Data from lightning detection sensors and dual-polarized weather radars are used for high-resolution analysis of hailstorms developing around the Ore Mountains ridge. Measured radar and lightning parameters provide information on storm dynamics and are indicative of severe hailfall occurrence. Our case study deals with a multicellular hailstorm that developed during a summer afternoon near the ridge of the Ore Mountains from Germany to Czechia.

On 27 June 2022, hailstorms formed on both the windward and leeward sides of the Ore Mountains ridge. The multicellular storm evolved in warm air ahead a warm front advancing to the east. Severe hail was reported in many places, with hailstones up to 5 cm in diameter. And high lightning activity was detected.

Both radar and lightning detection data were used to study the hailstorm evolution thoroughly. Lightning data from the ENTLN network provide lightning activity parameters from the entire area of interest. Radar data from the Czech weather service C-band radar network are covering the area of interest as well. Data from the X-band radar situated on the Milesovka hill are used in the radar range, which is 30 km from the Milesovka observatory. This measurement does not cover the entire study area of the storms, but provides more detailed information, also due to the frequent RHI scanning in the scanning strategy.

How to cite: Skripniková, K. and Sokol, Z.: A case study of hailstorm dynamics during mountain ridge crossing, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15921, https://doi.org/10.5194/egusphere-egu25-15921, 2025.

Thunderstorm-related severe weather, particularly hail, causes extensive damage to life and infrastructure across Europe. However, the effect of a warmer climate on the occurrence of hail is still not fully understood. To date, most projections of hail occurrence under future climate scenarios have relied on hail proxies derived from global and regional climate models that use parameterized representations of convection. Recently, convection-permitting regional climate simulations with a high computational resolution of 2 km, using the COSMO model with the online hail diagnostic HAILCAST, have provided new insights. The simulations revealed spatially contrasting changes in hail frequency under a 3°C global warming scenario, showing a substantial decrease in summer hail frequency in southwestern Europe and an even larger increase in central and eastern Europe. In this study, we leverage these high-resolution model outputs to assess future projections of hail occurrence. Specifically, we compare differences in hail day frequencies between a warmer future climate and the present day climate as derived from (i) traditional hail proxies using environmental variables (e.g. CAPE and wind shear) and (ii) the HAILCAST online diagnostic. Through this comparison, we aim to better understand two key questions: (1) how accurately hail proxies capture the spatial and temporal patterns of hail occurrence in comparison to HAILCAST, and (2) whether the relationship between environmental variables and hail occurrence remains stationary under changing climatic conditions.

How to cite: Thurnherr, I., Wilhelm, L., Cui, R., Feldmann, M., Beer, S., Schär, C., and Wernli, H.: Comparison of future hail trends across Europe based on the HAILCAST diagnostic and hail proxies, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16917, https://doi.org/10.5194/egusphere-egu25-16917, 2025.

Hailstorms can cause a lot of damage for agriculture and property. Therefore, efforts exist to mitigate hail damage by means of seeding a developing hailstorm with ice nucleating particles. Motivated by the Swiss hail mitigation campaign, we examined the impact of silver iodide (AgI) perturbations on a convective storm observed over northern Switzerland on July 6, 2019. We evaluated the effectiveness of an early seeding strategy and investigated the concept of beneficial competition, where increased number of INPs lead to the formation of smaller, less damaging hailstones. We used the Consortium for Small-Scale Modeling Regional Weather and Climate Model (COSMO) to simulate this case. AgI particles were added as a prognostic variable to the hailstorm during its cumulus stage and were released in the updraft region near the cloud base with concentrations ranging from 0.2/cm3 to 2000/cm3 in ensemble simulations. While seeding delayed the onset of precipitation, increased the graupel concentration and reduced supercooled liquid water, especially in the upper part of the convective cloud, no systematic change in the overall hail size has been found.

The lightning potential index (LPI) depends both on simultaneous occurrence of liquid water and ice species, as well as on the updraft strength. LPI increased in all seeding simulations in terms of intensity and spatial extent, because seeding increased the updraft strength and the graupel weighted ice mixing ratio.

How to cite: Lohmann, U., Papaevangelou, N., and Brülisauer, M.: Simulations of selective seeding of hailstorms over Switzerland: impacts for precipitation and lightning, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5635, https://doi.org/10.5194/egusphere-egu25-5635, 2025.

Mesoscale Convective Systems (MCSs) are organized collections of thunderstorms that typically consist of narrow, intense regions of convective precipitation alongside broader, lighter areas of stratiform precipitation. These systems are the primary contributors to extreme precipitation events across Europe (Da Silva & Haerter, 2023). While both convective and stratiform precipitation rates are expected to increase with temperature according to thermodynamic expectations (the Clausius-Clapeyron relationship), their statistical superposition may intensify at an even faster rate due to increased proportion of convective-type precipitation within MCSs under warmer conditions (Da Silva & Haerter, 2025, accepted).

Both the intensity and proportion of convective-type precipitation in MCSs play a critical role in determining flood risks, but the spatial and temporal organization of convection is equally significant for shaping the characteristics and severity of flooding. Larger, long-lived clusters of convection within MCSs are indeed more likely to trigger severe flooding compared to smaller, isolated clusters.

In this study, we analyze the spatio-temporal characteristics of convective clusters within MCSs. MCSs are identified and tracked using both radar precipitation data (RADOLAN radar; Bartels et al., 2004) and lightning records from the EUropean Cooperation for LIghtning Detection (EUCLID; Schulz et al., 2016) network over Germany. Convective-type precipitation is classified based on its proximity to lightning strikes. To explore links between these clusters and local environmental conditions, we incorporate data from German Weather Service (Deutscher Wetterdienst, DWD) weather stations and the ERA5 reanalysis dataset (Hersbach et al., 2020).

We measure the spatial clustering of convection within MCSs using two novel spatial organization indices that quantify deviations from random distributions. Our preliminary findings suggest that convective clusters within MCSs become wider at higher temperatures, consistent with observations of larger CAPE (Convective Available Potential Energy) environments. Additionally, we observe a geographic trend in the location of convective clusters: they are more frequently concentrated in the southern portions of MCSs. However, under warmer conditions, a larger fraction of MCSs exhibit convective clusters on their northern edges. We hypothesize that this shift is driven by stronger convective instability ahead of the northern flanks of MCSs at higher temperatures. This effect may be linked to increased near-surface baroclinicity and horizontal temperature gradients at warmer temperatures.

The temporal evolution of these convective clusters is further analyzed through the framework of directed percolation, a statistical physics approach that allows us to investigate the growth and connectivity of convective cells over time. Through this lens, we aim to better understand the lifecycle of convective clusters within MCSs, including their formation, propagation, and eventual dissipation. By combining spatial and temporal analyses, this study provides critical insights into how environmental conditions influence the organization of convection within MCSs, thereby advancing our ability to predict and mitigate flood risks in a warming climate.

How to cite: Da Silva, N., Monroy, D., Wilson, A., and Haerter, J.: Spatio-temporal aggregation of convective cell clusters in European MCSs, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18567, https://doi.org/10.5194/egusphere-egu25-18567, 2025.

The North American Monsoon (NAM) contributes the most significant amount of annual precipitation from July to September over northwestern Mexico. The increase in deep convection during the NAM is due to the emergence of extensive cumulonimbus cloud agglomerations, known as Mesoscale Convective Systems (MCSs). Previous studies suggest that upper tropospheric inverted troughs (IVs) (200 hPa), which occur over the NAM region, induce favorable environmental conditions for the development and intensification of MCSs. This talk shows the relationship of VIs with the organization of MCSs that occur over the NAM region. GOES-13 infrared satellite images (2010-2013) and the CLAUS database (1984-2008) are used to identify the trajectory of MCSs. ERA5 reanalysis data (1984–2008) characterize the atmospheric and thermodynamic conditions induced by VIs and upwellings. The results show that MCSs developed mainly in July and August, beginning to decline during September. A similar behavior is observed in the VIs, as most  of them originate from tropical upper tropospheric trough (TUTT) detachments. It is concluded that 22.8% of all MCSs formed during the study period interacted with VIs and produced more intense precipitation, unlike MCSs that did not interact. Although this percentage is small, MCSs that interacted with VIs induced more moisture to be transported to mid-atmospheric levels (500 hPa), compared to those that did not interact. Therefore, detecting these systems is essential to determine the existence of intense precipitation events over the NAM region.

How to cite: Dominguez, C.: The influence of inverted troughs on the formation of mesoscale convective systems during the North American Monsoon, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2623, https://doi.org/10.5194/egusphere-egu25-2623, 2025.

Doppler radar, FY-4B satellite data, wind profile radar, ERA-5 reanalysis data, automatic weather station and other multi-source observation data are used to analyze the causes and forecast deviation of a rainstorm on the eastern foot of Qinling mountains in Shaanxi province of China on 19^th July 2024, and the results indicated that the rainstorm had strong suddenness, locality and convection. The main water vapor and energy for the rainstorm were provided by the southwest low-level jet at 700 hPa, and the strengthening of the jet stream and the decrease in the height of the jet stream core had a good correlation with the occurrence of heavy precipitation. The mesoscale convective system(MCS) causing rainstorm was formed by the the merging and strengthening of locally generated cold clouds in the convergence and upward movement zone of 700 hPa jet stream front and mesoscale convective clouds moving from southwest to east. And the convective system continued to develop along the mountain direction, forming a "train effect" that strengthens local heavy rainfall. Topography played an important role in this rainstorm.  On the one hand, the Qinling Mountains block the low-level jet stream at 700hPa, causing water vapor to strongly accumulate on the windward slope and converge and rise. On the other hand, the Qinling Mountains produced a significant topographic uplift movement on the ground and southerly winds at 850 hPa, thereby strengthening precipitation together. The 24-hour precipitation forecast was significantly missed by several numerical models because of deviation of 700 hPa jet location and intensity, but the CMA-BJ model had good prediction on the falling area and intensity of heavy rainfall for 3 h. Conducting analysis of similar rainstorm events, summarizing the deviation characteristics of the numerical models, considering the triggering and maintenance mechanisms of convection, and the important role of terrain would help improve the forecasting and early warning capabilities of such rainstorm.

How to cite: Yiqing, X., Ming, L., and Yongyong, M.: Analysis on the Causes and Forecast Deviation of Rainstorm on the Eastern Foot of Qinling Mountains, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7625, https://doi.org/10.5194/egusphere-egu25-7625, 2025.

Previous studies have investigated the optimal configuration of squall lines affected by linear vertical wind shear and cold pools, as described by the RKW theory. However, the interaction dynamics between low-level jets (LLJs) with nonlinear vertical wind shear, and cold pools remain insufficiently understood. This study utilizes idealized numerical simulations to examine the impacts of LLJs on cold pools and their subsequent role in initiating convection, focusing on the sensitivity to LLJ height and strength. The simulations reveal that, under the influence of an LLJ, a cold pool typically evolves into a two-step structure with two distinct heads, and its intensity diminishes more rapidly compared to scenarios with quasi-linear shear or no wind. Two prominent regions of vertical velocity are identified near these heads, with oneassociated with the elevated cold pool head and elevated convection initiation. Variations in parcel lifting above and below the jet core arise from differences in horizontal vorticity induced by LLJ shear, resulting in two distinct clusters of parcel trajectories. A lower or weaker LLJ leads to earlier convection initiation due to larger initial vertical velocity driven by LLJ-cold pool interactions. Convection intensity peaks when the LLJ height aligns with levels of high CAPE and when vorticity pairing above the jet core reaches its optimal state, consistent with RKW theory.

How to cite: Du, Y., Fu, D., and Yang, H.: The Interaction Between Low-level Jets and Cold Pools and Their Impacts on Convection Initiation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5421, https://doi.org/10.5194/egusphere-egu25-5421, 2025.

Mesoscale convective systems (MCSs) frequently occur over southern China during early summer, often leading to significant precipitation and associated socioeconomic impacts. This study investigates the differences in MCS frequency and related precipitation in southern China before and after the onset of the South China Sea summer monsoon (SCSSM) during 2001－2020, using high-resolution satellite data from the Global Precipitation Measurement mission and iterative rain cell tracking (IRT) method that combines cloud and precipitation criteria.

Our analysis indicates that during two pentads after the SCSSM breaks out, the frequency of MCSs significantly decreases in southern China, especially over the middle and lower reaches of Yangtze River basin (MLYRB). Accordingly, the heavy rainfall amounts decrease sharply. For instance, the number of grids with hourly precipitation between 10 and 30 mm drops by over 40% over MLYRB after the monsoon onset. It is found that the remarkable weakening of lower-level vertical wind shear and abnormal descending motion over southern China are unfavorable for the formation of MCSs. Corresponding to the SCSSM onset, on the one hand, atmosphere warms much less over the tropical oceans, including the SCS, than over the extratropical lands, resulting in smaller magnitude of meridional air temperature gradient and a subsequent decrease in vertical wind shear. On the other hand, the lower-level northerly winds induced by the SCSSM onset result in the suppressed ascent flows over southern China.

How to cite: Li, W., Lu, R., and Wang, L.: The Influence of South China Sea Summer Monsoon Onset on Mesoscale Convective Systems in Southern China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5357, https://doi.org/10.5194/egusphere-egu25-5357, 2025.

Mesoscale Convective Systems (MCSs) account for over 50% of annual rainfall across the tropics and many regions of the subtropics and midlatitudes. They are often associated with strong lightning and extremely heavy rainfall, which can lead to floods. Consequently, comprehending the spatio-temporal attributes and long-term trends of MCSs is crucial for better preparedness to avoid natural hazards in current and future climatic conditions. In contrast to other MCS hotspots around the globe, investigations on the long-term alterations of MCSs in South Asia are still scarce. Our work employs high-resolution satellite brightness temperature and precipitation data, together with a novel MCS tracking method (PyFLEXTRKR), to generate a database of MCS occurrences in South Asia during the last two decades (2001-2020), and then perform a detailed analysis to understand their climatological characteristics, the environmental conditions favouring them and the long term trend. Bay of Bengal, Western Ghats, and Southern Peninsula of India are the sites where the highest number of MCSs develop. Additionally, there is a distinct seasonality in MCS activity, with the summer monsoon season seeing the highest formation of convective systems. During this season, especially over the Bay of Bengal, we observe the strong characteristics of the MCS, such as its larger area, longer duration, and greater rain rate. To a certain degree, the formation of small (<10⁴ km²) and medium-sized (10⁴ km²–4.4 x 10⁴ km²) MCSs is equally dispersed between the ocean and land, whereas the large (4.4 x 10⁴ km²–1.6 x 10⁵ km²) MCSs form mostly across the oceans. On the other hand, the Super (> 1.6x10⁵ km²) MCSs are exclusively found over the Bay of Bengal, primarily during the monsoon season. The percentage of rainfall contributed by MCSs over South Asia varies with the seasons, and the highest amount is received during the monsoon season. There is also a disparity over land and ocean, with land areas receiving 40%-50% and oceans receiving 55%-65% of their annual mean rainfall. Compared to the other types of convections, the MCSs contribute to the major fraction of total rainfall produced over both land and ocean, especially towards the higher magnitudes of rainfall. The ability to produce strong rainfall by any MCS strongly depends on its spatial extent (r_corr = 0.83 (0.88) over land (ocean)), followed by lifetime (r_corr = 0.48 (0.65) over land (ocean)), whereas the brightness temperature is negatively correlated weakly (r_corr = -0.24 (-0.37) over land (ocean)). The analysis of the large-scale environmental conditions reveals a gradual build-up of favorable conditions six hours before the initiation, with a noticeable increase within a 100-kilometer radius just three hours before the initiation. Our analysis shows that within the last 20 years, MCSs have increased in frequency and spatial extent. Furthermore, the precipitation associated with MCS has shown a notable upward trend. The rising frequency and severity of MCSs are propelled by increasingly conducive water vapor rich environment, which are expected to escalate with global warming. This could significantly impact the hydroclimate of South Asia, particularly the probability of severe events.

How to cite: Paul, D. and Dubey, S. K.: Comprehending the Attributes and Strengthening of Mesoscale Convective Systems over South Asia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2340, https://doi.org/10.5194/egusphere-egu25-2340, 2025.

High-resolution simulations, both at mesoscale and microscale, have become increasingly prevalent, often leveraging high-resolution terrain datasets. However, terrain-following coordinate models can encounter numerical instabilities in regions where terrain slopes exceed critical thresholds, generally around 35º. To address this issue, terrain smoothing is typically required. Current approaches usually involve applying global smoothing methods across the entire domain, which inevitably results in a loss of terrain detail and resolution to prevent numerical instabilities in regions where it is not necessary. Moreover, as the model resolution increases, the number of grid points with steep slopes grows, underscoring the need for alternative terrain smoothing strategies.

This study presents the development and implementation of a local terrain smoothing approach designed to mitigate numerical instabilities in a mesoscale model (the WRF model) and a microscale model (NCAR-RAL’s GPU-accelerated FastEddy^® LES model). Various smoothing techniques were evaluated, including both simultaneous and sequential approaches. Following a thorough performance analysis—considering the number of iterations required for convergence, computational cost, and, most importantly, the degree of terrain distortion—the most effective method was selected and implemented. The final approach applies a Gaussian filter (σ = 25) over a 3x3 grid centered on each steep-slope point, with a blending factor of 0.2 at the edges. This ensures that the central point is smoothed while the surrounding points retain 80% of their original terrain characteristics. Each steep slope is addressed individually but processed simultaneously across iterations. A higher blending factor results in greater terrain distortion, while a lower blending factor significantly increases computational time, often preventing convergence within the imposed iteration limit.

This terrain smoothing method has been fully implemented in FastEddy^® and is now used operationally and routinely within the model. This implementation will be made publicly available in the next release of FastEddy^®, hosted on GitHub (https:// github.com/NCAR/FastEddy-model, starting with version 3.0). For WRF, the method has been integrated as an additional step in the WPS workflow, following the execution of the geogrid program. The proposed local smoothing approach helps preventing the occurrence of CFL errors in high-resolution simulations over complex terrain without relying on excessively high values of the time off-centering parameter (epssm) to dampen vertically propagating sound waves, which can lead to excessive high-frequency damping, negatively impacting the accuracy of the simulations.

In conclusion, this study presents a simple yet effective method for avoiding terrain-driven numerical instabilities in high-resolution simulations, ensuring the maximal preservation of terrain resolution in both microscale and mesoscale models. This approach can be easily applied to other models, offering a straightforward solution to enhance numerical stability while maintaining high-resolution terrain features in diverse simulation environments.

Acknowledgements: The authors acknowledge the ECCE project (PID2020-115693RB-I00) of the Ministerio de Ciencia e Innovación/Agencia Estatal de Investigación (MCIN/AEI/10.13039/501100011033). ERL thanks her predoctoral contract FPU (FPU21/02464) to the Ministerio de Universidades of Spain.

How to cite: Montávez, J. P., Raluy-López, E., Muñoz-Esparza, D., and Sauer, J.: A Local Terrain Smoothing Approach for Stabilizing Microscale and High-Resolution Mesoscale Simulations: Application to FastEddy® and WRF, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17755, https://doi.org/10.5194/egusphere-egu25-17755, 2025.