Analysis and Numerical Simulations of the Saharan Air Layer and Its Effect on Easterly Wave Disturbances

Abstract

A Conceptual model of the Saharan Air Layer (SAL) and easterly wave disturbance is presented in light of diagnostic analyses of dust outbreaks. Numerical simulations of the SAL were carried out to 5 days for two case studies using the Penn State/NCAR limited-area tropical model. The region of simulations encompasses North Africa and the eastern tropical Atlantic Ocean. One set of simulations used a horizontal resolution of 220 km. Analysis of the simulations emphasize the structure of the SAL and easterly wave disturbance and evaluation is made with reference to available observations and a conceptual model. Because both cases are similar, emphasis of the sensitivity tests is placed on the August 1974 case only. These tests include the effect of enhancing the SAL in the initial conditions, the role of surface sensible heating, the role of latent heating in the atmosphere, and the effect of heating due to radiative warming of the aerosol. A fine-mesh simulation of 110 km was also made to resolve the mesoscale features of the SAL. Topics treated in the discussion include 1) the interaction of the SAL with attendant easterly wave disturbances, 2) the frontal structure of the SAL along the leading and southern boundary of the SAL, 3) forcing of vertical motions and the transverse/vertical circulations in the SAL front, 4) the nature of the anticyclonic curvature of the SAL plume along the coast of Africa and 5) the role of aerosol radiative heating in preserving the characteristics of the SAL as it moves toward the west. A significant conclusion is that the SAL contributes to forcing of vertical motions and cumulus convection and is therefore important (if not necessary) in the initial development of some easterly wave disturbances. Without surface heating over the Sahara or a proper initialization of the desert mixing layer, atmospheric forcing tends to be very much weaker than for the cam where a deep SAL is present.