What is the mechanism for the organization of cloud cover by land-surface induced flows?

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. The source of convective initiation is usually attributed to convergence, but not enough consideration has been given to what exact mechanism leads to enhanced convection within these convergence zones, nor whether the vertical velocities caused by mesoscale convergence zones are significant.

There are three general mechanisms that could explain how mesoscale convergence promotes convective initiation: A purely dynamical response due to increased vertical velocities from the mesoscale convergence; a thermodynamic response due to moisture convergence from a cool, moist region (lower Bowen ratio) to a warmer drier region (higher Bowen ratio); or mesoscale convergence may be of secondary importance, and the observed organization could simply be attributed to higher turbulence over warmer regions.

An idealised modelling 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.

The model data shows that the optimum conditions for the initiation of convection are found on the warm side of the land-surface boundaries. The equivalent potential temperature, closely linked to CAPE, is initially higher over the forest, due to a lower PBL height reducing entrainment. In the afternoon, however, the land-surface induced flow advects and vertically mixes high equivalent potential temperature air over the crop boundary. This reduces the vertical equivalent potential temperature flux, leading to a domain-wide peak at the convergence zones, thus promoting convective development. Furthermore, physical convergence forces persistent vertical velocities of up to 0.5 ms^-1. Both these mechanisms combine to enhance the initiation of convection at the crop boundaries. The relative importance of these two mechanisms depends on the synoptic conditions. When convective inhibition is weak, the thermodynamic conditions at the convergence zone are most important, as the triggering of convection is easily accomplished. However, when the thermodynamic profile inhibits convection, the mesoscale updraughts become essential for triggering, in order to break through the inhibiting barrier. At the same time, subsidence over the forest produces a warm capping layer over the boundary layer top that suppresses convection over the forest throughout the afternoon.