Inclusion of Mesoscale Updrafts and Downdrafts in Computations of Vertical Fluxes by Ensembles of Tropical Clouds

Abstract

Ensembles of convective clouds, especially in the tropics, often have widespread precipitating anvils associated with their deeper convective clouds. Mesoscale downdrafts occur below the middle-level bases of the anvils, and mesoscale updrafts may occur within the anvils themselves. Often, the special characteristics of these mesoscale anvil vertical motions are not taken into account in diagnosing or parameterizing the large-scale effects of cloud ensembles. This paper describes calculations of the differences in diagnosed ensemble mass and heat fluxes obtained when mesoscale motions are accounted for in comparison to when they are not. One dimensional models are used to represent both the convective-scale updrafts and downdrafts and the mesoscale anvil updrafts and downdrafts occurring within the cloud ensemble. All ensembles are constrained to account for the observed convective and anvil precipitation in Phase III of the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE). The inclusion of reasonable amounts of either mesoscale updraft or mesoscale downdraft motion or some combination of the two leads to the diagnosis of 15?20% less ensemble mass transport at low levels and 20?30% more mass flux aloft. Diagnosed heat transports in the mid to low troposphere are increased substantially by the inclusion of mesoscale downdrafts and decreased by the inclusion of mesoscale updrafts. These opposing effects on the beat flux cancel if moderate amounts of both mesoscale updraft and downdraft motions are included, and the resulting heat flux in this case differs little from one diagnosed without accounting for the mesoscale motions.