Investigation of Biogeophysical Feedback on the African Climate Using a Two-Dimensional Model

Abstract

A 2-D zonally averaged, time-dependent climate model has been developed to study the biogeophysical feedback for the climate of Africa. A numerical scheme has been specifically designed for the model to ensure the conservation of mass, momentum, energy, and water vapor. A control experiment has been carried out in which the solar zenith angle was varied from 15 June to 30 July. The simulated results are presented using averages over the last 30 days. The simulated temperature, humidity, and winds for July mean conditions compare reasonably, well with zonally averaged, observed values. A vegetation layer has been incorporated in the present 2-D climate model. Using the coupled climate-vegetation model, we performed two tests involving the removal and expansion of the Sahara Desert. Results show that variations in the surface conditions produce a significant feedback to the climate system. The feedback from the land surface to the atmosphere affects not only precipitation and cloud cover, but also temperature, radiation budgets, and wind fields. The simulation responses to the temperature and zonal wind in the case of an expanded desert agree with the climatological data for African dry years. Perturbed simulations have also been performed by changing the albedo only, without allowing the variation in the vegetation layer. In this case, the model is unable to reproduce the observed temperature, humidity, and wind fields over the African continent for both dry and wet years. We show that the variation in latent heat release is significant and is related to changes in the vegetation cover in a number of ways. As the desert is expanded, the decrease in latent heat is much larger than the increase in sensible heat generated by the hot surface. The specific humidity in the atmosphere decreases due to less evaporation from the ground and a reduction in the horizontal convergence of water vapor transport. As a result, precipitation and cloud cover are reduced.