Deciphering the rainfall trends over the Western Ghats situated along the west coast of India has been the subject of several recent studies. However, less attention is rendered, particularly to understanding the spatial characteristics of atmospheric features associated with widespread and elevation-dependent extreme rainfall occurrences over this region and this study intends to provide some insights into this aspect. This study observes a rising trend in extreme rainfall events over the Western Ghats during the 1979–2020 period, consistent with earlier investigations. The extreme rainfall events on the windward side located below and above 500 m above sea level exhibit different background circulation signatures, such as mean wind speeds of low-level jets and moist static stability. The extreme rainfall events seen below [above] 500 m above sea level occur in the backdrop of mesoscale [large-scale] monsoon circulation. A Froude number analysis further elucidates the importance of Western Ghats foothill topography in complementing the development and spatial segregation of extreme rainfall occurrences.

How to cite: Kooran, R. M. J., Vellore, R., Varikoden, H., Tamilselvam, G., and Krishnan, R.: Elevation-dependent characteristics of widespread rainfall extremes along the Western Ghats, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-526, https://doi.org/10.5194/egusphere-egu24-526, 2024.

Global warming and the associated climate change, unequivocally attributable to human activities and greenhouse gas emissions, will very likely intensify in the near future, resulting in the increased occurrence of extreme weather events, such as heatwaves. Summer heatwaves are widely studied to explore their thermodynamic precursors and their effects on health and natural ecosystems, while winter warm spells (WWS), defined by the Expert Team on Climate Change Detection and Indices (ETCCDI) as “a sequence of at least six consecutive days when the daily maximum air temperature exceeds the calendar day 90th percentile of the probability density distribution of the reference period", are still scarcely studied.

In this contribution, the temporal and spatial variability of WWS that occurred over the Italian Peninsula during the period 1993-2022 is investigated. The identification of WWS is carried out by examining the wintertime (December, January, February) maximum daily temperatures measured in eight Italian airport sites, belonging to different Köppen-Geiger climatological classes.

The WWS events involving the whole Italian territory or only northern/central/southern Italy are detected. It is interesting to note that although exceeding the 90th percentile of the daily maximum temperature is quite frequent, only one winter warm spell that affected the entire Italian territory is detected over the period 1993-2022. In the period under investigation, the synoptic conditions associated with WWS over Italy or a portion of the peninsula are, on average, characterised by anticyclonic systems centred on the western Mediterranean, responsible for persistent high-pressure conditions over Italy, subsidence and, therefore, exceptional warming.

Finally, the period length threshold used for the detection of WWS is reduced from six to three days. The outcomes suggest that, in orographically heterogeneous areas such as Italy, the definition of WWS provided by ETCCDI allows for capturing only synoptic scale events, losing information on moderate warm spells, which can have important implications on health and natural ecosystems. Therefore, for regional studies on complex terrain, it would be advisable to reduce the time threshold for the identification of WWS to three days.

This study is supported by the Boundary layer Air Quality-analysis Using Network of Instruments (BAQUNIN) project, funded by ESA, which allowed the establishment of one of the first observatories in the world to involve several passive and active ground-based instruments installed in multiple locations and managed by different research institutions. Moreover, this research is part of the activities envisaged in the “uRban hEat and pollution iSlands inTerAction in Rome and possible miTigation strategies” (RESTART) project, funded by the Italian Ministry for University and Research as a Project of National Interest (PRIN2022). RESTART aims to explore the urban heat island and the urban pollution island in Rome (Italy), providing a series of mitigation strategies, including tailored nature-based solutions, and ready-to-use guidelines for the improvement of well-being and liveability in urban environments.

How to cite: Di Bernardino, A., Iannarelli, A. M., Casadio, S., and Siani, A. M.: Spatial-temporal variation of winter warm spells in Italy over the period 1993-2022 , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1719, https://doi.org/10.5194/egusphere-egu24-1719, 2024.

Over the Baltic Sea, a semi-closed sea with a complex shoreline, direction of wind and its rapid changes within short time period can have significant influence on the resulting hydrodynamic processes. Highest air pressure gradient and therefore the strongest wind speeds over Baltic are usually the result of low pressure systems moving across the sea or in its vicinity. These events are strongly connected to a large scale atmospheric circulation in Northern Hemisphere and usually are generated over north-western Atlantic.

In this study we examine the meteorological conditions associated with high waves in the Gulf of Gdańsk (southern Baltic Sea). We selected 34 extreme storm events in five distinctly different locations within the area of interest based on the significant wave height (SWH) for years 1958–2001. The analysis of these events will be presented in a separate publication. Based on the 1-hour atmospheric pressure fields over north-eastern Europe, we traced the trajectories of low pressure centres for each storm event. These trajectories were subsequently classified based on their common characteristics and the impact they exert on the wind wave field in the region.

We identify and analyse four cyclone paths: two representing the most common trajectories (P1 and P2) associated with extreme wind wave conditions in the Gulf of Gdańsk, and two unique trajectories (P3 and P4) that occurred only once during the analysed period. Trajectories P3 and P4 are highly atypical for storm events in the area.

Path P1 is characterised by a low pressure centre moving from west to east across Scandinavia and through the Baltic, a pattern arising from zonal circulation that dominates the region. Storms generated by lows travelling along this path are among the most severe in the Gulf of Gdańsk, with SWH reaching nearly 9 m. Path P2 typically originates over the Norwegian Sea, following a NW-SE trajectory across the Baltic. This path has been infrequently considered in relation to wind waves in the southeastern Baltic, more commonly being associated to storm surges along the eastern Polish coast.

Path P3 features a low pressure system moving northward in the Atlantic, along the western coast of the Scandinavian Peninsula. During this particular storm (12–14 January 1984) the lowest SWH was recorded out of all 34 analysed events. The low pressure system following path P4 (9-12 April 1986) moved south of the Baltic Sea along the W-E trajectory. Its relative position to the Gulf of Gdańsk resulted in different wind directions compared to those in storms following path P1. At one of our selected analysis points, northeastern winds have the longest fetch. Therefore it is not surprising that this storm generated the highest significant wave height at this location for the entire study period.

This study utilised modelled wind wave and atmospheric data resulting from project HIPOCAS. The meteorological dataset was produced using the REMO atmospheric model (Jacob and Podzun 1997), based on NCEP reanalysis data. The wind wave dataset was created with the WAM wave model (Cieślikiewicz & Paplińska-Swerpel 2008).

How to cite: Cupial, A. and Cieslikiewicz, W.: Low pressure paths and their relation to the variability of extreme wind waves in the Southern Baltic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1839, https://doi.org/10.5194/egusphere-egu24-1839, 2024.

As an archipelagic nation, the Philippines is highly vulnerable to the adverse impacts of weather-related hazards, such as extreme precipitation events. Despite the typical dry conditions associated with the warm phase of the El Niño-Southern Oscillation (ENSO) in the tropical Pacific Ocean, extreme precipitation days still occur in the Philippines during boreal winters. This paradoxical occurrence of extreme precipitation days during the typically dry conditions associated with the warm phase of ENSO underscores the complexity of the processes leading to extreme precipitation in this region. It is important to comprehensively understand the mechanisms driving extreme precipitation in the Philippines. In this talk, we will present an analysis of boreal winter extreme precipitation over the Philippines and its associated circulation features from the synoptic scale to the sub-seasonal scale. It is found that tropical cyclones and depressions play a crucial role in resulting in extreme precipitation days in the Philippines during ENSO winters. The tracks of tropical cyclones and depressions show different characteristics in extreme precipitation days between El Niño and La Niña winters. The physical explanation will be provided for the above features based on observational analysis. A parallel analysis for weak precipitation days in the Philippines will be conducted to help understand the conditions for different precipitation events.

How to cite: Zhang, W.-J. and Wu, R.: Why does extreme precipitation occur in the Philippines during El Niño winters?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2348, https://doi.org/10.5194/egusphere-egu24-2348, 2024.

Atmospheric heat waves pose a threat to natural ecosystems and society, which is projected to become more severe due to anthropogenic global warming. However, the mechanisms that determine the formation of heat waves are not yet sufficiently understood. In particular, there is still quite some debate about the relative contribution of three key processes: horizontal temperature transport, adiabatic heating due to subsidence, and diabatic heating. Here, we quantify these processes from a Lagrangian persepctive using a method that provides essential Lagrangian information about the atmospheric flow on an Eulerian grid. The method is based on the advection of passive tracer fields and includes a relaxation term. For each grid point at any time, the method allows us to decompose a temperature anomaly into the effect of horizontal transport across climatological temperature gradients, the combined effect of vertical transport across climatological temperature gradients and adiabatic heating, and the parcel-based diabatic heating. The tracer method thus provides a field-based view on the three processes under discussion. We analyse several recent heat wave episodes and quantify the contributions from horizontal transport, vertical transport, and diabatic heating. We then continue to analyse whether and to what extent these absolute fields are anomalous with respect to their corresponding climatologies. It turns out that the anomaly-based perspective leads to significant differences regarding the relative importance of the various processes compared to the perspective in terms of absolute fields. Our work complements previous studies based on trajectories, which generally considered significantly fewer air masses and did not take into account a climatological background. The results further our knowledge on important mechanisms and drivers of heat waves, which in turn may help to improve their forecasts.

How to cite: Mayer, A. and Wirth, V.: Lagrangian characterization of heat waves: The perspective matters, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2779, https://doi.org/10.5194/egusphere-egu24-2779, 2024.

Midlatitude Europe stands out as a prominent heatwave hotspot, characterized by accelerated upward trends in both summer surface air temperature and heatwave days. Remarkably, these trends surpass the global land average by approximately 2.6 and 2.3 times since 1979. Through dynamic adjustments applied to reanalysis datasets, we found that one-third of these trends resulted from externally forced circulation changes, characterized by a zonal dipolar circulation exhibiting an anticyclonic pattern over Europe. These observed circulation changes are primarily induced by a Rossby wave response triggered by the warming of sea surface temperatures in the North Atlantic, resembling the Atlantic Multidecadal Variability pattern. The ensemble simulations from the sixth phase of the Coupled Model Intercomparison Project indicate that these sea surface temperatures are dominated by the greenhouse gases, with additional contributions from a reduction in aerosols. Additionally, the stronger air temperature response in midlatitude Europe to the reduced aerosols further amplify summer warming, contributing to the rapid increased frequency of heatwave days. These findings offer evidence of important anthropogenic forcing impacts on the rapid surge of heatwaves in Europe, with important implications for potential adaptation strategies and risk management.

How to cite: Yin, Z., Yang, S., and Dong, B.: Amplified Midlatitude European Heatwave Trends Linked to Anthropogenic Forced Atlantic Multidecadal Variability-like Warming and Decreased Aerosol Emissions       , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3230, https://doi.org/10.5194/egusphere-egu24-3230, 2024.

Extreme precipitation from tropical cyclones (TCs) commonly results in fatalities and expensive property damage in both coastal locations and hundreds of miles inland. Recent studies have found a decreasing trend in TC inner core precipitation and an increasing trend in outer rainband precipitation using about 20 years of precipitation data from satellite products. Most modeling studies that have looked at the response of TC precipitation to climate change found either an increase in the inner core only or an increase in both the inner core and outer rainbands; however, the models were too coarse to resolve detailed TC precipitation structures. This work uses high-resolution Weather Research and Forecasting (WRF) simulations to explore how three-dimensional TC convective structures and precipitation respond to both idealized and more realistic warming scenarios. TCs in the idealized simulations show increasing precipitation in both the TC inner core and outer rainbands with warmer SST, in disagreement with the decrease in inner core precipitation over time in observations. This suggests that either these model simulations are too idealized to simulate the observed decrease in inner core precipitation or the models are missing a key physical process happening in real-world TCs. It also suggests that the disagreements between TC precipitation trends with warming in models and satellite observations are not caused by insufficient model resolution. Results from these idealized simulations will be compared to TCs in 20-year-long historical and future climate runs in regional WRF.

How to cite: Stansfield, A. and Rasmussen, K.: Investigating Changes in Tropical Cyclone Inner Core and Outer Rainband Precipitation in Models under Warming Scenarios, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4194, https://doi.org/10.5194/egusphere-egu24-4194, 2024.

The summer of 2022/23 in Argentina marked an unprecedented period with 10 heat wave events (HWs), primarily affecting the central region. This study characterises the four most extensive HWs of that season through synoptic, thermodynamic, and attribution analyses.

Regarding the synoptic conditions, we find that mid-level anticyclonic anomalies were essential for the occurrence of these HWs. Although they exhibited different characteristics, the high-pressure system was quasi-stationary in three of them, while it was transient in the other one. This atmospheric pattern interacted with the South Atlantic Convergence Zone and the South American Low-Level Jet, and played a fundamental role in the amplification of the HWs, providing a warming of ~+2°C (compared to what would be obtained by a random circulation).

The terms of the thermodynamic equation reveal that diabatic processes were the main drivers of daily temperature changes in the analysed HW events. Horizontal advection also made an important (albeit secondary) contribution, particularly during a tropical air mass intrusion over central and northern Argentina.

Local feedbacks were also important for understanding the events. North-central Argentina is a region of strong land-atmosphere coupling, and all HWs were preceded by soil moisture deficits impinged by three consecutive La Niña years. Using the flow-analogue technique, we quantify that the contribution of dry soils to warming was ~1°C (with respect to that of wet soils), suggesting that soil moisture deficits caused an increase in sensible heat fluxes and amplification of warming.

Finally, we find that recent climate change has also exacerbated HW intensities by +0.5 to +1.2°C. Atmospheric circulation patterns similar to those observed during the events cause warmer conditions in the present than in the past, mostly due to temperature trends rather than changes in the intensity of weather systems.

Our analysis reveals that the exceptionally high temperatures were a result of various factors, including the rare occurrence of a three-year La Niña-induced drought. Recent studies suggest that consecutive La Niña events may become more common due to climate change. Northeastern Argentina would be particularly affected by this shift due to its large response to ENSO and land-atmosphere interaction. Additionally, ongoing temperature trends are expected to accelerate, contributing to the intensification of HWs. Consequently, it is expected that similar extreme summers become more frequent in the 21st century.

How to cite: Collazo, S., Suli, S., Zaninelli, P., García-Herrera, R., Barriopedro, D., and Garrido-Pérez, J. M.: Argentina Breaks Heat Records in the 2022/23 Warm Season: Influence of Synoptic Circulation, Local Feedbacks, and Climate Change, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5262, https://doi.org/10.5194/egusphere-egu24-5262, 2024.

Sub-hourly heavy precipitation events (SHHPs) associated with regional low-pressure (RegL) systems represent a significant natural hazard, frequently causing significant losses over a wide range of natural systems and socioeconomic sectors in mainland Portugal, such as in agriculture (e.g., viticulture). This study provides a preliminary identification of the main dynamic and thermodynamic drivers of the SHHP events associated with RegL systems, also providing a systematised and comprehensive assessment of the atmospheric conditions that are at their genesis. This study uses observations from operational automated surface weather stations (WSs), maintained by the Portuguese Weather Service (Instituto Português do Mar e da Atmosfera, IPMA) for the period 2000–2022 (23 years), with a 10 min temporal resolution, with special emphasis on three weather stations that are representative of the different climatic regions of mainland Portugal. The southern region of Portugal exhibits higher precipitation variability, characterized by a greater occurrence of extreme events on fewer rainy days. The research establishes a connection between SHHPs and low-pressure systems situated to the west of the Iberian Peninsula. These systems display a cold core, especially pronounced at mid-levels, and a positive vorticity anomaly extending from the upper troposphere to lower levels. These conditions induce differential positive vorticity advection in the upper and middle levels (increasing with height), thereby favouring rising motion to the east of the low-pressure systems (over western Iberia). Additionally, these systems facilitate moisture advection over western Iberia at lower levels, highlighted by the positive anomalies of 2-m dew point temperatures and promote instability conditions, diagnosed by different instability indices (Convective available potential energy, Total-Totals index, and K-index). Lastly, the total column cloud ice water revealed higher values for the heavier precipitation events, relative to the values of total column cloud liquid water, suggesting that it may be a useful predictor of such events. The combination of analysed conditions, suggests that some of these SHHPs may be associated with cut-off lows, however, this hypothesis should be validated in a future study.

How to cite: Cruz, J., Belo-Pereira, M., Fonseca, A., and Santos, J.: Analysis of dynamic and thermodynamic drivers linked to heavy sub-hourly precipitation events associated with regional low-pressure systems in mainland Portugal, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6104, https://doi.org/10.5194/egusphere-egu24-6104, 2024.

Extreme rainfall events (ERF) and their associated regimes are among the major catastrophes that have a worldwide impact including India which result in large-scale flooding and immense socio-economic loss. During the southwest monsoon season, these extremes are becoming a frequent norm in the hilly and mountainous regions of the country such as Assam which received one of the most historical ERFs during June 14-17, 2022. The present study investigates the ERF implementing the Weather Research and Forecasting (WRF) model using two different land use and land cover data sets (i.e. ISRO and USGS) and three different sets of physical parameterization schemes (i.e. planetary boundary layer, cumulus, and microphysics) with a lead time up to 96 hours. Further ahead, the performance of rainfall in bias-corrected ensembles (BCE) along with a suite of model ensembles is rigorously quantified up to the district level in terms of its intensity, and distribution. Results suggest that among all the ensembles (E1-E10), USGS (E6 - E10) has underestimated rainfall (140-260 mm/day) compared to ISRO (150-280 mm/day), including the heavy rainfall (HR), very heavy rainfall (VHR), and extremely heavy rainfall (EHR) categories over the upper Assam division (UAD) and lower Assam division (LAD) of Assam throughout the 96 hours. Afterward, rigorous statistical analysis in terms of frequency distribution, Taylor diagram, and benchmark skill scores is carried out to elucidate the model biases for all the ensembles. Throughout the 96 forecast hour, BCE E5(E10) is noted with the distinct realistic(underestimated) representation of model rainfall bias over all the subdivisions of Assam. The findings of the present study encapsulate the critical role of the ensembles of physical parameterization schemes, along with proper LULC, and the BCE approach is required to overcome challenges to improve the skills of ERF, particularly over complex terrains of Indian subcontinent such as Assam.

How to cite: Vishwakarma, V. and Pattnaik, S.: Role of Land Use Land Cover and Skilful Prediction of Rainfall Using Bias Corrected Ensemble during Extreme Rainfall Event, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7096, https://doi.org/10.5194/egusphere-egu24-7096, 2024.

Heatwaves are periods of extremely high temperature. They are one of the most hazardous extreme meteorological events for the developed countries. Droughts caused by heatwaves can lead to crop failures. Human mortality due to cardiovascular diseases and drowning significantly increases during heatwaves. High temperatures can lead to infrastructure damage. Electrical grid overload might occur due to high air conditioning use. Railroad tracks expand due to heat, which can lead to irreversible damage. Overall, the longer and more intense is a heatwave, the more impact it has on society. Identifying historical heat extremes and evaluating their return period is important to better prepare for similar events in the future.

In this work historical air temperature observation data in Latvia since 1966 was used to develop a statistical model. The model was used to evaluate yearly cycle of probability distribution of temperature related meteorological variables. Temperature related variables chosen in this work were mean temperature, highest and lowest maximum temperature, highest and lowest minimum temperature. Daily temperature is autocorrelated in time, which makes calculation of return periods of heatwaves different from calculations of return periods of just daily mean temperature. Therefore, distributions of these temperature variables were calculated for periods with length ranging between 1 and 30 days.

Usually, probability distribution functions of meteorological variables are calculated based on the reference period assuming normal distribution. The model used in this study considered downsides of such approach. First, during the summer distribution of daily mean temperature in Latvia is skewed towards high temperatures, therefore normal distribution is not suitable as the probability function. Therefore, in this work skewed Student-t distribution was used for temperature. Second, due to climate change temperature has increased. Therefore, it is more likely that heatwaves are identified in the recent years. To solve this problem, a trend was added to the mean of the probability distribution.

Based on the statistical model most extreme heat events between May and September in Latvia were identified. For the specific event return period is highly dependent on the analysed temperature variable. For example, during July 2021 heatwave record for the highest nighttime temperature was broken (record was broken once again on 6^th of August 2023). Such high lowest daily temperature on 21^st June has a return period of 4300 years. However, daily mean temperature reached on 21^st June 2021 has return period of 186 years and daily maximum temperature has return period of only 41 years.

The most intensive heatwave events were chosen for further analysis to investigate mechanisms that have caused them. Trajectories of air parcels that bring warm air into the region were identified using Lagrangian tracing backwards in time. Air temperature, potential temperature and humidity of air parcels was traced along the trajectories to identify the mechanisms behind the extreme heat. Modelling of the events was performed using WRF to establish issues that arise when forecasting such events.

How to cite: Pogumirskis, M. and Sīle, T.: Identifying most extreme heatwaves in Latvia based on a skewed temperature distribution., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7708, https://doi.org/10.5194/egusphere-egu24-7708, 2024.

The compound extremes of rainfall and temperature increase globally under the warming climate. Over the past few decades, the frequency and intensity of compound extremes, such as extreme rainfall events and heat waves, have increased across various climatic zones in India. This study aims to analyze the changes in these compound extremes at specific thresholds (percentiles) from CMIP6-based Multi-Model Ensemble (MME). We further examine the future changes in extremes in different time frames, i.e., near (2015-2040), mid (2041-2070), and far (2071-2099) future at annual and seasonal time scales. The projected extremity under the new Shared Socioeconomic Pathways (SSPs) showed an increasing trend in consecutive dry (wet) stages over an increase in northern (central) parts of India. Intense and frequent heat waves are mainly concentrated over the north-central region under the low to high-emission scenarios. Also, India's northern, central, and western regions may experience more extremity under high-emission (SSP5-8.5) scenarios that highlight the importance of developing long-term adaptation and mitigation strategies aimed at reducing climate vulnerability.

How to cite: Chaubey, P. K. and Attada, R.: Projected changes in Compound Extremes under low to high emission SSPs Scenarios, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14265, https://doi.org/10.5194/egusphere-egu24-14265, 2024.

Recent prolonged heat waves in South Korea have raised questions about the key factors influencing their duration. This study investigates the potential physical drivers affecting the duration of Korean summer heat waves over 50 years (1973–2022), categorizing two types of events: short-term (5–7 days) and long-term (≥16 days) events. Using JRA-55 reanalysis data, we examine the distinct characteristics of both event types in relation to components contributing to the surface energy budget. Additional attention is given to the role of soil moisture and clouds in interacting with these components. The primary cause of both heat wave events is an increase in net shortwave radiation flux, attributed to anticyclonic circulation over South Korea, resulting in decreased clouds. Nonetheless, notable differences emerge: the short-term event exhibits a rapid recovery in all-altitude clouds, while the long-term event displays a slower recovery in low-level clouds. Continuously fewer low-level clouds allow much more incoming solar radiation, mainly contributing to the prolonged heat waves. This is linked to a dry atmosphere and weak atmospheric instability, which inhibits the development of lower-level clouds. Moreover, long-term events also exhibit a sudden increase in clouds at 100–200 hPa, intensifying the trapping effect on outgoing longwave radiation in the atmosphere, and subsequently leading to surface warming. This study enhances a comprehensive understanding of the mechanisms behind prolonged summer heat waves in South Korea, providing valuable insights into the complex interplay of atmospheric components.

How to cite: Cho, M., Kim, H.-R., and Choi, Y.-S.: Clouds as the Primary Driver of Recent Prolonged Summer Heat Waves in South Korea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14652, https://doi.org/10.5194/egusphere-egu24-14652, 2024.

The Hanjiang River basin (HJB), a representative monsoon-influenced basin in China, is alternately influenced by the southwest monsoon and southeast monsoon throughout the year. However, the variation characteristics of extreme precipitation in the basin, the specific relationship between extreme precipitation and monsoons, and the relative contribution of different monsoons, remains unclear in the HJB. Using multiple datasets, this study analysed the variability of various extreme precipitation types in the HJB during 1985–2020, investigated their relationships with different monsoon indicators and assessed the contribution of natural factors on the extreme precipitation variance. The results reveal a nonsignificant increasing trend in rainstorms within the basin, the severe convective rainstorm days accounted for a high proportion of the total rainstorm days, and the spatiotemporal characteristics of different rainstorm types are different. The rainstorm indicators are significantly correlated with the monsoon within the HJB and are positively correlated with the SWM, particularly its intensity. The SEM exhibits a weak correlation with rainstorms and has limited explanation for the variance in rainstorms. The intensity of SWM within the HJB shows a significant positive correlation with all rainstorm indicators. It is also detected as the most important indicator for explaining the interannual variation in rainstorms, explaining 41.97% and 39.45% of the variance in daily rainstorm frequency and totals, which comprises more than half of the total explanatory portion. The explanation of circulation factors for the variance in daily rainstorms is higher than that for severe convective rainstorms. In addition to monsoons, the Pacific Decadal Oscillation and the Southern Oscillation Index also have a great contribution to the rainstorm variability in the HJB.

How to cite: Jing, Y. and Shi, P.: Change characteristics and influencing factors of extreme precipitation events in Hanjiang River Basin, China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17718, https://doi.org/10.5194/egusphere-egu24-17718, 2024.

Motivated by heat wave’s impacts on human health and the economy, research on this type of extreme event generally focuses on near surface variables. In this study, we broaden the view by looking at the effects on the vertical structure of the atmospheric boundary layer. Previous research by Miralles et al. (2014) reports extreme boundary layer heights during the severe heat waves of 2003 and 2010 and suggests a correlation between mean potential temperature in the lower atmosphere and the boundary layer height. We investigate whether these findings are common to European heat waves in general with the aim of getting insights into their formation and persistence. To get a comprehensive analysis, we compare summer time vertical profiles taken from reanalysis, namely CERRA and COSMO-REA6, as well as German radio sonde observations between 2014 and 2018. We follow the method of Szemkus et al. (in press) to extract extremal spatial patterns of two meter temperature from the reanalyses. This information is used to define heat waves. Furthermore, the atmospheric boundary layer heights in all three data sources are estimated either by the well established Bulk-Richardson-Number based method or a self-developed machine learning approach. We then compare empirical distributions of boundary layer heights during heat waves and normal conditions on a domain wide scale and grid point wise to account for regional differences. Additionally, we also extract extremal spatial patterns from the height data using the aforementioned method to compare them to the patterns found for temperature. The results of our work could possibly be used to improve the discriminability of different severity levels of heat waves or to formulate a heat wave measure that is not based solely on surface variables.

How to cite: Fohrmann, T., Friederichs, P., and Hense, A.: Investigating the impact of European heat waves on the lower atmosphere, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18245, https://doi.org/10.5194/egusphere-egu24-18245, 2024.

The impact of severe atmospheric events on human society and activities is becoming a primary issue, considering that the actual trend of climate change is expected to increase the frequency and intensity of such events. Despite exceptional advancements that have been done in the Numerical Weather Prediction field in the last decades, there is still a lack of knowledge on features of atmospheric phenomena at the lower and less energetic scales. The present contribution focuses on the genesis mechanism and evolution of periodic secondary vortices, of the von Karman wake type, that arise downwind of thunderstorm supercells. These structures develop at intermediate altitudes, are often less energetic and have a shorter lifetime than the principal supercell. Therefore, they are hardly captured by meteorological observations, but they can play an important role in transporting kinematic and thermal qualities, also anticipating the impact of the supercell itself. 

To address this topic, a real case has been studied and numerically reproduced: the thunderstorm supercell generated on 5th September 2015 over the Gulf of Naples (Italy), which was of exceptional intensity for the Mediterranean area. The analysis of multi-platform data (including data from ERA5, ground and space-borne radar, and local measurements) enabled the identification of the secondary vortices of interest and to derive the geometric, thermal and kinematic characteristics of the system. A simplified model of the supercell was then designed and used to set up a Large-Eddy Simulation, based on computational fluid dynamics techniques, to directly solve the physics of most large and energetic scales of motion. 

The numerical experiment reproduced the fundamental structure of the supercell and its well-known features, including interactions with the tropopause (e.g. overshooting top, anvil, hydraulic jump, gravity waves). The wake of secondary vortices downwind of the main body of the supercell was reproduced and analysed, and the mechanism of generation of these turbulent structures was described.  

To the best of the authors' knowledge, this is the first contribution integrating direct observations and Large-Eddy Simulation numerical simulations to analyse secondary vortex trails behind storm supercells. 

How to cite: Cintolesi, C., Grenzi, M., Di Sabatino, S., and Pocù, F.: Unraveling the genesis of von Kármán vortices behind storm supercell by numerical simulations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19150, https://doi.org/10.5194/egusphere-egu24-19150, 2024.

Underground space floods are a complex type of calamity mainly caused by increasing rainfall and temperature, along with urbanization and population growth necessitating the creation of underground spaces such as subway stations, underground parking lots, underground garages, etc. The causes above have contributed to the frequent and severe appearance of floods in underground spaces in recent decades. The situation is complicated, so further study and research are still needed.

This work applied numerical simulation using Fluent software to investigate, analyze, and compare the impact of altering the magnitude and type of inlet velocities on the underground space's flooding characteristics. Two types of velocities, fixed and transient velocities, were involved in the investigation.

How to cite: Elhamamy, W. and Ni, G.: Velocity inlet variation impact on flooding in underground spaces, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2332, https://doi.org/10.5194/egusphere-egu24-2332, 2024.

We introduce the concept of atmospheric deserts, air masses that originated as hot and dry boundary layers in semi-arid or desert source regions. When they are advected to regions with moister and cooler boundary layers, they can cap the local boundary layers, eliminate cloudiness, and lead to the buildup of heat underneath. Heat waves can occur when atmospheric deserts are present over a target region for several days. Thunderstorm formation can be suppressed where the capping lid is strong, but where it is punctured, thunderstorms can erupt violently.

We illustrate this new concept with a case study from mid-June 2022 when an atmospheric desert was advected from its source region in North Africa towards Europe. With the Lagrangian analysis tool (LAGRANTO), approximately 200 million trajectories are traced, tracking the path of the air mass and the development of its properties as it  progresses towards and across Europe over the course of 5 days. By the end of the study period the atmospheric desert extends from the Atlantic to Eastern Europe and as far north as Sweden. k-means-clustering identifies four typical pathways that the trajectories follow. Most of the atmospheric desert air is modified along the way, with exception of one pathway for which air remains well mixed and forms an elevated mixed layer.

Thunderstorms erupted along a line parallel to the northwestern edge along the surface temperature front, but were mainly absent in the core region of the atmospheric desert. A heat wave affected large parts of Europe, from the Iberian Peninsula to Central Europe. Temperatures set new records, for example in some parts of Eastern Germany. Potential temperatures in some locations even became as high as the ones of the overlying atmospheric desert air.

How to cite: Fix, F., Mayr, G., Stucke, I., and Zeileis, A.: Looking under the Lid: Understanding the Influence of Atmospheric Deserts on Heat Wave and Thunderstorm Formation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8999, https://doi.org/10.5194/egusphere-egu24-8999, 2024.

Snowstorms are the dangerous weather event that effect daily life. In 24^th January 2022 Istanbul is affected by a major snow storm, this storm caused financial loses and suspended the both public transportations and flights. For now this phenomena is similar to Polar Lows. Polar Lows are mesoscale phenomena that can produce gust force wind speed, their horizontal scale is 200 to 1000 km, lifetime of Polar Lows last hours to daily scale and they form and intensify in the presence of upper level trough, air-sea or sea-ice temperature difference, potential vorticity anomalies. In this study snowstorm occurred in Western Black Sea will be analyzed using Numerical Weather Prediction model called Weather Research and Forecasting Model and what kind of environment the low developed will be discussed. The upper level cut-off cyclone reached -44 °C in 24 January 2022 which creates a favorable environment for polar low formation and intensifies over the relatively warmer sea. WRF results suggested that wind speed and sea surface temperature minus T500 satisfied the determined criterion for polar lows with 20 m/s and 50 °C respectively. Diabatic processes are another mechanism that enhances the polar lows and, in this study, it is seen they were both equally contributing the polar low and cross sectional analysis showed the warm-core structure and deep moist convection in 24 January 2022 12UTC. However, questions still remaining such as baroclinic instability, condensational heating and other mechanisms that may contribute polar low intensification needs to be investigated comprehensively. From our results the polar low did not intensify because it is not remained over the relatively warm sea long time like the Arctic region and Japan Sea. From our knowledge this is the first study of polar low in Black Sea and it requires more investigation and sensitivity experiments like changing the model parameterization, increasing sea surface temperature etc. to understand polar low structure further more.

How to cite: Sahinoglu, S. and Asilhan Sirdas, S.: Evaluation of WRF Simulations of Snow Storm and Polar Low Over Western Black Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12169, https://doi.org/10.5194/egusphere-egu24-12169, 2024.