Multi‐scale description of a Sahelian synoptic weather system representative of the West African monsoon

A 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 field 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 field 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 Aïr 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. Copyright © 2002 Royal Meteorological Society