations. The most severe day was October 29^th in the Valencia region, with rainfall accumulations higher than 300 mm in a notable area and locally registering 771 mm in 24 hours. In addition, the Turís official weather station recorded numerous national records for rainfall intensity. Moreover, the convective system developed 11 tornadoes (two of them with intensity IF2) and large hail (~ 5 cm). The social impact of the floods in Valencia was very high, with more than 16.5 billion euros of damage to infrastructure (roads, railways, etc.), housing and croplands, as well as 225 fatalities.

In this survey, we focus on Valencia’s floods on October 29^th. Here, by performing model simulations with the WRF-ARW model and using a storyline approach, we find an enhancement in intensity and a significant increase in extreme accumulated rainfall area (e.g., 100 mm, 180 mm, 200 mm, and 300 mm) caused by current anthropogenic climate change conditions compared to preindustrial ones. Moreover, the enhanced available water vapor content played a central role, while CAPE, diabatic heating, and stronger vertical velocities boosted convective processes. A deeper warm cloud layer and elevated graupel concentration reveal microphysical mechanisms that enhanced precipitation volumes in a warmer climate, exceeding Clausius-Clapeyron scaling.

This study highlights the growing risks in the Mediterranean area and the urgent need for effective adaptation in urban planning to reduce the hydrometeorological extremes in human-induced climate change.

How to cite: Calvo-Sancho, C., Díaz-Fernández, J., González-Alemán, J. J., Halifa-Marín, A., Miglietta, M. M., Azorín-Molina, C., Prein, A. F., Montoro-Mendoza, A., Bolgiani, P., Morata, A., and Martín, M. L.: Anthropogenic Climate Change Attribution to the Valencia’s deadly floods, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-504, https://doi.org/10.5194/ems2025-504, 2025.

Within the global effort to understand climate extremes and their projected changes, the response of hailstorms to global warming remains unclear. Hailstorms are difficult to model due to their low frequency of occurrence, scarcity and short duration of available observations. Moreover, the spatio-temporal scales involved in hail formation are small, requiring high resolution modelling. In this study, we present a novel approach to model future changes in hailstorm frequencies by combining reanalysis data with climate model projections, in an attempt to overcome the issue of coarse model resolution. We identify a set of 84 storm-related parameters, which are found to be good hail predictors and can be estimated by current global or regional climate model simulations. The hail predictors were retrieved from 3-hourly time scale ERA5 reanalysis data during 1999-2023, and calibrated against a hail dataset (Laviola et al. 2022) derived from passive microwave satellite observations over the whole Mediterranean basin (5W-35E, 25N-50N). A multi-model of 9 CMIP6 simulations was developed, selecting models with daily output, spatial resolution finer than 100 km and availability of variables (relative humidity, geopotential height, temperature and u- and v-component of wind) used to calculate the 84 hail predictors. ERA5-based hail predictors were projected to the end of the century using CMIP6 multi-model trends, adopting a trend-based scaling approach. The proposed technique provides a simple yet robust framework for assessing future changes in the occurrence of hail events. Hail climatologies were also compared to those of the extreme ETCCDI indices. Results of the assessment indicate that the multi-model is able to simulate the observed hail climatology over the Mediterranean with good accuracy (pattern correlations of 0.90), particularly capturing the main maxima over the Po Plain and over the central Mediterranean sea between Sicily and Greece. Eventually, a strong connection was found between large-scale atmospheric circulation (represented by the Euro-Atlantic weather regimes of Grams et al. 2017) and hailstorm occurrence, as also highlighted by ten exceptional hailstorms (>10 cm) occurring during summer 2023.

How to cite: Cortesi, N., Cassardo, C., Laviola, S., Monte, G., Capozzi, V., and Arnone, E.: Climate extremes in the Mediterranean region: the challenges of predicting hailstorms with current climate model simulations, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-314, https://doi.org/10.5194/ems2025-314, 2025.

Hailstorms are among the most damaging weather phenomena impacting people's life and human activities. Climate change is expected to modify the environmental conditions in which hailstorms typically develop, affecting low-level moisture and convective instability, microphysical processes, and vertical wind shear. This is particularly true in a climatic hotspot such as the Mediterranean area, which is one of the most hail-prone areas of the Earth. Despite their relevance and associated risk in the Mediterranean, open issues remain concerning the identification of the synoptic-scale meteorological conditions favourable to hailstorms’ occurrence.

Using the high-resolution (1°x1°) daily satellite-based hail climatology developed in Laviola et al. (2022), this work aims to identify the main summer large hail (>2 cm) spatial patterns in the Northern Italy (44.0-47.0°N, 6.0-14.0°E) and the associated atmospheric types. To pursue such goals, Principal Component Analysis and Cluster Analysis have been employed. Several atmospheric fields retrieved from ERA5 have been considered to characterize the large-scale atmospheric patterns. To mitigate possible inconsistencies in the available dataset, the period 2014-2023 has been selected for this analysis.

Main results shows that the spatial distribution of large hail events in Northern Italy follows three main clusters that appear to be relatively well separated among them. In the first cluster, the hail episodes mainly affect the northeastern Italy; in the second cluster, the hail events are concentrated over the Po Plain, whereas in the third cluster the hailstorm generally affect the northwestern sector. The atmospheric circulation schemes associated with the first two spatial clusters (ACS  1 and ACS 2) are linked by the presence of a trough over the western Mediterranean basin, which advects a warm and moist southwestern flow over northern Italy, and by a thermal boundary at 850-hPa level. The ACS related to the third cluster (ACS 3) is characterized by a cyclonic area over western Europe, associated with a strong thermal boundary over France with a hot subtropical southwestern flow over Italy. Another key meteorological feature of the identified synoptic patterns lies in the anomalous intense water-vapor transport in the 2-5 km atmospheric layer, favouring large liquid water content. From a comparison between 2014-2018 and 2019-2023 sub-periods, it emerged an increase of the relative contribution to the total number of hail events provided by ACS 3 (from 19.6 to 31.8%). This means that hail events are increasing in the northwestern Italy in terms of frequency of occurrence, both in absolute and relative terms.

References

Laviola, S.; Monte, G.; Cattani, E.; Levizzani, V. Hail Climatology in the Mediterranean Basin Using the GPM Constellation (1999–2021). Remote Sens. 2022, 14, 4320. https://doi.org/10.3390/rs14174320.

How to cite: Capozzi, V., Fucci, A., Budillon, G., Fusco, G., Arnone, E., Cortesi, N., Monte, G., and Laviola, S.: Identification of the atmospheric types that promote summer hail events in the Northern Italy, EMS Annual Meeting 2025, Ljubljana, Slovenia, 7–12 Sep 2025, EMS2025-549, https://doi.org/10.5194/ems2025-549, 2025.