An Analysis of Convective System on a 100-km Scale during GATE

Abstract

Usig data on a 100 km-scale from Phase III of GATE, the kinematic and thermodynamic, properties of the mesoscale environment in which convective clouds of varying intensities exist are investigated. Classifications into disturbed and suppressed modes is based on both radar echo and rainfall data. Relative vorticity and horizontal divergence are found to be scale-independent under suppressed conditions with about equal magnitudes over the GATE A/B-, B- and C-scales. Under these conditions, relative vorticity was generally anticyclonic on all scales. However, during disturbed periods, vorticity was cyclonic with the smallest (100 km) scale having the largest values in the 450?200 mb layer, with an upscale decrease. C-scale vorticity budgets are not determined, but C-scale vorticity values are shown to be consistent with estimates by others of the apparent vorticity source on the A/B-scale. A simplified theory using the circulation theorem is presented. Budgets of heat and moisture show that the vertical advection and condensational heating terms dominate, especially for deep convection. During suppressed periods, heat and moisture transfer by cloud-scale eddies was mainly upwards; both fluxes were upwards and very large under disturbed conditions. In order to calculate the moisture budget, Betts' transient cloud model was used for suppressed conditions. This has been extended here to deep precipitating convection; the observed rainfall rate is used as the primary parameter determining Betts' convective mass flux.