Properties of a simulated convective boundary layer over inhomogeneous vegetation

Abstract

Using a high-resolution boundary-layer model, effects of heterogeneity over a mesoscale flat domain are investigated through a series of idealised model simulations. In these simulations two different land surface types (bare soil and vegetation) are arranged in two different patterns. It is found that the effect of heterogeneity remains significant with geostrophic winds of 10 m s−1. However, when the geostrophic wind direction is perpendicular to the alignment of the surface heterogeneities (with alignment here defined as the direction along the edges of a sequence of surface patches), higher winds tend to weaken the coherent circulations caused by the surface heterogeneities. The vertical winds generated by the mesoscale circulations associated with the surface heterogeneities are on the order of 0.5 cm s−1. When the geostrophic wind direction is perpendicular to the alignment of the surface heterogeneities over a three-strip surface type, the mesoscale pattern in horizontal velocity is also pronounced, with significant fluctuations at the interfaces between two different surface patches. The heights at which the heterogeneity effects on potential temperature and winds vanish are well above the convective boundary layer top and reach at least 3.3 Zi under light winds, but depend on the wind speed and directions as well as the orientation of surface heterogeneities. Finally, the implications of the surface heterogeneity for initiation of deep convection have been explored for a surface consisting of two-dimensional strips of alternating soil and vegetated surfaces aligned in the north–south direction. For this surface pattern, the interaction between westerly background winds and the secondary circulation sets up conditions which favour the initiation of deep convection at the eastern, downwind edge of the soil strip. Copyright © 2010 Royal Meteorological Society