Abstract
Due to the extremely low moisture content of the air and the underlying desert surface over North Africa during the summertime, solar radiation works effectively in heating the desert surface. Because the earth's surface receives more radiative energy from the sun than it loses in the form of infrared radiation during the day, a large amount of radiative energy is transferred from the ground to the atmosphere in the form of sensible heat flux. The horizontal differences in the upward flux of sensible heat between North Africa and equatorial Africa build strong surface baroclinic patterns.
A real-data forecast using a multi-level primitive-equation model was designed to study the influences of solar radiation and the associated diurnal variations in surface temperatures on the dynamics and energetics of the African low-level circulations. A surface energy budget which included solar radiation, infrared radiation, sensible heat flux, and latent heat flux was employed in estimating surface temperatures. The effects of cloudiness on radiation processes were also considered, and heat and moisture exchanges due to turbulence inside the boundary layer were parameterized. The upward transfer of heat from the boundary layer to higher levels in the atmosphere was carried out by a dry convective adjustment.
Two numerical experiments, one with solar radiation and another without, were performed and the energetics of each forecast were examined. Results clearly indicated the important role played by solar radiation in the maintenance of low-level motions. With solar heating, which preserved the rather unique thermal structure in North Africa, the model produced a very realistic forecast in the circulation patterns. Without solar heating the model substantially underpredicted the intensity of wind circulation over North Africa. Large diurnal variation in response to solar heating was noted in the dynamic fields and model energetics.
