What is the Mechanism for the Modification of Convective Cloud Distributions by Land Surface–Induced Flows?

Abstract

The aim of this study is to determine the mechanism that modulates the initiation of convection within convergence zones caused by land surface?induced mesoscale flows. An idealized modeling approach linked quantitatively to observations of vegetation breezes over tropical Benin was used. A large-eddy model was used with a prescribed land surface describing heterogeneities between crop and forest over which vegetation breezes have been observed. The total surface fluxes were constant but the Bowen ratio varied with vegetation type. The heterogeneous land surface created temperature differences consistent with observations, which in turn forced mesoscale winds and convection at the convergence zones over the crop boundaries. At these convergence zones optimum conditions for the initiation of convection were found in the afternoon; the equivalent potential temperature was higher in the convergence zones than over anywhere else in the domain, due to reduced entrainment, and the mesoscale convergence produced a persistent increase in vertical wind velocities of up to 0.5 m s?1 over a 5?10-km region. The relative importance of these two mechanisms depended on the synoptic conditions. When convective inhibition was weak, the thermodynamic conditions at the convergence zone were most important, as the triggering of convection was easily accomplished. However, when the thermodynamic profile inhibited convection, the mesoscale updrafts became essential for triggering in order to break through the inhibiting barrier. At the same time, subsidence over the forest produced a warm capping layer over the boundary layer top that suppressed convection over the forest throughout the afternoon.