Field observations such as those of the HAPEX-Sahel project and JET2000 have been used to demonstrate large-scale diurnal variability over the West African Sahel region. A GCM and reanalyses are here used to study this variability at smaller-scale in the region.

We present a case study simulating the atmospheric dynamics of the development of a tropical disturbance that formed from an AEW (African Easterly Wave) into the precursor of Hurricane Alberto in 2000. The Met Office global Unified Model (UM) is used and its simulations are compared to the ECMWF ERA-40 reanalyses. We track the AEW and the local development of synoptic vortices as they cross the West African region over a period of three days. For this period we show vortex intensification modulated by the diurnal variation. Interaction between vortices, and the corresponding effects on moist convection, are discussed. Finally, the sensitivity of the UM to initial vortex structures is also described.