Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
High spatial and temporal variability of mesoscale moisture fields is still a challenge for quantitative precipitation forecast within numerical weather prediction (NWP) models especially over ocean regions where observations are lacking. This study presents the comparison between integrated water vapor over the Mediterranean Sea determined from shipborne GPS, the NWP ALADIN/Météo-France model and MODIS retrieval during a 4-month campaign (autumn 2008). While moisture prediction of the NWP remains accurate during most of the observation period, there are significant periods with offsets between GPS data and model predictions. We analyze such events and discuss the associated meteorological situation. Copyright © 2012 Royal Meteorological Society
ISSN: 1530-261X

hal-00757776
https://hal.archives-ouvertes.fr/hal-00757776
DOI : 10.1002/asl.391

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
The Convective and Orographically-driven Precipitation Study (COPS) carried out in summer 2007 over northeastern France and southwestern Germany provided a fairly comprehensive description of the low-troposphere water-vapour field, thanks in particular to the deployment of two airborne differential absorption lidar systems. These lidar observations were assimilated using the 3D-Var assimilation system of the Application of Research to Operations at MEsoscale (AROME) numerical weather prediction mesoscale model. The assimilation was carried out for the period 4 July-3 August by running a three-hour forward intermittent assimilation cycle. First, the impact of the lidar observations was assessed by comparing the analyses with a set of more than 200 independent soundings. The lidar observations were found to have a positive impact on the analyses by reducing the dry bias in the first 500 m above ground level and by diminishing the root-mean-square error by roughly 15% in the first km. Then the impact of the lidar observations was assessed by comparing the precipitation forecasts (obtained with and without the lidar observations for the period 15 July-2 August) with the gridded precipitation observations provided by the Vienna Enhanced Resolution Analysis. In general, the impact was found to be positive but not significant for the 24 h precipitation and positive and significant for the 6 h precipitation, with an improvement lasting up to 24 h. Some selected case studies show that the improvement was obtained through a better depiction of convection initiation or through a more accurate positioning of the precipitation systems
ISSN: 0035-9009

hal-00659497
https://hal.archives-ouvertes.fr/hal-00659497
DOI : 10.1002/qj.1864

Éditeur(s) : HAL CCSD
Résumé : Meteorological phenomena with scales lower than hurricanes, from extreme rainfalls to pollution peaks, represent important environmental and health risks in the Caribbean. These examples show the need to downscale large-scale meteorological data to island-scale fields.The main goal of the present numerical study is the analysis of the meso- and micro-scale mechanisms inducing diurnal and nocturnal circulations in the Guadeloupe archipelago. The Weather Research and Forecasting model is employed. The micro-scale results are obtained by the coupling of the 1-km simulated fields and the Large Eddy Simulation method. Then, in order to analyze the dispersion of a pollutants plume, the micro-scale outputs with 111 m of resolution are used to force the lagrangian particles dispersion model FLEXPART. The study has been conducted for three real cases of weather types linked with strong, medium and weak trade winds.The results of simulations, validated with available observed data, permitted to simulate all local circulations, including a nocturnal counter-current flow which may induce a pollution peak in the urban area of Pointe-à-Pitre city.These works, which may be applied to other Lesser Antilles islands, suggest many environmental and energy applications.
Les phénomènes météorologiques d'échelles inférieures aux cyclones, depuis les pluies intenses, jusqu'à la dégradation de la qualité de l'air, ont pour les Caraïbes une importance aiguë en termes de risques naturels ou de risques sur la santé. Ces exemples attestent de la nécessité d'utiliser des méthodes de descentes d'échelle pour exploiter l'information météorologique de grande échelle et dériver des scénarios locaux appliqués aux territoires insulaires. Les travaux présentés ont pour principaux objectifs la simulation numérique et l'analyse des mécanismes méso- et micro-échelles qui induisent des circulations locales diurnes et nocturnes sur l'archipel de la Guadeloupe, à l'aide du modèle numérique météorologique Weather Research and Forecasting. Les résultats des simulations micro-échelles sont obtenus par le couplage des champs modélisés à 1 km avec la méthode Large Eddy Simulation. Les sorties micro-échelles, à 111 m, sont ensuite utilisées pour forcer le modèle lagrangien de dispersion de particules FLEXPART afin d'étudier la dispersion d'un panache de polluants. L'étude a été menée pour trois cas réels de situations météorologiques correspondant à des alizés soutenus, moyens et faibles. Les résultats des simulations, après validation, et comparaison aux données d'observation disponibles, permettent de restituer l'ensemble des circulations locales et soulignent l'existence d'un écoulement nocturne à contre courant susceptible d'induire une forte pollution sur la zone urbaine de Pointe-à-Pitre. Cette étude, transposable aux autres îles des petites Antilles, laisse envisager de nombreuses applications environnementales et énergétiques.
https://hal.archives-ouvertes.fr/tel-01074600

Droits : info:eu-repo/semantics/OpenAccess
tel-01074600
https://hal.archives-ouvertes.fr/tel-01074600
https://hal.archives-ouvertes.fr/tel-01074600/document
https://hal.archives-ouvertes.fr/tel-01074600/file/rcece_thesis.pdf

Éditeur(s) : HAL CCSD European Geosciences Union
Résumé : International audience
Impact of GPS (Global Positioning System) data assimilation is assessed here using a high-resolution numerical weather prediction system at 2.5 km horizontal resolution. The Zenithal Tropospheric Delay (ZTD) GPS data from mesoscale networks are assimilated with the 3DVAR AROME data assimilation scheme. Data from more than 280 stations over the model domain have been assimilated during 15-day long assimilation cycles prior each of the two studied events. The results of these assimilation cycles show that the assimilation of GPS ZTD with the AROME system performs well in producing analyses closer to the ZTD observations in average. Then the impacts of assimilating GPS data on the precipitation forecast have been evaluated. For the first case, only the AROME runs starting a few hours prior the triggering of the convective system are able to simulate the convective precipitation. The assimilation of GPS ZTD observations improves the simulation of the spatial extent of the precipitation, but slightly underestimates the heaviest precipitation in that case compared with the experiment without GPS. The accuracy of the precipitation forecast for the second case is much better. The analyses from the control assimilation cycle provide already a good description of the atmosphere state that cannot be further improved by the assimilation of GPS observations. Only for the latest day (22 November 2007), significant differences have been found between the two parallel cycles. In that case, the assimilation of GPS ZTD allows to improve the first 6 to 12 hours of the precipitation forecast.
ISSN: 0992-7689

hal-00403847
https://hal.archives-ouvertes.fr/hal-00403847
https://hal.archives-ouvertes.fr/hal-00403847/document
https://hal.archives-ouvertes.fr/hal-00403847/file/angeo-27-2739-2009.pdf

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
Scientific interest in urban meteorology has increased because highly populated areas experience high vulnerability to pollution or heavy rain. However, compared to urban air quality or urban heat island (UHI) processes, the urban water vapour cycle is poorly understood because it has been investigated less due to the lack of upper-air measurements and the high sensitivity of surface measurements to local heterogeneities. In this paper, surface measurements of wind, temperature, pressure and humidity, as well as integrated water vapour (IWV) from GPS and MODIS and numerical simulations, have been used to investigate the urban cycle of water vapour in May and June 2004 during the VAPIC field experiment in the Paris area. The surface data show the typical characteristics of an urban area with the absence of water vapour sources and a UHI of about 6 °C at night. The urban IWV distribution differs completely, with an urban IWV excess on average between 1600 and 0600 UTC (with a maximum of about 1.5 kg m−2 at 0600 and 1700 UTC). No IWV difference between the urban and rural areas is found in the middle of the day. The numerical simulations reproduce accurately the urban IWV anomaly. Shallow surface wind convergence associated with the UHI during nighttime provides moisture from the rural areas. Urban areas are therefore under wind convergence for most of the time. The rural water vapour sources and the depth of the convergence control the amplitude of the urban IWV excess. At about 1200 UTC, entrainment at the top of the urban boundary layer is the key process that inhibits the urban IWV excess observed at night
ISSN: 0035-9009

hal-00406370
https://hal.archives-ouvertes.fr/hal-00406370
DOI : 10.1002/qj.446

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
The benefit of assimilating high spatial and temporal resolution Global Positioning System (GPS) zenith tropospheric delay (ZTD) observations within a high-resolution numerical weather prediction (NWP) system is evaluated during IOP 9 of the Convective and Orographically-induced Precipitation Study (COPS) campaign. In the framework of the international COPS field experiment, which took place from June to August 2007, a dense GPS network was deployed in the Vosges mountain region in eastern France and in the Black Forest region in southwestern Germany. GPS ZTD observations collected during the COPS field campaign, in addition to the European operational GPS data, are assimilated into the AROME/3D-Var system. We investigate the impact of assimilating these GPS ZTD observations on the forecast of convective systems that travelled over eastern France during the three days of COPS IOP 9 (18-20 July 2007). The results show a clear positive impact on short-range quantitative precipitation forecasts when GPS ZTD observations are assimilated. The impact is most important on 19 July 2007 (IOP 9b). The present study benefits from additional validation measurements collected during the COPS campaign. In particular we find that the assimilation of ZTD data induces changes in the low-to-middle tropospheric water vapour vertical structure that are consistent with water vapour measurements from radiosonde and airborne lidar. As most of the improvement is gained by assimilating GPS ZTD observations from the European operational GPS network, this study opens up encouraging horizons for the assimilation of GPS ZTD in new high-resolution operational NWP systems
ISSN: 0035-9009

hal-00424350
https://hal.archives-ouvertes.fr/hal-00424350
DOI : 10.1002/qj.508