MECHANISMS FOR THE MESOSCALE ORGANIZATION OF TROPICAL CLOUD CLUSTERS IN GATE PHASE-III .1. SHALLOW CLOUD BANDS

Mesoscale cloud clusters are a frequently observed feature of the trop ical atmosphere and are primarily responsible for the observed large-s cale vertical mass flux. Given that the forcing for such convection co mes 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 pe rsistent organization of cloud clusters into the scale 10-50 km presen ts an important problem. In this article, Part I, two- and three-dimen sional numerical simulations of convection under a capping inversion a t 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 simulat ions is one of several instances reported by LeMone and Meitin of meso scale cloud bands during GATE Phase III. These observations are remark able in that the cloud bands are shallow, yet possess a rather large h orizontal periodicity in the range 15-30 km. The profile in this case is typical of the disturbed conditions of GATE Phase III, where the Af rican easterly jet has undergone rotation due to a passing easterly wa ve trough. The result is a turning wind profile where the mean shear i n the boundary layer is perpendicular to the shear in the lower tropos phere, a case that leads to line organization. It is shown that mesosc ale organization of the shallow cloud bands can be attributed to a mec hanism where the scale selection is modified by the presence of deep g ravity 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.