SUMMARYA reference case of a Sahelian weather system observed during the Hydrological Atmospheric Pilot Experiment, HAPEX-SAHEL, in August 1992, is described from a seasonal viewpoint as well as from synoptic and convective system viewpoints. It is shown that the case-study is representative of the climatology at all these scales and presents many interacting scales and physical processes. At intraseasonal scale, the monsoon onset is characterized by an abrupt shift of precipitation together with a latitudinal migration of the African easterly jet (AEJ) and convection. At the month and day scales, the convective activity occurs in an apparent zonal break of the tropical easterly jet. The month of August 1992 exhibits intense synoptic activity. The vorticity eld is characterized by northerly (dry) and southerly (wet) components located at 850 hPa on each side of the AEJ. Their intraseasonal modulation on a period of 20 to 40 days leads to active and break phases of the synoptic activity. Around 21 August, the 700 hPa vorticity eld features the propagation of a typical easterly wave with a westward propagation of a cyclonic circulation followed by an anticyclonic circulation. Convective activity occurs mainly ahead of the 700 hPa vorticity maximum with the formation of a squall line on Aṏ r mountains propagating south-westward at 15 m s ¡1 . The convective system propagates about twice as fast as the vortex core, in contrast with the convection in the European Centre for Medium-Range Weather Forecasts re-analysis which stays in phase with the vorticity. The squall line corresponds to the largest contributor to the systems passing in August 1992 over the HAPEX-SAHEL region; its environmental conditions and its effects on the atmosphere including the surface parameters are presented.

No abstract

Reliable, real-time river flow forecasting in Africa on a time scale of days can provide enormous humanitarian and economic benefits. This study investigates the feasibility of using daily rainfall estimates based on cold cloud duration (CCD) derived from Meteosat thermal infrared imagery as input to a simple rainfall-runoff model and also whether such estimates can be improved by the inclusion of information from numerical weather prediction (NWP) analysis models. The Bakoye catchment in Mali, West Africa has been used as a test area. The data available for the study covered the main months of the rainy season for three years. The rainfall estimates were initially validated against gauge data. Improvements in quality were observed when information relating to African Easterly Wave phase and storm type was included in a multiple linear regression (MR) algorithm. The estimates were also tested by using them as input to a rainfall-runoff model. When contemporaneous calibrations from raingauges were available for calibration, both CCD-only and MR rainfall estimates gave more accurate river flow forecasts than when using raingauge data alone. In the absence of contemporaneous calibrations, the performance was reduced but the MR did better than the CCDonly input in all years. The use of satellite-derived vegetation index did not improve the quality of the river flow forecasts.Key words satellite rainfall estimates; river flow; hydrological model; NWP model; multiple regression; Africa Estimation par satellite des précipitations pour la prévision des débits fluviaux en Afrique. I: Estimations des précipitations et prévisions hydrologiques Résumé Prévoir en Afrique, avec fiabilité et en temps réel, le débit fluvial à échéance de quelques jours, peut apporter d'énormes bénéfices humanitaires et économiques. Cette étude porte sur la possibilité d'utiliser des estimations journalières de précipitation fondées sur la durée de nuages à sommet froid (DNSF) dérivée d'images infrarouge thermique Meteosat, comme entrée d'un modèle pluie-débit. Le but de l'étude est également de savoir si de telles estimations peuvent être améliorées en incluant des informations provenant de modèles numériques de prévision météorologique. Le bassin de Bakoye au Mali, Afrique de l'Ouest, a été utilisé en tant que test dans cette région. Les données disponibles pour l'étude couvrent les principaux mois de la saison pluvieuse durant trois ans. Les estimations des pluies ont été initialement validées par des données pluviométriques au sol. Une amélioration de la qualité a été observée lors de l'incorporation d'informations concernant la phase des ondes d'est africaines et le type d'orage dans un algorithme de régression multiple. Les estimations ont aussi été testées en tant qu'entrées d'un modèle pluie-débit. Lorsque des calibrations pluviométriques simultanées étaient diponibles, les précipitations estimées par le DNSF seul et la régression multiple ont donné des débits fluviaux plus précis que lorsque seules les données pluviométriques o...

While climate science has made great progress in the projection of weather and climate information, its uptake by local communities remains largely elusive. This paper describes two innovative approaches that strengthen understanding between the providers and users of weather and climate information and support-appropriate application: (1) knowledge timelines, which compare different sources and levels of certainty in community and scientific weather and climate information; and (2) participatory downscaling, which supports users to translate national and regional information into a range of outcomes at the local level. Results from piloting these approaches among flood-prone communities in Senegal and drought-prone farmers in Kenya highlight the importance of co-producing 'user-useful' climate information. Recognising that disaster risk management actions draw on a wide range of knowledge sources, climate information that can effectively support community-based decision-making needs to be integrated with local knowledge systems and based on an appreciation of the inherent uncertainty of weather and climate information.

No abstract