Multiscale local forcing of the Arabian Desert daytime boundary layer, andimplications for the dispersion of surface-released contaminants

Four 6-day simulations of the atmospheric conditions over the Arabian Deser t during the time of the 1991 detonation and release of toxic material at t he Khamisiyah, Iraq, weapons depot were performed using a mesoscale model r un in a data-assimilation mode. These atmospheric simulations are being emp loyed in a forensic analysis of the potential contribution of the toxic mat erial to so-called Gulf War illness. The transport and concentration of suc h surface-released contaminants are related strongly to the planetary bound ary layer (PBL) depth and the horizontal wind speed in the PBL. The product of the PBL depth and the mean wind speed within it is referred to as the v entilation and is used as a metric of the horizontal transport within the P BL. Thus, a corollary study to the larger forensic analysis involves employ ing the model solutions and available data in an analysis of the multiscale spatial variability of the daytime desert PBL depth and ventilation as the y are affected by surface forcing from terrain elevation variations, coasta l circulations, and contrasts in sui face physical properties. The coarsest computational grid spanned the entire northern Arabian Desert and surrounding areas of the Middle East, and represented the large-scale P BL modulation by the orography. The PDL depths were greatest over the high elevations of the western Arabian Peninsula and over the Zagros Mountains i n western Iran and were shallowest over water bodies and the lower elevatio ns in the Tigris-Euphrates Valley. Higher-resolution grids in the nest (the smallest grid increment was 3.3 km) showed that the PBL depth minimum in t he Tigris-Euphrates Valley was likely a consequence of compensating subside nce associated with the thermally forced daytime upward motion over the Zag ros Mountains to the east in Iran, with possible contributions from an elev ated mixed layer. Further local modulation of the daytime desert PDL occurr ed as a result of the inland penetration of the coastal sea-breeze circulat ion on the west side of the Persian Gull, where PBL depths were suppressed as far as 100 km inland. On the finest scales, significant PBL-depth variab ility resulted from surface thermal differences associated with contrasts b etween barren desert and partially vegetated desert. The average 1.500 LT ventilation over the Arabian Desert for the 6-day peri od varied spatially from less than 4000 m(2) s(-1) to over 24 000 m(2) s(-1 ). This range represents over a factor-of-6 variation in the ability of the atmosphere to transport contaminants away from a source region.