THERMODYNAMIC BUDGET OF THE 5 DAY WAVE OVER THE SAHARAN DESERT DURING SUMMER

A spectral analysis of the domain averaged height field at 1 000 hPa surface over the Sahara using ECMWF data reveals a major oscillation of about five days. A composite analysis technique has then been developed which permits to emphasize the major characteristics of the evolution of the thermal low over the Sahara during a five day period in the summer season. This analysis shows that the composite structure of the thermal field reveals a well mixed layer with an almost constant potential temperature and specific humidity from the surface up to about 650 hPa. The computation of the vertical velocity and the horizontal divergence reveals a presence of a two cell vertical circulation over the thermal low region. Although the heat low over the Saharan desert appears as a shallow low pressure area confined into the mixed layer, the entire troposphere seems to be dynamically active. In the surface layer, the response of the height field to the temperature field has a lag time of about one day. Adjustment of the theoretical net radiative heating rate have been applied assuming dust to have an effect on both shortwave and longwave irradiances. The results compare reasonably well with measurements obtained from the literature, and provide an improvement for the surface heat fluxes as compared to those deduced from conventional radiative transfer model. The thermodynamic budget over the desert region reveals that the contribution of the advective terms is to stabilize the mixed layer by removing the excess of heat from the surface layer. Furthermore, the destabilization of the mixed layer seems to result from a thermal unbalance between the net radiative heating and the convergence of the eddy sensible heat flux during the formative stage of the thermal low. Downward motion developing above the mixed layer during the occurrence of the heat low seems to be responsible for the cooling of the surface layer and a decrease of the surface temperature, which marks the decaying phase of the thermal wave. A qualitative inference of the top of the dust layer from the large scale variables has been attempted, and a simple scheme for the parameterization of the net solar radiation as a function of the large scale apparent heating is proposed. The results appear to be a reasonable approach for the definition of the net radiative heating. The moisture budget reveals that water vapor is supplied to the domain via horizontal advection from the region of the West African monsoon. Furthermore, its vertical distribution appears to be controlled by the complex vertical eddy motions in the dry thermals which develop over the warm Saharan desert during the afternoon hours.