Analysis and numerical simulations of the Saharan Air Layer and its effect on easterly wave disturbances

• Published in: Journal of the atmospheric sciences 1988-11, Vol.45 (21), p.3102-3136
• Main Authors: MOHAN KARYAMPUDI, V, CARLSON, T. N
• Format: Article
• Language: English
• Subjects: Earth, ocean, space; Exact sciences and technology; External geophysics; General circulation. Atmospheric waves; Meteorology
• ISSN: 0022-4928; 1520-0469
• DOI: 10.1175/1520-0469(1988)045<3102:aansot>2.0.co;2

A conceptual model of the Saharan air layer (SAL) and easterly wave disturbances is presented in light of diagnostic analyses of dust outbreaks. Numerical simulations of the SAL were conducted to 5 days for two case studies by 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 emphasizes the structure of the SAL and easterly wave disturbances, 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 Aug. 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 attributable 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 much weaker than for the case where a deep SAL is present.