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Heavy precipitation is a challenging phenomenon with high impact on human lives and infrastructure, and thus a better modelling of its characteristics can improve understanding and simulation at climate timescales. The achievement of convection-permitting modelling (CPM) resolutions (Δx<4 km) has brought relevant advancements in its representation. However, further research is needed on how the very high resolution and switching-off of the convection parameterization affects the representation of processes related to heavy precipitation. In this study, we evaluate reanalysis-driven simulations for the greater Alpine area over the period 2000–2015 and assess the differences in representing heavy precipitation and other model variables in a CPM setup with a grid size of 3 km and a regional climate model (RCM) setup at 25 km resolution using the COSMO-CLM model. We validate our simulations against high-resolution observations (E-OBS (ENSEMBLES observations), HYRAS (Hydrometeorologische Rasterdatensätze), MSWEP (Multi-Source Weighted-Ensemble Precipitation), and UWYO (University of Wyoming)). The study presents a revisited version of the precipitation severity index (PSI) for severe event detection, which is a useful method to detect severe events and is flexible for prioritizing long-lasting events and episodes affecting typically drier areas. Furthermore, we use principal component analysis (PCA) to obtain the main modes of heavy precipitation variance and the associated synoptic weather types (WTs). The PCA showed that four WTs suffice to explain the synoptic situations associated with heavy precipitation in winter, due to stationary fronts and zonal flow regimes. Whereas in summer, five WTs are needed to classify the majority of heavy precipitation events. They are associated with upper-level elongated troughs over western Europe, sometimes evolving into cutoff lows, or with winter-like situations of strong zonal circulation. The results indicate that CPM represents higher precipitation intensities, better rank correlation, better hit rates for extremes detection, and an improved representation of heavy precipitation amount and structure for selected events compared to RCM. However, CPM overestimates grid point precipitation rates, which agrees with findings in past literature. CPM systematically represents more precipitation at the mountain tops. However, the RCMs may show large intensities in other regions. Integrated water vapour and equivalent potential temperature at 850 hPa are systematically larger in RCM compared to CPM in heavy precipitation situations (up to 2 mm and 3 K, respectively) due to wetter mid-level conditions and an intensified latent heat flux over the sea. At the ground level, CPM emits more latent heat than RCM over land (15 W m -2), bringing larger specific humidity north of the Alps (1 g kg -1) and higher CAPE (convective available potential energy) values (100 J kg -1). RCM, on the contrary simulates a wetter surface level over Italy and the Mediterranean Sea. Surface temperatures in RCM are up to 2 ∘ C higher in RCM than in CPM. This causes outgoing longwave radiation to be larger in RCM compared to CPM over those areas (10 W m -2). Our analysis emphasizes the improvements of CPM for heavy precipitation modelling and highlights the differences against RCM that should be considered when using COSMO-CLM climate simulations. [ABSTRACT FROM AUTHOR]

Heavy precipitation over western Germany and neighboring countries in July 2021 led to widespread floods, with the Ahr and Erft river catchments being particularly affected. Following the event characterization and process analysis in Part 1, here we put the 2021 event in the historical context regarding precipitation and discharge records and in terms of the temporal transformation of the valley morphology. Furthermore, we evaluated the role of ongoing and future climate change on the modification of rainfall totals and the associated flood hazard, as well as implications for flood management. The event was among the five heaviest precipitation events of the past 70 years in Germany. However, consideration of the large LAERTES-EU regional climate model (RCM) ensemble revealed a substantial underestimation of both return levels and periods based on extreme value statistics using only observations. An analysis of homogeneous hydrological data of the last 70 years demonstrated that the event discharges exceeded by far the statistical 100-year return levels. Nevertheless, the flood peaks at the Ahr river were comparable to the reconstructed major historical events of 1804 and 1910, which were not included in the flood risk assessment so far. A comparison between the 2021 and past events showed differences in terms of the observed hydro-morphodynamic processes which enhanced the flood risk due to changes in the landscape organization and occupation. The role of climate change and how the 2021 event would unfold under warmer or colder conditions (within a -2 to +3 K range) was considered based on both a pseudo global warming (PGW) model experiments and the analysis of an RCM ensemble. The PGW experiments showed that the spatial mean precipitation scales with the theoretical Clausius–Clapeyron (CC) relation, predicting a 7 % to 9 % increase per degree of warming. Using the PGW rainfall simulations as input to a hydrological model of the Ahr river basin revealed a strong and non-linear effect on flood peaks: for the +2 K scenario, the 18 % increase in areal rainfall led to a 39 % increase of the flood peak at gauge Altenahr. The analysis of the high-resolution convection-permitting KIT-KLIWA RCM ensemble confirmed the CC scaling for moderate spatial mean precipitation but showed a super CC scaling of up to 10 % for higher intensities. Moreover, the spatial extent of such precipitation events is also expected to increase. [ABSTRACT FROM AUTHOR]

The July 2021 flood in central Europe was one of the five costliest disasters in Europe in the last half century, with an estimated total damage of EUR 32 billion. The aim of this study is to analyze and assess the flood within an interdisciplinary approach along its entire process chain: the synoptic setting of the atmospheric pressure fields, the processes causing the high rainfall totals, the extraordinary streamflows and water levels in the affected catchments, the hydro-morphological effects, and the impacts on infrastructure and society. In addition, we address the question of what measures are possible to generate added value to early response management in the immediate aftermath of a disaster. The superposition of several factors resulted in widespread extreme precipitation totals and water levels well beyond a 100-year event: slow propagation of the low pressure system Bernd, convection embedded in a mesoscale precipitation field, unusually moist air masses associated with a significant positive anomaly in sea surface temperature over the Baltic Sea, wet soils, and steep terrain in the affected catchments. Various hydro-morphodynamic processes as well as changes in valley morphology observed during the event exacerbated the impact of the flood. Relevant effects included, among many others, the occurrence of extreme landscape erosion, rapidly evolving erosion and scour processes in the channel network and urban space, recruitment of debris from the natural and urban landscape, and deposition and clogging of bottlenecks in the channel network with eventual collapse. The estimation of inundation areas as well as the derived damage assessments were carried out during or directly after the flood and show the potential of near-real-time forensic disaster analyses for crisis management, emergency personnel on-site, and the provision of relief supplies. This study is part one of a two-paper series. The second part puts the July 2021 flood into a historical context and into the context of climate change. [ABSTRACT FROM AUTHOR]

Regional and local wind systems are often complex, particularly near coastal areas with a highly variable orography. Thus, the realistic representation of regional wind systems in weather and climate models is of strong relevance. Here, we evaluate the ability of a 13‐year convection‐permitting climate simulation in reproducing the interaction of several regional summer wind systems over the complex orography in the eastern Mediterranean region. The COSMO‐CLM simulations are driven by hourly ERA‐5 reanalysis and have a spatial resolution of 2.8 and 7.0 km. The simulated near‐surface wind fields are compared with unique very high‐resolution wind observations collected within the "Dead Sea Research Venue" project (DESERVE) and data from the Israel Meteorological Service synop network. The high‐resolution COSMO‐CLM simulations largely reproduce the main characteristics of the regional wind systems (Mediterranean and Dead Sea breeze, slope winds in the Judean Mountains and winds along the Jordan Rift valley), whereas ERA‐5 is only able to represent the Mediterranean Sea breeze. The high‐resolution simulations substantially improve the representation of regional winds, particularly over complex orography. Indeed, the 2.8 km simulation outperforms the 7.0 km run, on 88% of the days. Two mid‐July 2015 case studies show that only the 2.8 simulation can realistically simulate the penetration of the Mediterranean Sea Breeze into the Jordan Rift valley and complex interactions with other wind systems like the Dead Sea breeze. Our results may have profound implications for regional weather and climate prediction since very high‐resolution information seems to be necessary to reproduce the main summertime climatic features in this region. We envisage that such simulations may also be required at other regions with complex orography. [ABSTRACT FROM AUTHOR]

Extreme precipitation is a weather phenomenon with tremendous damaging potential for property and human life. As the intensity and frequency of such events is projected to increase in a warming climate, there is an urgent need to advance the existing knowledge on extreme precipitation processes, statistics and impacts across scales. To this end, a working group within the Germany-based project, ClimXtreme, has been established to carry out multidisciplinary analyses of high-impact events. In this work, we provide a comprehensive assessment of the 29 June 2017 heavy precipitation event (HPE) affecting the Berlin metropolitan region (Germany), from the meteorological, impacts and climate perspectives, including climate change attribution. Our analysis showed that this event occurred under the influence of a mid-tropospheric trough over western Europe and two shortwave surface lows over Britain and Poland (Rasmund and Rasmund II), inducing relevant low-level wind convergence along the German–Polish border. Over 11 000 convective cells were triggered, starting early morning 29 June, displacing northwards slowly under the influence of a weak tropospheric flow (10 m s -1 at 500 hPa). The quasi-stationary situation led to totals up to 196 mm d -1 , making this event the 29 June most severe in the 1951–2021 climatology, ranked by means of a precipitation-based index. Regarding impacts, it incurred the largest insured losses in the period 2002 to 2017 (EUR 60 million) in the greater Berlin area. We provide further insights on flood attributes (inundation, depth, duration) based on a unique household-level survey data set. The major moisture source for this event was the Alpine–Slovenian region (63 % of identified sources) due to recycling of precipitation falling over that region 1 d earlier. Implementing three different generalised extreme value (GEV) models, we quantified the return periods for this case to be above 100 years for daily aggregated precipitation, and up to 100 and 10 years for 8 and 1 h aggregations, respectively. The conditional attribution demonstrated that warming since the pre-industrial era caused a small but significant increase of 4 % in total precipitation and 10 % for extreme intensities. The possibility that not just greenhouse-gas-induced warming, but also anthropogenic aerosols affected the intensity of precipitation is investigated through aerosol sensitivity experiments. Our multi-disciplinary approach allowed us to relate interconnected aspects of extreme precipitation. For instance, the link between the unique meteorological conditions of this case and its very large return periods, or the extent to which it is attributable to already-observed anthropogenic climate change. [ABSTRACT FROM AUTHOR]

Heavy precipitation over western Germany and neighboring countries in July 2021 led to widespread floods, with the Ahr and Erft river catchments being particularly affected. Following the event characterization and process analysis in Part 1, here we put the 2021 event in the historical context regarding precipitation and discharge records, and in terms of the temporal transformation of the valley morphology. Furthermore, we evaluated the role of ongoing and future climate change on the modification of rainfall totals and associated flood hazards as well as implications for flood management. The event was among the five heaviest precipitation events of the past 70 years in Germany. However, considering the large LAERTES-EU regional climate model (RCM) ensemble revealed a substantial underestimation of return values and periods based on extreme value statistics using only observations. An analysis of homogeneous hydrological data of the last 70 years demonstrated that the event discharges exceeded by far the statistical 100-year return values. Nevertheless, the flood peaks at the Ahr River were comparable to the reconstructed major historical events of 1804 and 1910, which were not included in the hazard assessment of flood risk so far. A comparison between the 2021 and past events showed differences in terms of the observed hydro-morphodynamic processes which enhanced the flood risk due to changes in the landscape organization and occupation. The role of climate change and how the 2021 event would unfold under warmer or colder conditions (within a -2K to +4 K range) was analyzed based on pseudo-global-warming (PGW) model experiments. These showed that the spatial mean precipitation scales to first order with the theoretical Clausius-Clapeyron (CC) relation predicting a 7 to 9 % increase per degree warming. Using the PGW rainfall simulations as input to a hydrological model of the Ahr river basin revealed a strong and non-linear effect on flood peaks: For the +2K scenario, the 18 % increase in areal rainfall led to a 39 % increase of the flood peak at gauge Altenahr. The analysis of the high-resolution convection-permitting KIT-KLIWA RCM ensemble confirmed the CC-scaling for moderate spatial mean precipitation but showed a super CC-scaling of up to 10 % for higher intensities. Moreover, also the spatial extent of such precipitation events is expected to increase. [ABSTRACT FROM AUTHOR]

The July 2021 flood in central Europe was one of the five costliest natural disasters in Europe in the last half century with estimated total damage of EUR 32 billion. This study investigates the complex interactions between meteorological, hydrological, and hydro-morphological processes and mechanisms that led to the exceptional flood. Furthermore, we present our estimates of the impacts in terms of inundation areas, traffic disruptions, and economic losses. The estimation of inundation areas as well as the derived damage assessments were carried out during or directly after the flood, and show the potential of near-real-time forensic disaster analyses for crisis management, emergency personnel on-site, and the provision of relief supplies. The superposition of several factors resulted in widespread extreme precipitation totals and water levels well beyond a 100-year event: slow propagation of the low pressure system Bernd, convection embedded in a mesoscale precipitation field, unusually moist air masses associated with a significant positive anomaly in sea surface temperature over the Baltic Sea, wet soils, and steep terrain. Various hydro-morphodynamic processes as well as changes in valley morphology observed during the event exacerbated the impact of the flood. Relevant effects included, among many others, the occurrence of extreme landscape erosion, rapidly evolving erosion and scour processes in the channel network and urban space, recruitment of debris from the natural and urban landscape, deposition and clogging of bottlenecks in the channel network with eventual collapse. This study is part one of a two-paper series. The second part puts the July 2021 flood into a historical context and into the context of climate change. [ABSTRACT FROM AUTHOR]

Extreme precipitation is a weather phenomenon with tremendous damaging potential for property and human life. As the intensity and frequency of such events is projected to increase in a warming climate, there is an urgent need to advance the existing knowledge on extreme precipitation processes, statistics and impacts across scales. To this end, a working group within the German-based project ClimXtreme, has been established to carry out multidisciplinary analyses of high-impact events. In 5 this work, we provide a comprehensive assessment of a selected case, affecting the Berlin metropolitan region (Germany) on 29 June 2017, from the meteorological, impacts and climate perspectives, additionally estimating the contribution of climate change to its extremeness. Our analysis shows that this event occurred under the influence of a mid-tropospheric trough over western Europe and two short-wave surface lows over Britain and Poland, inducing relevant low-level wind convergence along the German-Polish border. Several thousand convective cells were triggered in the early morning of 29 June, displacing northwards slowly under the influence of a weak tropospheric flow (10 m s-1 10 at 500 hPa). A very moist and warm southwesterly flow was present south of the cyclone over Poland, in the presence of moderate Convective Available Potential Energy (CAPE). We identified the soil in the Alpine-Slovenian region as the major moisture source for this case (63 % of identified sources). Maximum precipitation amounted to 196 mm d-1, causing the largest insured losses due to a heavy precipitation event in the period 2002 to 2017 (C60 Mill.) over the area. A household-level survey revealed that the inundation duration was 4 to 12 15 times larger than other surveyed events in Germany in 2005, 2010 and 2014. The climate analysis showed return periods of over 100 years for daily aggregated precipitation, and up to 100 years and 10 years for 8 h and 1 h aggregations, respectively. The event was the 29th most extreme event in the 1951-2021 climatology in terms of severity and the second with respect to the number of convective cells triggered from 2001 to 2020 over Germany. The conditional attribution demonstrated that warming since the pre-industrial era caused a small, but significant increase of 4 % in total precipitation and 10 % for extreme intensities. 20 The aerosol sensitivity experiments showed that increased anthropogenic aerosols induce larger cloud cover and probability of extreme precipitation (> 150 mm d-1 ). Our analysis allowed relating interconnected aspects of extreme precipitation. For instance, the link between the unique meteorological conditions of this case and its climate extremeness, or the extent to which this is attributable to already-observed anthropogenic climate change. [ABSTRACT FROM AUTHOR]

Heavy Precipitation Events (HPEs) are a challenging atmospheric phenomenon with a high impact on human lives and infrastructures. The achievement of high-resolution simulations for Convection Permitting Modelling (CPM) has brought relevant advancements in the representation of HPEs in climate simulations compared to coarser resolution Regional Climate Models (RCM). However, further insight is needed on the scale-dependency of mesoscale precipitation processes. In this study, we aim at evaluating reanalysis-driven climate simulations of the greater Alpine area in recent climate conditions and assessing the scale-dependency of thermodynamical processes influencing extreme precipitation. We evaluate COSMO-CLM simulations of the period 1971--2015, at resolutions of 25 km (RCM) and 3 km (CPM) downscaled from ERA-40 and ERA- interim. We validate our simulations against high-resolution observations (EOBS, HYRAS, MSWEP, and UWYO). In the methodology, we present a revisited version of the Precipitation Severity Index (PSI) useful for extremes detection. Furthermore, we obtain the main modes of precipitation variance and synoptic Weather Types (WTs) associated with extreme precipitation using Principal Component Analysis (PCA). PCA is also used to derive composites of model variables associated with the thermodynamical processes of heavy precipitation. The results indicate a good detection capability of the PSI for precipitation extremes. We identified four WTs as precursors of extreme precipitation in winter, associated with stationary fronts or a zonal flow regimes. In summer, 5 WTs bring heavy precipitation, associated with upper-level elongated troughs over western Europe, sometimes evolving into cut-off lows, or by winter-like situations of strong zonal circulation. The model evaluation showed that CPM (3 km) represents higher precipitation intensities, better rank correlation, better hit rates for extremes detection, and an improved representation of heavy precipitation amount and structure for selected HPEs compared to RCM (25 km). CPM overestimates grid point precipitation rates especially over elevated terrain fostered by the scale-dependency of convective dynamic processes such as stronger updrafts and more triggering of convective cells. However, at low altitudes, precipitation differences due to resolution are explained through the scale-dependency of thermodynamic variables, where the largest impact is caused by differences in surface moisture up to 1 g kg-1 . These differences show a predominant north-south gradient where locations north of the Alps show larger (lower) surface moisture and precipitation in CPM (RCM) and locations south of the Alps show larger (lower) humidity and precipitation in RCM (CPM). The humidity differences are caused by an uneven partition of latent and sensible heat fluxes between RCM and CPM. RCM simulates larger emissions of latent heat flux over the Sea (25 W m-2 more), and CPM emits larger latent heat over land (15 W m-2 more). In turn, RCM emits larger surface sensible heat fluxes over land (30 W m-2 more), showing a warmer surface (0.5 to 1°C) than CPM. These results provide evidence that CPM is a powerful tool for obtaining accurate high-resolution climate information also pointing at the different scale-dependency of dynamic and thermodynamical precipitation processes at high and low terrain. [ABSTRACT FROM AUTHOR]

Heavy precipitation (HP) constitutes a major meteorological threat in the western Mediterranean (WMed). Every year, recurrent events affect the area with fatal consequences for infrastructure and personal losses. Despite this being a well-known issue widely investigated in the past, open questions still remain. Particularly, the understanding of the underlying mechanisms and the modeling representation of the events must be improved. One of the major goals of the Hydrological Cycle in the Mediterranean Experiment (HyMeX; 2010–2020) has been to advance knowledge on this topic. In this article, we present an overview of the most recent lessons learned from HyMeX towards an improved understanding of the mechanisms leading to HP in the WMed. The unique network of instruments deployed as well as the use of finer model resolutions and coupled models provided an unprecedented opportunity to validate numerical model simulations, develop improved parameterizations, and design high-resolution ensemble modeling approaches and sophisticated assimilation techniques across scales. All in all, HyMeX, and particularly the science team heavy precipitation, favored the evidencing of theoretical results, the enrichment of our knowledge on the genesis and evolution of convection in a complex topography environment, and the improvement of precipitation forecasts. Illustratively, the intervention of cyclones and warm conveyor belts in the occurrence of heavy precipitation has been pointed out, and the crucial role of the spatiotemporal distribution of atmospheric water vapor for the understanding and accurate forecast of the timing and location of deep convection has been evidenced, as has the complex interaction among processes across scales. The importance of soil and ocean conditions and the interactions among systems were highlighted, and such systems were specifically developed in the framework of HyMeX to improve the realism of weather forecasts. Furthermore, the benefits of cross-disciplinary efforts within HyMeX have been a key asset in bringing our knowledge about heavy precipitation in the Mediterranean region a step forward. [ABSTRACT FROM AUTHOR]

Heavy precipitation (HP) constitutes a major meteorological threat in the western Mediterranean (WMed). Every year, recurrent events affect the area with fatal consequences on infrastructure and personal losses. Despite this being a well-known issue, widely investigated in the past, still open questions remain. Particularly, the understanding of the underlying mechanisms and the modelling representation of the events must be improved. One of the major goals of the Hydrological cYcle in the Mediterranean eXperiment (HyMeX; 2010-2020) has been to advance knowledge on this topic. In this article we present an overview of the most recent lessons learned from HyMeX towards improved understanding of the mechanisms leading to HP in the WMed. The unique network of instruments deployed, the use of finer model resolutions and of coupled models, provided an unprecedented opportunity to validate numerical model simulations, to develop improved parameterizations, designing high-resolution ensemble modelling approaches and sophisticated assimilation techniques across scales. All in all, HyMeX and particularly the science team heavy precipitation favoured the evidencing of theoretical results, the enrichment of our knowledge on the genesis and evolution of convection in a complex topography environment, and the improvement of precipitation forecasts. Illustratively, the intervention of cyclones and warm conveyor belts in the occurrence of heavy precipitation has been pointed out, the crucial role of the spatio-temporal distribution of the atmospheric water vapor for the understanding and accurate forecast of the timing and location of deep convection has been evidenced, as well as the complex interaction among processes across scales. The importance of soil and ocean conditions and the interactions among systems were highlighted and such systems were specifically developed in the framework of HyMeX to improve the realism of weather forecasts. Furthermore, the benefits of cross-disciplinary efforts within HyMeX have been a key asset in bringing a step forward our knowledge about heavy precipitation in the Mediterranean region. [ABSTRACT FROM AUTHOR]

Gaining insight into the interaction between atmospheric moisture and convection is determinant for improving the model representation of heavy precipitation, a weather phenomenon that causes casualties and monetary losses in the western Mediterranean region every year. Given the large variability of atmospheric moisture, an accurate representation of its distribution is expected to reduce the errors related to the representation of moist convective processes. In this study, we use a diagnostic approach to assess the sensitivity of convective precipitation and underlying mechanisms during a heavy precipitation event (Hydrological cycle in the Mediterranean eXperiment Intensive Observation Period; HyMeX IOP16) to variations of the atmospheric moisture spatio-temporal distribution. Sensitivity experiments are carried out by nudging a homogenized data set of the Global Positioning System-derived zenith total delay (GPS-ZTD) with sub-hourly temporal resolution (10 min) in 7 and 2.8 km simulations with the COnsortium for Small-scale MOdeling in CLimate Mode (COSMO-CLM) model over the western Mediterranean region. The analysis shows that (a) large atmospheric moisture amounts (integrated water vapour; IWV ∼ 40 mm) precede heavy precipitation in the affected areas. This occurs 12 h prior to initiation over southern France and 4 h over Sardinia, north-eastern Italy and Corsica, which is our main study area. (b) We found that the moisture is swept from the Atlantic by a westerly large-scale front associated with an upper level low on the one hand and evaporated from the Mediterranean Sea and north Africa on the other. The latter moisture transport occurs in the 1 to 4 km layer. (c) COSMO-CLM overestimated the atmospheric humidity over the study region (Corsica), and this was, to a good extent, corrected by the GPS-ZTD nudging. This reduced maximum precipitation (- 49 % for 7 km and - 16 % for 2.8 km) drastically, considerably improving the precipitation representation in the 7 km simulation. The convection-permitting simulation (2.8 km) without the GPS-ZTD nudging already did a good job in representing the precipitation amount. (d) The two processes that exerted the largest control on precipitation reduction were the decrease of atmospheric instability over Corsica (convective available potential energy; CAPE - 35 %) and the drying of the lower free troposphere bringing additional dry air entrainment. In addition, the 7 km simulation showed a weakening of the represented low-pressure system and the associated cyclonic wind circulation. This ultimately reduced the intensity and number of convective updrafts represented over the island. These results highlight the large impact exerted by moisture corrections on precipitating convection and the chain of processes leading to it across scales. [ABSTRACT FROM AUTHOR]

Gaining insight on the interaction between atmospheric moisture and convection is determinant to improve the model representation of heavy precipitation, a weather phenomenon that every year brings casualties and important monetary losses in the western Mediterranean region. Given the large variability of atmospheric moisture, an accurate representation of its distribution is expected to reduce the errors related to the representation of moist convective processes. In this study, we assess the sensitivity of precipitating convection and underlying mechanisms during a heavy precipitation event (HyMeX intensive observation period 16) to corrections of the atmospheric moisture spatio-temporal distribution. Sensitivity experiments are carried out by nudging a homogenised data set of GPS-derived Zenith Total Delays (GPS-ZTD) with sub-hourly frequency (10 minutes) in 7 km and 2.8 km simulations with the COSMO-CLM model over the western Mediterranean region. The analysis shows that (a) large atmospheric moisture amounts (Integrated Water Vapour ~ 40 mm) precede heavy precipitation at the affected areas. This occurs 12 h before initiation over southern France and 4 h over Sardinia, north eastern Italy and Corsica (our main study area). (b) We found that the moisture is transported on the one hand, swept by a westerly large-scale front associated with an upper-level low and on the other hand evaporated from the Mediterranean Sea and north Africa. The latter moisture transport occurs in the < 1 km to 4 km layer and has been identified for this event for the first time. (c) COSMO-CLM overestimated the atmospheric humidity and precipitation amount over the study region (Corsica) and this was, to a good extent, corrected by the GPS-ZTD nudging by reducing noticeably both quantities, bringing results closer to observations. (d) The two processes that exerted the largest control on precipitation were the reduction of atmospheric instability over the island (CAPE -35%) and the drying of the lower free troposphere bringing more dry air entrainment. Besides, the 7 km simulation showed a stronger impact for large-scale dynamical lifting at the target area, given a weakening of the represented low-pressure system and the associated wind circulation. This reduced ultimately, the intensity and number of convective updrafts represented over the island. These results highlight the large impact exerted by moisture corrections on precipitating convection and the chain of related processes leading to it across scales. Additionally. The modelling experiments demonstrated the benefit of sub-hourly GPS-ZTD nudging to improve the modelling of precipitation. [ABSTRACT FROM AUTHOR]

The presented investigation focuses on the role of atmospheric water vapour in shaping the intensity, structure, location and timing of atmospheric convection. The aim of this thesis is, therefore, gaining knowledge and reducing uncertainty associated with the interaction between atmospheric moisture and convection, assessing the impact of nudging Global Positioning System (GPS)- derived Zenith Total Delays (ZTDs) in different resolution simulations of heavy precipitation from seasonal to event scales. Analysis of atmospheric simulations in a realistic set-up with the wellestablished Consortium for Small-scale Modelling (COSMO) model are carried out on different temporal and spatial scales. Simulations from seasonal to event scale are produced to improve the understanding of the mechanisms of convection and moisture interaction. A unique GPS-derived ZTD data set, a proxy of column atmospheric moisture, merging data from more than 25 European networks, with a temporal resolution of ten minutes is nudged to improve the representation of the atmospheric humidity distribution. The moisture nudging sensitivity experiments are done on different horizontal resolution simulations (7 km, 2.8 km and 500 m) to assess the sensitivity of the model in different configurations, from convection parameterized to convection-permitting, reaching the micro-a scale. Nudging column atmospheric moisture improved the humidity wet bias present in COSMO in the simulation of the autumn 2012 period in the WMed. The root mean square error, the agreement index and the mean absolute error scores improved in seasonal and event scale simulations using 7 km, 2.8 km and 500 m grid spacings. Past studies have also reported a wet bias in COSMO (Schraff et al., 2008; Cress et al., 2012; Devidasrao, 2012). The column moisture nudging could not, however, correct biases over particular high-terrain stations (e.g. the Alps, the Pyrenees) given the misrepresentation of the model surface height. Neither could the column moisture nudging correct humidity biases which remained in the order of 1 g kg􀀀1 at particular pressure levels. Errors in this order in the vertical distribution have already been reported as crucial for convective precipitation representation (Weckwerth et al., 1996; Khodayar et al., 2016b; Chazette et al., 2015b). This issue, present in all grid length types, could be overcome by jointly nudging column moisture observations with operational radiosondes. This was demonstrated for a heavy precipitation event of the autumn 2012 period. The autumn 2012 seasonal mean values, endured an Integrated Water Vapour (IWV) reduction between -3% and -8% as a result of the column moisture nudging. The largest seasonal IWV reduction took place over the Iberian Peninsula and Italy (IT) and the lowest over France (FR) .The outcome for instability as given by CAPE and for the total cloud cover was an intense reduction of about 􀀀10 J kg􀀀1 and of 􀀀4%, respectively. All in all, these impacts reduced the total precipitation amount over land (with decreases ranging between -9% and -65% which were largest for FR and shortest for IT. Regarding the dependence on the used model grid-spacing, the precipitation reduction was larger in the 7 km simulations (between -23% and -65%) than in the 2.8 km simulations (between -9% and -34%). The analysis on the seasonal conditions as well as of two Heavy Precipitation Events (HPEs) of the period showed that the convective processes enduring the largest impact to the column moisture nudging were instability and drying (moistening) of the lower free-troposphere. The latter had stringent impacts on entrainment/detrainment processes and buoyancy reduction (intensification) of the convective plume. Besides, to a lower extent, the humidity corrections affected the distribution of the large-scale pressure distribution with a tendency for higher geopotential heights at 500 hPa and surface pressure. This effect was observed over France only, due to the larger number of GPS stations and that typically low pressure systems are located over France during autumn. About 90% of the HPEs in the autumn 2012 period were found to be preceded by IWV monotonic increases. In the mean, these increases were of 4 mm (with a standard deviation of 5mm) and lasted for 9 h in mean (9 h of standard deviation). When the most extreme precipitation events were considered (above percentile-90), the IWV increases preceding precipitation were largest (>10mm) and occurred for longer time periods (≈ 30 h). These results apply to the simulations using a 7 km and a 2.8 km grid over local target areas O(105 m) in the north WMed basin. The impact of simulating errors in the vertical distribution of humidity was studied in detail, simulating an HPE taking place on the 24-Sep-2012 over southern France jointly nudging observations of column atmospheric moisture and operational radiosondes. Model grid-spacings of 7 km, 2.8 km and 500 m were employed in this experiment. The results show that the humidity representation in the vertical distribution, as well as IWV are improved when applying the nudging of column moisture and radiosondes in a 500m grid. Especially for specific humidity below 900hPa. Moreover, nudging radiosonde information improves the representation of the structure, location and amount of precipitation over the affected areas, for every used grid-spacing type. This thesis provides novel insights on how IWV increases precondition heavy precipitation, the humidity wet bias in COSMO in the period, the atmospheric moisture impacts in convective processes and precipitation evolution and the improvement in moisture and precipitation representation by nudging GPS data and radiosondes in 500 m simulations. It provides deeper knowledge on COSMO responses and biases useful for users and developers of numerical prediction systems. The presented results demonstrate the benefit of using column moisture observations to correct the humidity biases with a large impact on precipitation. [ABSTRACT FROM AUTHOR]

The deployment of special instrumentation for the Hydrological Cycle in the Mediterranean Experiment (HyMeX) provides a valuable opportunity to investigate the spatio‐temporal variability of atmospheric water vapour across scales in relationship with the occurrence of Heavy Precipitation Systems (HPSs) in the north Western Mediterranean (WMed) during the Intensive Observation Period (IOP12), which is the focus of this investigation. High‐resolution convection‐permitting COSMO simulations complement the observational network and allow the calculation of on‐line trajectories. In addition to the presence of a favourable large‐scale situation and low‐level convergence, atmospheric moisture changes resulting in conditionally unstable air are identified as responsible for convective initiation (CI). All HPSs within the north‐WMed form in periods/areas of maximum integrated water vapour (IWV; 35–45 kg/m2) after an increase of about 10–20 kg/m2. The most intense events receive moisture from different sources simultaneously and show a sudden increase of about 10 kg/m2 between 6 and 12 h prior to the event, whereas in the less intense events the increase is larger, about 20 kg/m2, over a period of at least 24–36 h. Changes in the lower (∼900 hPa) and mid‐troposphere (∼700 hPa) control the evolution of the atmospheric moisture and the instability increase prior to CI. Spatial inhomogeneities in the lower boundary layer determine the timing and location of deep convection, whereas enhanced moisture in the mid‐troposphere favours intensification. Moister and deeper boundary layers, with updraughts reaching up to 2 km are identified in those pre‐convective environments leading to HPS, whereas dry, shallow boundary layers are found everywhere else. The build‐up time and vertical distribution of the moisture changes are found to be crucial for the evolution and severity of the HPSs rather than the amount of total column atmospheric moisture. This article investigates the spatio‐temporal variability of atmospheric water vapour across scales in relation to the occurrence of heavy precipitation systems in the northwestern Mediterranean. Synergies between different instruments, Global Positioning System (GPS) stations, radiosondes, airborne and ground‐based lidars and space‐borne observations from the Moderate Resolution Imaging Spectroradiometer (MODIS), among others, are used. High‐resolution convection‐permitting simulations complement the observational network. Daily mean spatial distribution of total column integrated water vapour (IWV) from MODIS (background) and GPS (circles) observations. Simulated COSMO‐7km and COSMO‐2.8km 950 hPa winds at 1200 UTC are superimposed. [ABSTRACT FROM AUTHOR]