16 research outputs found
Assessment of the water supply to Mediterranean heavy precipitation: a method based on finely designed water budgets
No full textOrigin of the moisture feeding the Heavy Precipitating Systems over Southeastern France
No full textIn the Northwestern Mediterranean region, large amounts of precipitation can accumulate over the coasts in less than a day. The present study aims at characterising the origin and the pathways of the moisture feeding such heavy precipitation. The ten Heavy Precipitating Events (HPEs) that occurred over the French Mediterranean region during the autumns of 2008 and 2009 are simulated with the non-hydrostatic research numerical model Meso-NH at 2.5 km, 10 km and 40 km horizontal resolution. <br><br> Using eulerian on-line passive tracers, high-resolution simulations (2.5 km horizontal resolution) show that the heavy precipitating systems are fed by a south-southwesterly to easterly low-level moist flow. It is typically 1000 m deep and remains almost unchanged all along an event. This low-level feeding flow crosses the most northwestern part of the Mediterranean in 5 to 10 h. <br><br> Larger-scale simulations (40 km and 10 km horizontal resolution) show that the moisture of the low-level feeding flow is provided by both evaporation of the Mediterranean Sea within the last 2 days before the HPE triggering and transport from remote sources in the lower half of the troposphere over more than 3 to 4 days. Local Mediterranean moisture is gained along the air mass low-level progress towards the Northwestern Mediterranean basin following two main branches along the Spanish coast and west of Sardinia. The Mediterranean Sea is the main moisture source when anticyclonic conditions prevail during the last 3 or 4 days before the HPE. When cyclonic conditions prevail before the HPE, the relative contribution of local and remote sources is more balanced. Remote moisture comes most of the time from the Atlantic Ocean. African tropical moisture is a less frequent but larger remote source
Sources et transports d'humidité pour les évènements de pluies intenses en région méditerranéenne : caractérisation et assimilation à mésoéchelle de radiances satellitaires infrarouges
No full textCe travail de thèse vise à progresser dans la compréhension et la prévision des épisodes de pluies intenses en région méditerranéenne. Cet objectif a été poursuivi sous deux angles.
Nous avons tout d'abord documenté les propriétés de l'alimentation en vapeur d'eau des systèmes fortement précipitants méditerranéens en identifiant son origine et les caractéristiques de son transport à l'aide de simulations de mésoéchelle. Sur un ensemble de 10 épisodes, diverses sources d'humidité ont été identifiées et leur contribution respective caractérisée. Les trajectoires privilégiées du transport d'humidité depuis les sources extérieures lointaines et au-dessus de la Méditerranée ont été déterminées.
Nous avons ensuite cherché à améliorer la description de l'humidité dans les modèles de prévision numérique du temps à échelle convective en proposant de nouveaux opérateurs d'observation plus réalistes pour l'assimilation des radiances satellitaires infrarouges. Cela a permis d'améliorer la simulation des équivalents-modèle de ces mesures dans la bande spectrale vapeur d'eau en filtrant les gradients d'humidité de fine échelle.This PhD work aimed at improving the understanding and forecasting of Mediterranean Heavy Precipitating Events (HPEs). This purpose was pursued focusing on two main issues.
Firstly, we addressed the question of the origin and transport of the moisture feeding the heavy precipitating systems. Based on a set of 10 HPEs, several moisture sources were identified and their contribution was characterized. The main trajectories of the moisture transport from these remote sources and over the Mediterranean Sea were determined.
The second issue investigated in this work is about the improvement of the description in convective-scale numerical weather prediction models of the moisture supply to HPEs. More realistic observation operators were designed for the assimilation of infrared satellite radiances. They improved the simulation of model-equivalent radiances in the water
vapor spectral band by filtering out the fine-scale humidity gradients having a smaller size than the observation resolution
Fine-scale numerical analysis of the sensitivity of the HyMeX IOP16a heavy precipitating event to the turbulent mixing-length parametrization
No full textRole of moisture patterns in the backbuilding formation of HyMeX IOP13 heavy precipitation systems
No full textInternational audienceMediterranean regions are regularly affected by heavy convective precipitation. During the Hydrological Cycle in the Mediterranean Experiment Intensive Observation Period 13 (HyMeX-IOP13), the multi-platform observation strategy allowed analysing the backbuilding convective systems which developed on 14 October 2012 as well as the associated moisture structures in the environment upstream of convection.The numerical simulation at 2.5-km horizontal resolution succeeds in reproducing the location and time evolution of the observed heavy precipitation systems and the main characteristics of the marine air mass. Convection develops in Southeastern France over the foothills closest to the coast when a moist conditionally unstable marine boundary layer topped by particularly dry air masses is advected inland. Cold air formed by evaporative cooling under the precipitating cells flows down the valleys slowly shifting the location of the backbuilding convective cells from the mountainsides to the coast and over the sea. Surface observations confirm that these simulated backbuilding mechanisms describe realistically the processes involved in the maintenance of the heavy precipitation event.A lagrangian analysis shows that the moisture supply to the convective system is provided by the moist conditionally unstable marine boundary layer while the dry air masses above are involved in the cold pool formation. Four days before the event, both, the dry and the moist air masses, come from the Atlantic Ocean in the lower half of the troposphere. The dry air mass involved in the cold pool formation results from both the advection of mid-level air masses and the drying of low-level air masses lifted up over Spain. For the moist air mass feeding the backbuilding convective systems, most of the air parcels overpass France before travelling almost 48 hours in the lowest 1000m above the Mediterranean. About 50 % of the moisture supply to the precipitating system originates from the evaporation over the sea
Mesoscale numerical analysis of the historical November 1982 heavy precipitation event over Andorra (Eastern Pyrenees)
No full textFrom 6 to 8 November 1982 one of the most catastrophic flash-flood events was recorded in the Eastern Pyrenees affecting Andorra and also France and Spain with rainfall accumulations exceeding 400 mm in 24 h, 44 fatalities and widespread damage. This paper aims to exhaustively document this heavy precipitation event and examines mesoscale simulations performed by the French Meso-NH non-hydrostatic atmospheric model. Large-scale simulations show the slow-evolving synoptic environment favourable for the development of a deep Atlantic cyclone which induced a strong southerly flow over the Eastern Pyrenees. From the evolution of the synoptic pattern four distinct phases have been identified during the event. The mesoscale analysis presents the second and the third phase as the most intense in terms of rainfall accumulations and highlights the interaction of the moist and conditionally unstable flows with the mountains. The presence of a SW low level jet (30 m s-1) around 1500 m also had a crucial role on focusing the precipitation over the exposed south slopes of the Eastern Pyrenees. Backward trajectories based on Eulerian on-line passive tracers indicate that the orographic uplift was the main forcing mechanism which triggered and maintained the precipitating systems more than 30 h over the Pyrenees. The moisture of the feeding flow mainly came from the Atlantic Ocean (7-9 g kg-1) and the role of the Mediterranean as a local moisture source was very limited (2-3 g kg-1) due to the high initial water vapour content of the parcels and the rapid passage over the basin along the Spanish Mediterranean coast (less than 12 h)
Convective initiation and maintenance processes of two back-building mesoscale convective systems leading to heavy precipitation events in Southern Italy during HyMeX IOP 13
No full textInternational audienceDuring Intensive Observation Period 13 (15–16 October 2012) of the first Special Observing Period of the Hydrological cycle in the Mediterranean Experiment (HyMeX), Southern Italy (SI) was affected by two consecutive heavy precipitation events (HPEs). Both HPEs were associated with multi-cell V-shaped retrograde regeneration mesoscale convective systems (MCSs). The life cycle of two MCSs in connection with their dynamic and thermodynamic environments were analysed using a combination of ground-based, airborne and spaceborne observations and numerical simulations. Rain gauges revealed that heavy precipitation occurred in two phases: the first one from 1300 to 1700 UTC (35 mm h−1) was caused by a V-shaped system initiating over the Tyrrhenian Sea in the early morning of 15 October. Convection was triggered by the low-level convergence between the south-westerlies ahead of an upper-level trough positioned over south-eastern France and very moist southerlies from the Strait of Sicily. The convection was favoured by high convective available potential energy (1500 J kg−1) resulting from warm and moist conditions at low levels associated with high sea surface temperatures in the Sicily Channel. In addition, humidity at mid-level was enriched by the presence of an elevated moisture plume from tropical Africa, favouring the efficiency of the convection to produce more precipitation. The second phase of heavy precipitation (2300 UTC on 15 October to 0200 UTC on 16 October, 34 mm h−1) was caused by a MCS initiating over Algeria around 1300 UTC, which subsequently traveled over the Strait of Sicily toward Sicily and SI. Convection was maintained by the combination of large low-level moisture contents and a marked convergence ahead of the cold front. Unlike other MCSs forming in the same region earlier on that day, this huge V-shaped system did affect SI because the strong upper-level flow progressively veered from southwesterly to south-southwesterly
Impact of upstream moisture structure on a back-building convective precipitation system in south-eastern France during HyMeX IOP13
No full textInternational audienceThe present study examines the impact of the environmental moisture structure in the lower troposphere (below 2 km above sea level, a.s.l.) on the precipitation development, observed in southern France during Intensive Observation Period (IOP) 13 of the first Special Observation Period of the Hydrological cycle in the Mediterranean Experiment (HyMeX SOP-1), through a series of sensitivity experiments using the non-hydrostatic mesoscale atmospheric numerical model (Meso-NH). The control simulation (CNTL) and all the other 12 sensitivity experiments examined in this study succeed in reproducing a heavy precipitation event (HPE) in the coastal mountainous region of Var in south-eastern France as observed. The sensitivity experiments are designed to investigate the response of the HPE to the variability of the water vapour content upstream in the moist marine atmospheric boundary layer (MABL) and the drier air above. The comparisons between CNTL and the 12 sensitivity experiments show how the life cycle of precipitation associated with the HPE, but also the upstream flow (over the sea), is modified, even for moisture content changes of only 1 g kg−1 below 2 km a.s.l. Within the low-level wind convergence between southerlies and south-westerlies, a small increase of moisture content in the MABL prolongs moderate precipitation (≥5 mm in 15 min) and enlarges the area of weak precipitation (≥1 mm in 15 min). The moistening in the 1–2 km a.s.l. layer, just above the MABL, prolongs the duration of moderate precipitation, for a similar total precipitation amount as in CNTL. The drier MABL and 1–2 km a.s.l. layer shorten the lifetime of precipitation and reduce the total precipitation amount with respect to CNTL. We also found that the moisture in the MABL has a stronger impact on producing enhanced precipitation (both in terms of amount and intensity) than the moisture just above (1–2 km a.s.l.). Also, it is worth noting that adding moisture in the MABL does not necessarily lead to enhanced precipitation amount. In moistening the MABL, the duration of moderate precipitation increases with increasing moisture as does the area covered by weak precipitation, while the area covered by the intense precipitation (≥30 mm) decreases. Despite a simplified moisture-profile modification approach, this study suggests that moisture structure in the lower troposphere is key for accurate prediction at short-term range of the timing and location of precipitation in the coastal mountainous region in southern France
Mesoscale numerical analysis of the historical November 1982 heavy precipitation event over Andorra (Eastern Pyrenees)
No full textFrom 6 to 8 November 1982 one of the most catastrophic flash-flood events was recorded in the Eastern Pyrenees affecting Andorra and also France and Spain with rainfall accumulations exceeding 400 mm in 24 h, 44 fatalities and widespread damage. This paper aims to exhaustively document this heavy precipitation event and examines mesoscale simulations performed by the French Meso-NH non-hydrostatic atmospheric model. Large-scale simulations show the slow-evolving synoptic environment favourable for the development of a deep Atlantic cyclone which induced a strong southerly flow over the Eastern Pyrenees. From the evolution of the synoptic pattern four distinct phases have been identified during the event. The mesoscale analysis presents the second and the third phase as the most intense in terms of rainfall accumulations and highlights the interaction of the moist and conditionally unstable flows with the mountains. The presence of a SW low level jet (30 m s-1) around 1500 m also had a crucial role on focusing the precipitation over the exposed south slopes of the Eastern Pyrenees. Backward trajectories based on Eulerian on-line passive tracers indicate that the orographic uplift was the main forcing mechanism which triggered and maintained the precipitating systems more than 30 h over the Pyrenees. The moisture of the feeding flow mainly came from the Atlantic Ocean (7-9 g kg-1) and the role of the Mediterranean as a local moisture source was very limited (2-3 g kg-1) due to the high initial water vapour content of the parcels and the rapid passage over the basin along the Spanish Mediterranean coast (less than 12 h)
Initiation and development of a mesoscale convective system in the Ebro River Valley and related heavy precipitation over northeastern Spain during HyMeX IOP 15a
No full textInternational audienceDuring Intensive Observation Period 15a (20 October 2012) of the first Special Observation Period of the Hydrological cycle in the Mediterranean Experiment, north-eastern Spain experienced heavy precipitation (130 mm in 24 h) associated with a retrograde regeneration mesoscale convective system (MCS) developing in the exit region of the Ebro River Valley (ERV). The life cycle of the MCS that brought intense hourly rainfall (34 mm) from the foothill of Iberian Plateau to the central Pyrenees, as well as the detailed structure of moist marine flow upstream, were analysed using a combination of ground-based, airborne and space-borne observations as well as model analyses. Over the Balearic Sea, the south-westerlies along the north-eastern flank of a surface low converged with south-easterlies from north Africa, creating a near surface moisture tongue in the region of the Balearic Islands, and a southeast-northwest oriented convergence line within a cloud cluster advecting from northern Africa. Airborne lidar measurements, acquired upstream of the ERV, evidenced water vapour mixing ratios in excess of 15 g kg−1 in the marine atmospheric boundary layer. In the mid-level (700 hPa), the presence of an elevated moisture plume from tropical Africa contributed for about one third to the large moisture content present over the western Mediterranean Sea. In this moist environment, the MCS was initiated over the orography of the north-eastern tip of the Iberian plateau, due to the combined influence of the approaching convergence line ahead of the surface low and the convergence resulting from weak north-westerlies channeled in the ERV and the easterlies impinging on the coastal range. After the initiation phase, the MCS further developed over the foothill of Iberian Plateau and moved into the ERV and along the southern flank of the Pyrenees, thanks to the penetration of the warm and moist maritime south-easterly flow through the narrow gap between the north-eastern part of the Iberian Plateau and the Catalan coastal range
