Simulations of the Saharan atmosphere—dependence on moisture and albedo

The sensitivity of an atmospheric general circulation model to changes in atmospheric water content, soil moisture content, radiation scheme and albedo have been investigated using the Meteorological Office global 11-layer model. Previous integrations had produced excessive rainfall in the Saharan region. This paper reports experiments which show the importance for a good Saharan simulation of the initial atmosphere and soil moisture specifications, of the radiation scheme and of the surface albedo specification. It is shown that a dry Sahara with realistic circulation patterns is simulated only if the surface and atmosphere are initially realistically dry and the model's longwave radiation is dependent on atmospheric water vapour content. With a moist atmosphere, introduced either initially or by rapid evaporation from a wet soil, or by using a radiation scheme without water vapour dependence, there is increased net downward radiation which enhances ascent and convective activity and leads eventually to excessive rainfall in the desert region with a reversal of the usual circulation pattern to give 700 mb westerly winds at 5°–15°N associated with a cyclonic circulation over the Saharan region which draws in moist air from the south. As expected from Charney's hypothesis on the maintenance of deserts, the use of more realistic albedos tends to transfer rainfall from regions of higher albedo (e.g. deserts) to regions of lower albedo (e.g. forests). An extension of Charney's hypothesis to include moisture is proposed in which any increase in diabatic heating, whether due to reduced albedo or increased moisture availability, must be compensated by increased cold advection; in the tropics this typically occurs through ascent of potentially colder air leading to rainfall if the air is moist enough or the ascending layer sufficiently deep. Some implications of the possibility of two distinct summer circulation patterns for the theory of past North African climate are discussed.