Indices de convection et précipitations quotidiennes en Afrique de l'Ouest (un pas vers la prévision de la pluie à Dakar à quelques jours d'échéance)

PARIS12-CRETEIL BU Multidisc. (940282102) / SudocSudocFranceF

Rainfall distribution over the Fouta Djallon — Guinea

International audienceThe main goal of the present paper is to discuss the asymmetry of the rainfall distribution over the mountain range of the Fouta Djallon — Guinea — in relation with orography and local atmospheric circulation. The Fouta Djallon is the main West African hydrographic source area. The two principal rivers (Niger and Senegal) take birth in this region where, in addition, one of the highest cumulative rain heights in the world (> 10 m) is observed. The data used are images of the Meteosat satellite, reanalyses of NCEP/NCAR (National Center for Environmental Prediction/National Center for Atmospheric Research) and ECMWF (European Centre for Medium-Range Weather Forecasts), and observations from ground-based raingauge networks. Over the Fouta Djallon, the convective activity is found to increase in frequency and intensity due to the orographic forcing of water vapor in an area of convergence between monsoon and Harmattan fluxes; heavy rain is released over the southwest side of the range. A conceptual model is proposed suggesting that convective systems developing over or east of the range are advected westward of the generating area, resulting in a strong pluviometric contrast and a maximum offshore

A cyclogenesis index for tropical Atlantic off the African coasts

International audiencehe westward moving Soudano-Sahelian mesoscale convective systems (MCS) frequently reach and cross the Atlantic Coast. At the end of their continental route, most MCS weaken and vanish over the ocean, near the coast, while others strengthen. The latter play an important part in the genesis of some Atlantic tropical cyclones. In the present paper, following the work of Gray, an index liable to be associated with the coast-crossing MCS cyclonic evolution is built. The data used in this work are observations by the Dakar-Yoff radar, reanalyses of NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research), outgoing long wave radiation at the top of the atmosphere, and the resources of the National Hurricane Center data base. Several terms describing the variation of individual meteorological parameters are first analysed and then combined into an index of cyclogenesis or ICY. Combination of vertical vorticity at 925 hPa and potential vorticity at 700 hPa is notably found to be a good factor to discriminate between strengthening and weakening MCS over the near Atlantic. A good correlation between the ICY maximum and the beginning of the MCS cyclogenesis is observed. This index enables discrimination of the simultaneous presence of two separate cyclonic perturbations over the Atlantic. These results show that the sole variable ICY is useful to detect a cyclogenesis process in progress in a Sahelian MCS

Coastal observations of weather features in Senegal during the African Monsoon Multidisciplinary Analysis Special Observing Period 3

International audienceDuring 15 August through 30 September 2006 (Special Observing Period 3, SOP3), key weather measurements are obtained from ground and aircraft platforms during the African Monsoon Multidisciplinary Analysis campaign. Key measurements are aimed at investigating African easterly waves (AEWs) and mesoscale convective systems in a coastal environment as they transition to the eastern Atlantic Ocean. Ground and aircraft instruments include polarimetric radar, a coarse and a high-density rain gauge network, surface chemical measurements, 12 m meteorological measurement, broadband IR, solar and microwave measurements, rawinsonde, aircraft dropsonde, lidar, and cloud radar measurements. Ground observations during SOP3 show that Senegal was influenced by 5 squall lines, 6 Saharan air layer intrusions, and 10 AEWs. Downstream tropical cyclones developed were associated with the passage of four AEWs. FA-20 aircraft measurements of microphysical aspects of 22 September squall line and several nondeveloping AEWs over the extreme eastern Atlantic Ocean are presented