Cumulus Transports in a Tropical Wave Composite for Phase III of GATE

Abstract

A diagnostic study has been carried out of convective transports in tropical African wave disturbances that occurred during GATE. Data from the Reed et al. (1977) three-dimensional composite of eight wave disturbances during Phase III of GATE have been used in the analysis. In the compositing procedure the wave trough has been defined as the position of maximum relative vorticity at 700 mb, and eight categories in the cast-west direction and seven latitude bands (separated by 4° latitude) have been assigned based on this center. The wave composite combines data from both ocean and land regions. Precipitation observations for the wave are compared to precipitation rates given by the cloud layer model to determine the magnitudes of the cumulus updraft and downdraft mass fluxes. The findings for the innermost latitude bands confirm results from earlier studies that downdraft mass fluxes are significant in regions of deep tropical convection. In the most active categories of the wave the magnitude of the downdraft mass flux at cloud base is found to be as great as two-thirds the magnitude of the updraft mass flux at cloud base. Important meridional differences in the response of deep convection to large-scale forcing (low-level convergence) have been found. At and north of the central latitude band (11.5°N) the maximum deep cloud activity occurs about 10–20 h after the maximum low-level convergence, whereas to the south the response of deep clouds to large-scale forcing is nearly instantaneous. These differences are explained in terms of the meridional variation in the mean moisture stratification. The important role of shallow trade wind cumuli in moistening the lower troposphere in advance of the trough region at the central latitude is discussed.