EFFECTS OF SATURATED AND DRY LAND SURFACES ON THE TROPICAL CIRCULATION AND PRECIPITATION IN A GENERAL-CIRCULATION MODEL

A comprehensive rhomboidal-15 general circulation model with idealized boundary conditions is used to investigate the effects of interactions between the tropical circulation and continental climate on the precipitation distribution. Sea surface temperatures are fixed and zonally uniform and, along with the solar forcing, establish Perpetual solstice conditions. Clouds are also prescribed and zonally uniform. Experiments with dry and saturated land surfaces are compared with an all-ocean control integration, The winter hemisphere of the saturated continent is cooler than the prescribed ocean surface at the same latitude, and the summer hemisphere is warmer. When the surface is dry, the maximum summer hemisphere warming is four times larger than in the saturated surface case and extends into the winter hemisphere. The ITCZ is shifted farther into the summer hemisphere and enhanced near the coasts over the saturated continent, but it is interrupted in crossing the dry surface. The modification of the precipitation distribution over the saturated land surface can be understood by considering the low-level flow. Over the dry surface, however, low-level horizontal moisture convergence and precipitation patterns are unrelated. The extreme dryness of the surface and the atmosphere below 830 mb eliminates condensation in the lower troposphere despite the increased instability of the tropical atmosphere. Condensation in the middle troposphere also decreases over the western half of the continent.