Aerosol's optical and physical characteristics and direct radiative forcing during a shamal dust storm, a case study

Dust aerosols are analyzed for their optical and physical properties during an episode of a dust storm that blew over Kuwait on 26 March 2003 when the military Operation Iraqi Freedom was in full swing. The intensity of the dust storm was such that it left a thick suspension of dust throughout the following day, 27 March. The synoptic sequence leading to the dust storm and the associated wind fields are discussed. Ground-based measurements of aerosol optical thickness reached 3.617 and 4.17 on 26 and 27 March respectively while the Angstrom coefficient, alpha(870/440), dropped to -0.0234 and -0.0318. Particulate matter concentration of 10 mu m diameter or less, PM10, peaked at 4800 mu g m(-3) during dust storm hours of 26 March. Moderate Resolution Imaging Spectroradiometer (MODIS) retrieved aerosol optical depth (AOD) by Deep Blue algorithm and Total Ozone Mapping Spectrometer (TOMS) aerosol index (AI) exhibited high values. Latitude-longitude maps of AOD and AI were used to deduce source regions of dust transport over Kuwait. The vertical profile of the dust layer was simulated using the SKIRON atmospheric model. Instantaneous net direct radiative forcing is calculated at top of atmosphere (TOA) and surface level. The thick dust layer of 26 March resulted in cooling the TOA by -60 Wm(-2) and surface level by -175 Wm(-2) for a surface albedo of 0.35. Slightly higher values were obtained for 27 March due to the increase in aerosol optical thickness. Radiative heating/cooling rates in the shortwave and longwave bands were also examined. Shortwave heating rate reached a maximum value of 2K day(-1) between 3 and 5 km, dropped to 1.5 K day(-1) at 6 km and diminished at 8 km. Longwave radiation initially heated the lower atmosphere by a maximum value of 0.2K day(-1) at surface level, declined sharply at increasing altitude and diminished at 4 km. Above 4 km long-wave radiation started to cool the atmosphere slightly reaching a maximum rate of -0.1 K day(-1) at 6 km.