A Diagnostic Study of Interaction of Cumulus Updrafts and Downdraf is with Large-Scale Motions in GATE

Abstract

A diagnostic method including cumulus updrafts and downdrafts is applied to GATE A/B-scale upper-air observations for Phase III to study the interaction of the cumulus convection with large-scale motions. Average results for Phase III show that the mass fluxes for deep and shallow clouds are larger than those for clouds with tops in the middle troposphere and downward mass fluxes equivalent to 40%-50% of updrafts convective mass fluxes are accomplished by cumulus downdrafts.
Correlation coefficients between deep, middle and shallow cloud mass fluxes and the large-scale vertical velocity are computed to examine the response of cumulus clouds to the large-scale forcing. The results show that deep clouds are highly correlated with the largescale vertical velocity at all levels, middle clouds have little correlations and shallow clouds are correlated only with the vertical velocity near cloud base. Maximum activity of deep clouds occurs nearly at the same time as maximum large-scale upward motion at higher levels but about 6 h after maximum low-level convergence. Strongest downdraft mass fluxes occur corresponding to maximum deep cloud activity. Shallow cloud mass fluxes have negative correlations with the low-level convergence indicating that shallow cloud activity is weak when the low-level convergence is large.
A compositing method is used to clarify relationships between the cumulus convection and the African wave disturbance observed during GATE. Maximum deep cloud mass flux is found in the region near the wave trough where the large-scale vertical velocity has a peak. This is also the region where largest downdraft cloud mass fluxes exist. Shallow cloud activity is weak near the wave trough but strong near the wave ridge. Weak subsidence with amplitude of a few mb h-1 is present nearly everywhere in the environment of the clouds in the wave disturbance, but slightly larger subsidence occurs in the regions where shallow cloud mass fluxes dominate.