Convection in an African Easterly Wave over West Africa and the Eastern Atlantic: A model case study of Helene (2006)

Convective systems over West Africa and the eastern Atlantic embedded in the African Easterly wave (AEW) out of which Hurricane Helene (2006) deloped, are investigated. The study is based on COSMO model runs with a horizontal resolution of 2.8 km. Over land, the developing, mature and decaying phases of a mesoscale convective system are identified. Over the ocean, the convection consists of short–lived systems. The structure of the convective systems over land and water differs significantly. Over land, the updraught, heating and vertical vorticity are tilted in the vertical. Over water, the convective cores are upright and collocated with positive relative vorticity. The atmosphere over land is characterised by drier air at midlevels, stronger shear, stronger downdraughts and the occurence of a distinct cold pool. The influence of the convective systems on their environment is assessed with potential temperature and relative vorticity budgets for regions encompassing the convective systems. During the convective system's life–cycle over land, the relative vorticity increases at the height of the African easterly jet due to the eddy flux term whereas over the ocean, the relative vorticity maximum moves downwards with time due to stretching. A positive low–level relative vorticity anomaly is seen over the ocean before the AEW trough crossed the West African coast. This low–level circulation developed as a positive low–level vorticity anomaly associated with the westward extension of the Saharan heat low moved over the Atlantic and southwestwards. In addition, positive vorticity associated with the low–level monsoon flow contributed to this circulation. The cyclogenesis of Hurricane Helene was initiated when the mid–level vorticity anomaly of the AEW moved over the low–level circulation, initiating convection and enhancing the low–level vorticity through vortex stretching.