Abstract
It is hypothesized that few African waves become tropical storms because they typically have an unfavorable thermal structure that cannot be sufficiently transformed during its propagation across the Atlantic Ocean by the condensational heating under normal moisture conditions. This hypothesis is tested in the context of a moist multilevel quasi-geostrophic model. The longitudinal variation of the background zonal flow over the Atlantic Ocean is incorporated in the model as a progressively changing zonal flow that an easterly wave would encounter as it propagates across the oceanic region. The dependence of the evolution of such a wave field for 10 days upon the various model parameters is evaluated by means of numerical integration.
A detailed comparison of the structural and energetic properties of the wave field in nondeveloping and developing cases supports the hypothesis. Specifically, the initial model easterly wave field has a cold core at the trough region centered at the lower tropospheric level, and another cold core ahead of the trough centered at 550 mb. For a wave to be able to intensify subsequently to great intensity within 10 days, the latter must be quickly transformed into a warm core within a few days. This could happen only under the accumulative influence of a sufficiently strong cumulus heating during the westward propagation of the wave. The specific humidity at the top of the surface moist layer in the model required for rapid intensification is found to be ∼13 g kg−1, which is a large but physically meaningful moisture condition.
