Abstract
Results f rum a series of numerical simulations are presented to show the effects of changes in albedo on rainfall in six areas, two each in Africa, Asia and North America. Each pair consists of a semi–arid area lying at the boundary between a major desert and an adjacent monsoonal region, and an area of the same size located within the monsoonal region itself. The sensitivity of the rainfall to the ground hydrology was determined by performing the albedo simulations with two different evapotranspiration parameterizations, one giving too high evaporation over land and the other giving negligible evaporation over land. In the high evaporation case, an albedo change from 0.14 to 0.35 caused large decreases of rainfall in all three of the semi–arid test areas and in two of the three monsoonal test areas. In the negligible evaporation case the simulations were performed only for the three semi–crid areas; in this case the albedo increase produced a significant decrease in rainfall in only one of the areas, the Sahel. Intercomparison of the high and negligible evaporation cases shows that changes in evaporation rate are as important as changes in albedo. Thus, in all but one of the six areas local evaporation was a major factor in influencing rainfall.
When appreciable evaporation occurs, the mechanism by which an increase of albedo reduces the rainfall is as follows. Initially, the increase of albedo acts to reduce the absorption of solar radiation by the ground and therefore the transfer of sensible, plus latent heat into the atmosphere. The resulting reduction in convective cloud tends to compensate for the increase of albedo by allowing more solar radiation to reach the ground, but it reduces the downward flux of longwave radiation even more, so that the net absorption of radiation by the ground, solar plus longwave, is decreased. Thus, with or without evaporation, the increase of albedo causes a net decrease of radiative flux into the ground and therefore a net decrease of convective cloud and precipitation.
