Impact of Shortwave Radiative Effects of Dust Aerosols on the Summer Season Heat Low over Saudi Arabia

Abstract

A two-stream scattering scheme based on the delta-Eddington approximation is incorporated into the Florida State University Limited Area Model for computing the shortwave radiative fluxes due to dust aerosols over the Saudi Arabian region and to study their impact on synoptic-scale systems and the diurnal cycle over the region. The radiative properties of dust corresponding to different categories of dustiness are determined from the results of field experiments. Satellite imagery and visibility are used to determine the intensity and extent of the dust layer. Two parallel simulations, one including the radiative effects of dust aerosols and the other without them, were made over a 6-day period starting with 1200 UTC 25 June 1979 using First GARP (Global Atmospheric Research Program) Global Experiment IIIb data analyses from ECMWF. A comparison of the two experiments shows that the dust aerosol radiative heating strengthens the heat low over Saudi Arabia. Furthermore, the radiative heating of the heavy dust concentrated at low levels during the dust outbreak episode protects the heat low from its possible destruction due to strong cold winds from the northwest. A significant improvement in the diurnal cycle of temperature at middle levels occurs with the introduction of dust aerosols. The extension of the dust layer over the Arabian Sea also warms the middle levels in the vicinity of the dust layer and cools the layer below it, thus intensifying the inversion above the monsoon flow. The presence of dust aerosols over the Arabian Sea is also found to affect the intensity of the low-level Somali jet and the diurnal cycle of the sea breeze. These model results are found to be consistent with observations.