Analyse multi-échelle du cycle de l'eau dans la mousson africaine à l'aide d'observations GPS

The West African Monsoon (WAM) regional climatic system is characterized by a strong seasonal cycle in humidity and precipitation and land-surface atmosphere interactions.The water cycle is a primordial element of the WAM, the study of which is a ma jor ob jective of the AMMA campaign (African Monsoon Multidisciplinary Analysis). Within this context, six GPS stations have been installed along the meridian climatic gradient in West Africa, beginning in 2005. This dissertation focuses on the analysis of atmospheric humidity through GPS data, while also proposing a more in-depth study of atmospheric water budgets based on different data sets resulting from the AMMA campaign. The analysis of temporal GPS series shows a spatial and temporal variability of water vapor (seasonal cycle, inter-seasonal variability with 10-20 day modes, synoptic scales and diurnal cycle) that is shown to be linked with the WAM atmospheric processes. The precision of this data also enables the identification of humidity biases in radiosounding data and through assimilation processes in meteorological prevision systems. A method is then developed to compute water budgets vertically integrated at regional scale based on a "hybrid" data set. This method combines satellite precipitation estimations, simulations from a grouping of land surface models, all of which are forced by the same precipitation and other elaborate products. The analysis of the functioning of the hydrological cycle at the surface-atmosphere interface allows us to confirm or not certain hypotheses that have been developed in the past, but especially to shed new light on the seasonal cycle and the interannual variability of water budgets and surface energy. We focus in particular on the coupling between the terms of these budgets (moisture convergence, precipitation, evaporation, runoff, soil moisture, and surface net radiation), which differ according to the scale in question. This dataset is then used as a reference in order to evaluate different terms of the water budget of an ensemble of meteorological models (reanalyses, operational analyses and forecasts from Météo France, ECMWF and NCEP). Important biases are diagnosed in the precipitation, evaporation, and moisture convergence of these models. Hypotheses are proposed regarding the origin of these biases (issues in the convection schemas, in the assimilation of soil moisture. . . ). For certain models (ex. ECMWF), we identify a retroaction in these biases, over the atmospheric circulation in the region of the Saharan depression. This work opens up towards possibilities of improvement to the water cycle in models of meteorological prevision and provides a hybrid dataset that is potentially useful in a further analysis of the interaction of processes at other scales(meso-scale and synoptic).La mousson d'Afrique de l'Ouest (MAO) représente un système climatique régional dont le cycle saisonnier est très marqué, notamment par l'humidité et les pluies de mousson et par la réponse de la végétation et des surfaces continentales. Le cycle de l'eau est un élément primordial de la MAO et son étude est un objectif majeur de la campagne AMMA (Analyses Multidisciplinaires de la Mousson Africaine). Dans ce contexte, six stations GPS ont été installées le long du gradient climatique méridien d'Afrique de l'Ouest dès 2005. Cette thèse porte d'une part sur l'analyse de l'humidité atmosphérique à l'aide des données GPS et propose d'autre part une étude plus approfondie des bilans d'eau dans l'atmosphère, à l'aide de différents jeux de données élaborés à partir des produits de la campagne AMMA (observations et modélisation). L'analyse des séries temporelles GPS met en évidence une variabilité spatiale et temporelle de la vapeur d'eau (cycle saisonnier, variabilité intra-saisonnière avec des modes de 10-20 jours, échelle synoptique et cycle diurne) dont le lien est fait avec les processus atmosphériques de la mousson. La précision de ces données permet également d'identifier des biais d'humidité dans les observations de radiosondages et de manière corrélée (via l'assimilation) dans les modèles de prévision météorologique. Une méthode est ensuite développée pour calculer des bilans d'eau intégrés verticalement à l'échelle de l'Afrique de l'Ouest à partir d'un jeu de données "hybride". Elle combine des estimations de pluies satellitaires, des simulations d'un ensemble de modèles de transfert sol-végétation-atmosphère (TSVA) tous forcés par les mêmes pluies et d'autres produits élaborés de qualité. L'analyse du fonctionnement du cycle hydrologique à l'interface surface-atmosphère à l'aide de ce nouveau jeu de données, permet de confirmer ou d'infirmer certaines hypothèses élaborées dans le passé mais surtout d'apporter un éclairage nouveau sur le cycle saisonnier et la variabilité interannuelle des bilans d'eau et d'énergie à la surface. Nous mettons notamment en évidence des relations entre les termes de ces bilans (convergence d'humidité, pluie, évaporation, ruissellement, humidité dans le sol, et le rayonnement net) distinctes suivant les échelles considérées. Ce jeu de données est ensuite utilisé comme référence pour évaluer les différents termes du bilan d'eau d'un ensemble de modèles météorologiques (ré-analyses, analyses opérationnelles et prévisions des modèles de Météo-France, ECMWF et NCEP). Des biais importants sont diagnostiqués dans les précipitations, les évaporations et la convergence d'humidité de ces modèles. Des hypothèses sont proposées quant à l'origine de ces biais (défauts dans les schémas de convection, dans l'initialisation de l'humidité des sols et dans les données assimilées). Pour certains modèles (p.ex. ECMWF), nous identifions une rétroaction de ces biais sur la circulation atmosphérique dans la région de la dépression Saharienne. Ce travail ouvre des perspectives d'améliorations du cycle de l'eau des modèles de prévision météorologique et fournit un jeu de données hybride potentiellement utile pour analyser les intéractions des processus à d'autres échelles (méso-échelle et synoptique)

Impact of vegetation fires on tropospheric chemical composition in the Guinean Gulf and on megacities air quality.

International audienceIn the framework of the preparation of the "Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa" (DACCIWA) project, the tropospheric chemical composition in the megacities along the Guinean Gulf is studied using the WRF and CHIMERE regional models. Two simulations are performed for the May-July 2014 period, without and with biomass burning emissions. The impact of biomass burning from Central Africa is quantified for aerosol optical depth, gaseous species (ozone and carbon monoxide) and particulate matter with a mean mass median of diameter less than 10 μm (PM10, both concentrations and chemical composition). We show that vegetation fires in Central Africa represent an important contribution to air pollution in urbanized areas located in the Guinean Gulf. On average in July 2014, CO and O3 concentrations are increased in Abidjan (Ivory Coast) by 38.5% and 15.4% respectively. In Abidjan and Lagos (Nigeria), two of the biggest megacities in southern West Africa, a net increase of PM10 by 36.5% and 53.5% is quantified. The analysis of the chemical composition of PM10 shows that this increase is mainly related to an increase of Particulate Primary Matter and Particulate Organic Matter in the fine mode of the aerosol size distribution

Diurnal cycle of the West African Monsoon water cycle: regional and seasonal variability.

International audienceThe ground-based Global Positioning System (GPS) receivers deployed in the framework of AMMA provide an unprecedented insight into diurnal variations of precipitable water vapour over West Africa. They display a strong seasonal dynamics and distinct features along latitude. Overall, GPS data point to the significance of an afternoon maximum of PWV during the monsoon season. These data are used together with surface evapo-transpiration from the ALMIP land surface model simulations forced with TRMM 3B42v6 three-hourly precipitation data. The combination of the three datasets yields a comprehensive description of the atmospheric water cycle at a sub-daily timescale. A description of the monthly mean diurnal cycle in the water budget terms is discussed as a function of location and time. A focus is made on the pre- and post-onset periods. The diurnal cycle of the atmospheric processes involved are investigated with the help of high-resolution radiosonde data and the ECMWF model analysis and forecasts

Multi-model, multi-sensor estimates of global evapotranspiration: climatology, uncertainties and trends

International audienceEstimating evapotranspiration (ET) at continental to global scales is central to understanding the partitioning of energy and water at the earth's surface and the feedbacks with the atmosphere and biosphere, especially in the context of climate change. Recent evaluations of global estimates from remote sensing, upscaled observations, land surface models and atmospheric reanalyses indicate large uncertainty across the datasets of the order of 50% of the global annual mean value. In this paper, we explore the uncertainties in global land ET estimates using three process-based ET models and a set of remote sensing and observational based radiation and meteorological forcing datasets. Input forcings were obtained from International Satellite Cloud Climatology Project (ISCCP) and Surface Radiation Budget (SRB). The three process-based ET models are: a surface energy balance method (SEBS), a revised Penman-Monteith (PM) model, and a modified Priestley-Taylor model. Evaluations of the radiation products from ISCCP and SRB show large differences in the components of surface radiation, and temporal inconsistencies that relate to changes in satellite sensors and retrieval algorithms. In particular, step changes in the ISCCP surface temperature and humidity data lead to spurious increases in downward and upward longwave radiation that contributes to a step change in net radiation, and the ISCCP data are not used further. An ensemble of global estimates of land surface ET are generated at daily time scale and 0.5 degree spatial resolution for 1984-2007 using two SRB radiation products (SRB and SRBqc) and the three models. Uncertainty in ET from the models is much larger than the uncertainty from the radiation data. The largest uncertainties relative to the mean annual ET are in transition zones between dry and humid regions and monsoon regions. Comparisons with previous studies and an inferred estimate of ET from long-term inferred ET indicate that the ensemble mean value is reasonable, but generally biased high globally. Long-term changes over 1984-2007 indicate a slight increase over 1984-1998 and decline thereafter, although uncertainties in the forcing radiation data and lack of direct linkage with soil moisture limitations in the models prevents attribution of these changes

Diurnal cycle of the West African Monsoon water cycle: regional and seasonal variability.

International audienceThe ground-based Global Positioning System (GPS) receivers deployed in the framework of AMMA provide an unprecedented insight into diurnal variations of precipitable water vapour over West Africa. They display a strong seasonal dynamics and distinct features along latitude. Overall, GPS data point to the significance of an afternoon maximum of PWV during the monsoon season. These data are used together with surface evapo-transpiration from the ALMIP land surface model simulations forced with TRMM 3B42v6 three-hourly precipitation data. The combination of the three datasets yields a comprehensive description of the atmospheric water cycle at a sub-daily timescale. A description of the monthly mean diurnal cycle in the water budget terms is discussed as a function of location and time. A focus is made on the pre- and post-onset periods. The diurnal cycle of the atmospheric processes involved are investigated with the help of high-resolution radiosonde data and the ECMWF model analysis and forecasts

Atmospheric responses to anomalies of SST seasonal variations in the Northeastern Tropical Atlantic

International audienceThe north-eastern Tropical Atlantic sea surface temperature (SST) becomes very warm in boreal summer, north of the seasonal equatorial cold tongue, with a maximum in the vicinity of the InterTropical Convergence Zone (ITCZ). The ITCZ has a significant contribution in the functioning and partitioning of the water cycle over the ocean, but also over West Africa. Using the regional Weather Research and Forecasting Model (WRF), this study aims to describe and quantify the influence of the warm SST band on the ITCZ: two simulations examine independently the cases when the SST is not warming or not cooling regarding its regular seasonal evolution. It then allows to separate the influences of northern and southern SST fronts (where the meridional gradients are most intense) on surface winds and precipitation. The seasonal SST distribution impact on the ITCZ is indeed found to be very strong, with significant consequences on the moisture flux within the marine ITCZ and toward West Africa

Sensitivity testing of WRF parameterizations on air-sea interaction and its impact on water cycle in the Gulf of Guinea

International audienceA strong ocean–atmosphere coupling exists in the eastern equatorial region of the Tropical Atlantic at intraseasonal timescales, with a significant contribution in the functioning and partitioning of the water cycle in spring over the ocean, as well as later in the season over West Africa. Uncertainties in simulating the air-sea interaction in the Gulf of Guinea and its impact on the water cycle are studied using modeling experiments during spring-summer 2006 with the Weather Research and Forecasting model (WRF). Tested parameters include physical package of cumulus (Cu), planetary boundary layer (PBL), microphysics (MP) and radiative (RAD) schemes. The simulations are compared with satellite-based observations, ship-based radiosonde data and the state of art of atmospheric model reanalyses. Results show that Cumulus, Microphysics and Radiatives parameterizations exert a large influence in the simulated seasonal distribution of regional convective rainfall. Non-local PBL schemes are determinant to simulate the correct surface wind pattern and water vapor distribution in order to get realistic precipitation from intra-seasonal to diurnal scales, especially over the ocean where the nocturnal rainfall representation is improved

Seasonal influence of the sea surface temperature on the low atmospheric circulation and precipitation in the eastern equatorial Atlantic

International audienceThe air–sea interaction in the Gulf of Guinea and its role in setting precipitation at the Guinean coast is investigated in the present paper. This study is based on satellite observations and WRF simulations forced by different sea surface temperature (SST) patterns. It shows that the seasonal cold tongue setup in the Gulf of Guinea, along with its very active northern front, tends to strongly constrain the low level atmospheric dynamics between the equator and the Guinean coast. Underlying mechanisms including local SST effect on the marine boundary layer stability and hydrostatically-changed meridional pressure gradient through changes in SST gradient are quantified in WRF regarding observations and CFSR reanalyses. Theses mechanisms strongly impact moisture flux convergence near the coast, leading to the installation of the first rainy season of the West African Monsoon (WAM) system. The current study details the mechanisms by which the Atlantic Equatorial cold tongue plays a major role in the pre-onset of the boreal WAM