A NUMERICAL STUDY OF THE EVOLUTION, STRUCTURE AND ENERGETICS OF THE SAHARAN AIR LAYER (EASTERLY WAVES, EASTERN ATLANTIC, WEST AFRICA, DESERT, TROPICAL METEOROLOGY)

The evolution, structure and energetics of the Saharan Air Layer (SAL) and its influence on easterly wave disturbances are investigated in this study using a current tropical version of the Penn State limited-area model. Several numerical simulation experiments of five-day duration were carried out using the Geophysical Fluid Dynamic Laboratory's GATE data set for the period of 23-28 August 1974. A horizontal resolution of 220 km (coarse-mesh) and a higher resolution of 110 km (Fine-mesh) were used for the simulation experiments.; The simulated variables from the full physics coarse-mesh experiment compared favorably through the entire five-day period with the observed variables at Sal Island ((TURN)17(DEGREES)N, 23(DEGREES)W). Moreover, the model realistically simulated many of the features of the SAL in the conceptual model and of the observed easterly wave vertical structure.; Results of the high resolution sensitivity experiment show stronger mesoscale features related to the SAL and easterly waves than those of the corresponding coarse-mesh simulation. In particular, the anticyclonic rotation of the SAL is simulated well in the fine-mesh experiment. Dynamic instability of the middle-level jet appears to explain the anticyclonic looping of the trajectories within the SAL rather than the conservation of potential vorticity of the SAL plume.; The results of the coarse-mesh sensitivity experiments (performed by separately removing the data enhancement of the initial SAL structure, the surface heating over the Sahara and the latent heating from condensation) reveal that the SAL is important in the growth of the wave disturbances, including those that later become tropical cyclones over the eastern Atlantic. The reason appears to be that the presence of the SAL enhances the dual baroclinic-CISK mechanism. Further, the energetics of all the coarse-mesh simulations clearly indicate that the baroclinic energy conversion of eddy available potential energy to eddy kinetic energy is the most dominant form of energy transformation for the growth of wave disturbances rather than the barotropic energy conversion.