Mechanisms for the Mesoscale Organization of Tropical Cloud Clusters in GATE Phase III. Part I. Shallow Cloud Bands

Abstract

Mesoscale cloud clusters are a frequently observed feature of the tropical atmosphere and are primarily responsible for the observed large-scale vertical mass flux. Given that the forcing for such convection comes from widely separated horizontal scales (boundary-layer motions on the scale of 1 km and weak ascent on a scale of over 1000 km), the persistent organization of cloud clusters into the scale 10?50 km presents an important problem. In this article, Part I, two- and three-dimensional numerical simulations of convection under a capping inversion at 2 km are presented to demonstrate that the mechanism responsible for mesoscale organization of clouds requires neither deep convection nor large-scale forcing. The case study used as a basis for these simulations is one of several instances reported by LeMone and Meitin of mesoscale cloud bands during GATE Phase III. These observations are remarkable in that the cloud bands are shallow, yet possess a rather large horizontal periodicity in the range 15?30 km. The profile in this case is typical of the disturbed conditions of GATE Phase III, where the African easterly jet has undergone rotation due to a passing easterly wave trough. The result is a turning wind profile where the mean shear in the boundary layer is perpendicular to the shear in the lower troposphere, a case that leads to line organization. It is shown that mesoscale organization of the shallow cloud bands can be attributed to a mechanism where the scale selection is modified by the presence of deep gravity wave modes above the cloud layer. This particular case differs from earlier studies in that both trapped and propagating modes (each class possessing a distinct dominant horizontal scale) are excited in the free troposphere.