Numerical study of a Sahelian synoptic weather system: Initiation and mature stages of convection and its interactions with the large‐scale dynamics

Abstract

AbstractA 24 h multi‐scale simulation of the life cycle of a Sahelian squall line is presented. The initial and coupling fields have been taken from the European Centre for Medium‐Range Weather Forecasts re‐analysis (ERA‐15) after humidity corrections to avoid spurious absolute and conditional convective instabilities. Comparisons of model‐synthetic and Meteosat infra‐red radiances indicate that the simulated scenario of convection development and structure are realistic, though differences are found concerning the size of the squall line with a delay of 2–3 h. The simulated arc‐shaped structure and surface signature fit well with radar and ground station observations, respectively.The mature stage of the simulated squall line presents a cross‐line structure similar to conceptual models. An intense rear‐to‐front flow is generated below the trailing stratiform part. An acceleration of the African easterly jet (AEJ) is simulated behind the system, occurring across its full width. This leads to the generation of a dipole of counter‐rotating vortices. The resulting potential‐vorticity signature leads to local reinforcement of the barotropic instability on the north flank of the AEJ.This simulation is used to analyse the impact of the convection at synoptic‐scales and to compare it with ERA‐15. It suggests that the convection contributes to reinforce the monsoon at least up to 500 km. The monsoon inflow moves eastward and westward with time‐scales shorter and longer than 24 h, respectively. The tropical easterly jet intensifies to the south‐west of the convection area up to 700 km away. The AEJ is strongly intensified ahead of the trough where active convection occurs. Organized convection developing in the vicinity of the AEJ core induces weakening of the AEJ ahead of the squall line and strengthening behind it. A common result in the simulation and in ERA‐15 is that the AEJ core moves westward faster due to the convective activity. Copyright © 2002 Royal Meteorological Society