Determination of Mean Cumulus Cloud vorticity from GATE A/B-Scale Potential Vorticity Budget

Abstract

The effects of cumulus clouds on the large-scale potential vorticity field are investigated using GATE data. Clouds are found to modify the mean potential vorticity field not only through vertical mixing but also through the generation of potential vorticity by the release of latent beat. Overall, the dynamic effect and the thermodynamic effect of clouds are found to contribute about equally to the large-scale potential vorticity budget. A diagnostic method is also developed to determine mean cloud vertical vorticity profiles from observed large-scale potential vorticity sources. The method is applied to GATE AIB-scale potential vorticity budgets. The results show that 1) the mean cloud vorticity is of the same order of magnitude as the large-scale mean vorticity, despite the smallness of the horizontal scales of cumulus clouds, and 2) the mean cloud vorticity is smaller than the large-scale mean vorticity in the mean detrainment layer of the cloud population, and larger than the large-scale mean vorticity in the mean cloud entrainment layer. These properties are in agreement with the theoretical analysis presented in Choet al. (1979a).