Response of Cumulus Updraft and Downdraft to GATE A/B-Scale Motion Systems

Abstract

Large-scale mass, heat and moisture budgets have been computed over the GATE A/B-scale area during two priority periods in Phase 3. The computed budget results are well-correlated with the activities of the cloud clusters which develop and decay during the analyzed period. Strong upward motion exists with two maxima at 700 and 350 mb during the active period of the cloud cluster, but sinking motion appears in the middle troposphere during the dissipating period. Large amounts of apparent heat sources and moisture sinks occur in the whole troposphere and large amounts of heat energy are transported upward by cumulus clouds during the active period; but an apparent heat sink, moisture source and slight downward heat flux are observed in the middle troposphere during the dissipating period. A diagnostic method for determination of cloud properties has been improved by including downdraft effects and tested on the mean Marshall Islands data. In this method both spectral properties of updrafts and bulk properties of downdrafts are determined without prescribing downdraft parameters. The downdraft acts as a heat sink and a moisture source and neglect of the downdraft overestimates the mass flux of shallow clouds. The downdraft originates from air of the environment somewhere in the middle troposphere and extends below the cloud base keeping temperatures cooler than the surrounding air. This diagnostic method is applied to three different budget results in GATE which are classified according to the activities of the cloud clusters. Bimodal cloud mass flux distributions for updrafts with dominance of very shallow and very deep clouds are obtained during the period when the GATE area is not affected by cloud clusters. However, unimodal distributions are found with dominance of very deep clouds during the active stage of cloud clusters and with dominance of shallow clouds during the dissipating stage. Greater cloud downdraft mass flux is observed in GATE than in the Marshall Islands area and, specifically, net downward cloud mass flux results from the dominance of downdrafts during the dissipating period. The relation between vertical distributions of the downdraft mass flux and vertical shear of ambient winds is also discussed.