Initiation of West African squall lines

Summary It is proposed that squall lines are initiated through the growth (with time) of wave-like perturbations along the surface of discontinuity between the monsoon southwesterlies and the dry northeasterlies in West Africa. Out of the many possible modes of different growth rates, the modes with the largest amplification could block the 650-mb mid-tropospheric jet which, because it is cold, progressively sinks as it traverses West Africa from east to west. The distortion created by the blockage forces up parcels of convectively-unstable southwesterlies resulting in precipitation. Precipitation falls or partly evaporates into the underlying jet the latent heat of vaporisation being supplied by the jet. The jet, now cooler, sinks. While sinking, it can reach the surface of the earth and, due to the strong convergence created, a gust front is formed. The front, as a result of ascending southwesterlies, constitutes an area of vigorous convective activity which triggers off a self-regenerative mechanism of condensation, evaporation and sinking. This hypothesis, with others, is able to explain the predominance of highlands as source regions of squall lines, the close association between the propagation speed of squall lines and that of the mid-tropospheric jet, the observed overturning of the atmosphere after the passage of squalls and the possible effects of insolation and African easterly waves on the initiation process.With 2 Figures