Enhancement of Convective Precipitation by Mesoscale Variations in Vegetative Covering in Semiarid Regions

Abstract

It is hypothesized that planting bands of vegetation with widths of the order of 50?100 km in semiarid regions could, under favorable large-scale atmospheric conditions, result in increases of convective precipitation. These increases, which could be greater than those associated with the uniform vegetating of large areas, would occur through three major mechanisms. The first would be the modification of the environment to a state more conducive to the formation of moist convection through an increase of low-level moist static energy. This increase would be associated with a decrease in albedo, an increase in net radiation, and an increase in evapotranspiration. The second important mechanism would be the generation of mesoscale (horizontal scale of 20?200 km) circulations associated with the surface inhomogeneities created on this scale by the vegetation. The third mechanism would be the increase of atmospheric water vapor through decreased runoff and increased evaporation. A number of observational and theoretical studies which have a bearing on the above hypothesis are reviewed. Although individual studies may contain large uncertainties, taken together they provide considerable support for the hypothesis. In these studies, convective rainfall appears to be associated with increases in vegetation and with variations in surface characteristics in many parts of the world on scales ranging from 10 km to large fractions of continents. A review of recent agricultural research indicates that a variety of plants that thrive in semiarid regions (some under irrigation with saline water) could be suitable for cultivation. Many of these have potential economic value, which could defray or even exceed the cost of the cultivation. Finally, a preliminary estimate of the preferred horizontal scale of the vegetation bands is made using a linear model. For bands of width less than about 20 km, horizontal mixing limits the vertical penetration of the surface heating perturbation to heights too small to be effective in generating moist convection. For larger scales (widths ? 100 km), however, it appears that vertical circulations with order of magnitude 10 cm s?1 that extend to heights of 1 km or more are possible. When combined with increases in low-level moist static energy, circulations of this magnitude and scale appear to be capable of initiating and enhancing moist convection under appropriate atmospheric conditions. Further studies with more realistic models are necessary to obtain a more definitive evaluation of the hypothesis.