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Keywords:

  • mesoscale convective systems;
  • tropical cyclones;
  • relative vorticity budget;
  • AMMA;
  • COSMO

Abstract

Convective systems embedded in an African Easterly Wave (AEW) over West Africa and the eastern Atlantic in September 2006 are investigated. The study is based on COSMO model runs with a horizontal resolution of 2.8 km. The developing, mature and decaying phases of a mesoscale convective system over land are identified. Over the ocean, the convection consists of the development and decay of short-lived systems. There are significant differences between the structure of convection over land and water. 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 is drier at mid-levels over land, and stronger shear, downdraughts and a distinct cold pool occur. 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, the relative vorticity increased at the height of the African Easterly Jet over land due to the eddy-flux term, whereas the relative vorticity maximum over the ocean moved downwards with time due to stretching. High values of low-level relative vorticity were seen over the ocean before the AEW trough crossed the West African coast. The low-level circulation that 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. Copyright © 2010 Royal Meteorological Society