Evolution of AEWs and MCSs off West Africa observed during AMMA SOP-3 in September 2006

During the Special Observing Period (SOP) 3 of AMMA that was held at Dakar on the 15-28 September 2006, a non-developing case (the so-called “Perturbation D”) has been sampled with dropsondes on 3 successive days. These data provide information on the mesoscale environment of the “Perturbation D”. In order to further document the scale interaction processes involved in the evolution of this convective system, we made a composite analysis between Meteosat Second Generation (MSG) Brightness temperature in the second water vapor's channel n°6 (6.85 – 7.84µm) and analyses at pressure levels provided by the European Center of Mean Weather Forecast (ECMWF). The satellite images are used to localize and characterize the convection and ECMWF fields allow analyzing the synoptic processes such as the African Easterly Jet – AEJ, the African Easterly Wave – AEW, the monsoon flow and intrusion of dry Saharan air. Our preliminary conclusions are that the convective activity seems to be driven by the monsoon flow and consequently enhances the curvature of the AEW. The so-called “Perturbation D” didn't have a cyclogenetic evolution probably because of the interaction with dry air coming from the Sahara. Simulations with the non-hydrostatic model Meso-NH are conducted in order to more precisely analyze the processes involved. The activation of 3 embedded models with respective resolutions of 40-10-2.5km allows turning off the parameterization in the area of interest and the displacement of these models gives the possibility to study the disturbance for a long period. A special attention is given to the generation of vortices by the convective systems and how they merge and are recycled at synoptic level by the AEW, thus producing a long-lived vortex. This process occurred off the Guinea coast and the quasi permanent convective orograpically driven vorticies over Futa Jallon seem to be involved in this scale interaction process. The convective activity was very low when the so-called “Perturbation D” crossed the coast, which is a usual feature of West African disturbances at this stage of their propagation. Cyclogenesis is then commonly observed when a convective re-development occurs south of Cape-Verde Islands. This was the case of the so-called “Perturbation D” although too far northward and disconnected from the monsoon flow.

In order to fully document cyclogenesis off the West African coast two developing cases of Cape Verde Cyclones during 2006 have been considered: Debby and Helene. A similar composite analysis between MSG images and ECMWF fields has been performed. The major differences are the development of vorticity over the whole troposphere and enhanced monsoon flow, AEJ, AEW. Simulations with Meso-NH are also conducted and comparisons of the 3 cases are expected to bring some advances on the scale interaction processes involved in cyclogenesis.