Amélioration des estimations quantitatives des précipitations à hautes résolutions (comparaison de deux techniques combinant les observations et application à la vérification spatiale des modèles météorologiques)

Ces dernières années, de nombreux efforts ont été entrepris pour mieux comprendre les phénomènes précipitants parfois à l origine de crues de cours d eau et d inondations ravageuses. Courant 2009, un consortium auvergnat a été mis en place pour notamment surveiller et prévoir ces événements. Les travaux menés dans cette thèse visent d une part à améliorer les estimations quantitatives des précipitations (QPE) et d autre part à vérifier les prévisions issues de modèles numériques sur de petites zones d étude telles qu une agglomération. L observation des précipitations peut être réalisée à l aide soit d un pluviomètre qui fournit une mesure directe et précise de la quantité de pluie tombée au sol mais ne renseigne pas sur la variabilité spatiale des pluies soit d'un RADAR météorologique qui donne une représentation détaillée de la structure spatiale des précipitations mais dont les estimations sont sujettes à diverses erreurs d autant plus prononcées en régions montagneuses. Le premier défit de cette thèse a été de trouver la meilleure façon de combiner ces deux informations complémentaires. Deux techniques géostatistiques ont été sélectionnées pour obtenir la meilleur QPE : le krigeage avec dérive externe (KED) et la fusion conditionnée (MERG). Les performances de ces deux méthodes ont été comparées au travers de deux domaines d étude qui présentent des résolutions spatio-temporelles différentes. La seconde partie de cette thèse est consacrée à la mise en place d une méthodologie fiable permettant de comparer spatialement les champs de QPE alors reconstruits et les prévisions quantitatives des précipitations (QPF). L effort fut porté sur le modèle Weather Research et Forcasting (WRF). Une étude préliminaire a été réalisée pour tester les capacités du modèle et plus particulièrement des schémas de microphysique à reproduire la pluie. Cette étude assure ainsi l obtention de prévisions réalistes pour une application sur des cas réels. L appréciation de la qualité des QPF s est focalisée sur la quantification spatiale des erreurs de prévision en termes de structure, d intensité et de localisation des systèmes précipitants (SAL : Wernli et al. 2008, 2009).In the last decades, many efforts were made to better understand the origins of rain that sometimes lead to rivers runoff or devastating floods. In 2009, a consortium took place in Auvergne in order to observe and predict these events. These works were focused on the improvement of quantitative precipitation estimations (QPE) and the verification of numerical weather models over small areas such as urban environment. Rainfall measurement could be operated either by rain gauges which provides direct and precise rainfall estimations but unfortunately cannot capture the spatial variability or by using weather RADAR which provides a detailed spatial representation of precipitation but estimates are derived indirectly and are subject to a combination of errors which are most pronounced over complex terrain. The main issue of these works was to find the best way to combine both observational systems which are complementary as well. In order to obtain the more truthful fields of QPE, two geostatistical techniques were selected: the kriging with external drift (KED) and the conditional merging (MERG). The performances of these two methods have been experienced on two catchments with different spatial and temporal resolutions. The second part of these works is focused on a reliable method for QPE comparison and quantitative precipitation forecast (QPF). The main effort was focused on the Weather Research and Forecasting (WRF) model. A preliminary study was made to check the performances of the microphysics schemes of the model to ensure realistic forecasts for an application on real cases. The spatial verification of the model set up contains three distinct components that consider aspects of the structure, amplitude and location of the precipitation field (SAL : Wernli et al. 2008, 2009).CLERMONT FD-Bib.électronique (631139902) / SudocSudocFranceF

Development of a Detailed Microphysics Cirrus Model Tracking Aerosol Particles' Histories for Interpretation of the Recent INCA Campaign

International audienceCirrus clouds play an important role in the earth's energy balance. To quantify their impact, information is needed on their microstructure and more precisely on the number and size of the ice crystals. With the anthropogenic activity, more and more aerosol particles and water vapor are released even at the altitude where cirrus clouds are formed. Cirrus clouds formed in a polluted air mass may have different microphysical properties and, therefore, a different impact on the climate system via the changed radiative properties compared to background cirrus clouds. To study this aspect, the European project called the Interhemispheric Differences in Cirrus Properties due to Anthropogenic Emissions (INCA) measured the microphysical properties of cirrus clouds together with the physical and chemicals properties of aerosol particles in clean air (at Punta Arenas, Chile) and polluted air (at Prestwick, Scotland). The goal of the present work was to develop a detailed microphysics model for cirrus clouds for the interpretation and the generalization of the INCA observations. This model considers moist aerosol particles through the Externally Mixed (EXMIX) model, so that the chemical composition of solution droplets can be followed. Ice crystal formation is described through homogeneous or heterogeneous nucleation. The crystals then grow by deposition. With this model, the interactions between the microphysical processes, simulated ice crystal concentrations, and dimensional distributions of the INCA observations were studied, and explanations were provided for the observed differences between background and polluted cirrus clouds

A modelling study on the activation of small Aitken-mode aerosol particles during CIME 97

During February 1997, one of the 2 observational periods of CIME (cloud ice mountain experiment), a joint field experiment funded by the European Commission, took place on the summit of the Puy de Doˆme in the centre of France. During this experiment the droplet spectra were measured with an FSSP and the aerosol particles in the drops and in the interstitial particle phase were measured with a counterflow virtual impactor and a round jet impactor inside a windtunnel. Very low aerosol particle and drop concentrations were observed and particles as small as 25 nm in diameter were found to activate. Two datasets obtained on 15 February and 17 February were used to study the activation of the small Aitken-mode particles and the spectral form of the droplet spectrum and the scavenging fraction. Numerous sensitivity studies were performed investigating the roˆ le of the number density and chemical composition of the aerosol particles. The roˆ le of mixing inside the orographic cloud was studied by using a new technique. It considers the fact that the air arriving on the summit of the Puy de Doˆme is a mixture of air of different origins. Thus, it weighs the results of a spectral scavenging model (DESCAM or EXMIX) calculated along a number of individual trajectories. The weighing function is derived from tracer and trajectory studies with a 3-dimensional mesoscale model. The model was able to reproduce the activation of aerosol particles as small as 25 nm. It was caused by the low aerosol particle number concentrations. In general, we can conclude that the variability found in the sensitivity tests of the dynamical and chemical factors allows to reproduce the shape of the observed results. As too many free parameters exit at the moment we cannot quantify the contribution of each factor studied to the observed scavenging fraction, however, it seems that dynamics dominates

The catastrophic flash-flood event of 8–9 September 2002 in the Gard region, France: a first case study for the Cévennes–Vivarais Mediterranean Hydrometeorological Observatory

The Cévennes–Vivarais Mediterranean Hydrometeorological Observatory (OHM-CV) is a research initiative aimed at improving the understanding and modeling of the Mediterranean intense rain events that frequently result in devastating flash floods in southern France. A primary objective is to bring together the skills of meteorologists and hydrologists, modelers and instrumentalists, researchers and practitioners, to cope with these rather unpredictable events. In line with previously published flash-flood monographs, the present paper aims at documenting the 8–9 September 2002 catastrophic event, which resulted in 24 casualties and an economic damage evaluated at 1.2 billion euros (i.e., about 1 billion U.S. dollars) in the Gard region, France. A description of the synoptic meteorological situation is first given and shows that no particular precursor indicated the imminence of such an extreme event. Then, radar and rain gauge analyses are used to assess the magnitude of the rain event, which was particularly remarkable for its spatial extent with rain amounts greater than 200 mm in 24 h over 5500 km2. The maximum values of 600–700 mm observed locally are among the highest daily records in the region. The preliminary results of the postevent hydrological investigation show that the hydrologic response of the upstream watersheds of the Gard and Vidourle Rivers is consistent with the marked space–time structure of the rain event. It is noteworthy that peak specific discharges were very high over most of the affected areas (5–10 m3 s−1 km−2) and reached locally extraordinary values of more than 20 m3 s−1 km−2. A preliminary analysis indicates contrasting hydrological behaviors that seem to be related to geomorphological factors, notably the influence of karst in part of the region. An overview of the ongoing meteorological and hydrological research projects devoted to this case study within the OHM-CV is finally presented

The chemistry of sulfur and nitrogen species in a fog system A multiphase approach

Concentration and phase distribution of sulfur and nitrogen species during a particular fog episode in the Po Valley are experimentally described in this paper. Chemical measurements were carried out simultaneously at different heights within the fog layer, up to 50 m. Microphysical and meteorological parameters necessary for the description of the fog multiphase system were also concurrently measured as a function of height. The fog cycle (formation, evolution, dissipation) is described in terms of the total acidity of a unit volume of air containing gas species, interstitial aerosol particles and fog droplets. The fog system was not closed and input of acidic and basic components was observed during fog evolution. The driving force which determines the acidity of the fog multiphase atmospheric system was found to be the presence of NH 3 and its partitioning among the different phases. A strong decrease of fog water pH (from 5.6 down to 2.8) was observed during fog evolution and was attributed to a HNO 3 input to the system. These acidic and basic inputs are described in terms of a titration/back-titration process of the fog system. The SO 2 oxidation process in fog water was found to be of minor importance in determining the SO 4 = concentration within the fog system, due to both low SO 2 concentration and limited oxidant availability during the experiment. DOI: 10.1034/j.1600-0889.1992.t01-4-00005.

Venting of gases by convective clouds

International audienceA two-dimensional dynamic model with spectral microphysics and a spectral treatment of aerosol particle and gas scavenging (DESCAM) was used to estimate the transport of gases from the marine boundary layer to the free troposphere by a mediumsized warm precipitating convective cloud. In the simulation, three gases were considered, covering a range of Henry's law constants: an inert tracer, SO2, and H202. SO 2 was also used as the inert tracer by artificially suppressing any interaction with the cloud drops. The horizontal and vertical fluxes, their vertical means and the transport across the cloud boundaries were studied. It was calculated that for SO2 as an inert tracer 37 kg, for SO2 as a scavenged gas 34 kg, and for H202 12 kg were transported from the marine boundary layer across cloud base to the free troposphere for an estimated three-dimensional cloud. This represents a depletion of the marine boundary layer in the vicinity of the cloud by about 60%. After about half an hour of cloud life time, however, only 75% of the SO2 and only 30% of the H202 transported aloft still existed in the cloudy air. These residual gases could eventually participate in a long range transport if the cloud would dissipate. The rest had been scavenged by the cloud

Développement et évaluation d'un modèle tridimensionnel de nuage chaud à microphysique détaillée

CLERMONT FD-BCIU Sci.et Tech. (630142101) / SudocSudocFranceF

A review of our understanding of the aerosol–cloud interaction from the perspective of a bin resolved cloud scale modelling

International audienc

Développement d'un modèle tridimensionnel à microphysique détaillée (Application à la simulation de cas de convection moyenne et profonde)

La représentation des nuages est une source importante d'incertitude dans les modèles à échelle synoptique ou globale. Pour l'améliorer, la solution retenue consiste à développer un modèle de nuage le plus réaliste possible, pour pouvoir ensuite le comparer avec des représentations plus simplifiées et détecter leurs éventuelles faiblesses. Ainsi, un modèle de nuages tridimensionnel (3D) et à microphysique détaillée a été developpé à partir du modèle dynamique 3D de Clark et Hall (1991) et du modèle microphysique DESCAM (DEtailed SCAvenging Model) de Flossmann et al. (1985) et cette thèse traite plus particulièrement de la prise en compte de la phase glace dans ce modèle. Le module de microphysique froide implémenté a été validé par comparaison avec des mesures aéroportées et au sol. Le modèle DESCAM 3D étant aussi conçu pour l'étude des interactions aérosol-nuage, il a ensuite été utilisé pour étudier l'impact de la pollution particulaire sur la phase glace et les précipitationsCLERMONT FD-BCIU Sci.et Tech. (630142101) / SudocSudocFranceF

Cloud Processing of Aerosol Particles in Marine Stratocumulus Clouds

International audienc