Effect of non-spherical dust aerosol on its direct radiative forcing

The optical properties of spherical and non-spherical dust aerosols are calculated using the Lorenz-Mie theory and the combination of T-matrix method and an improved geometric optics method. The resulting optical properties are then applied in an interactive system that coupled a general circulation model with an aerosol model to quantitatively analyze the effect of non-spherical dust aerosol on its direct radiative forcing (DRF). Our results show that the maximum difference in dust instantaneous radiative forcing (IRF) between spherical and non-spherical particles is 0.27 W m(-2) at the top of the atmosphere (TOA) and appears over the Sahara Desert due to enhanced absorption of solar radiation by non-spherical dust. The global annual means of shortwave (longwave) IRFs due to spherical and non-spherical dust aerosols at the TOA for all sky are -0.62 (0.074) W m(-2) and -0.61 (0.073) W m(-2), respectively, and the corresponding values for clear sky are -1.16 (0.092) W m(-2) and -1.14 (0.093) W m(-2), which indicates that the non-spherical effect of dust has almost no effect on their global annual mean IRFs. However, non-spherical dust displays more evident influences than above on its atmospheric- and land-temperature adjusted radiative forcing (AF) at the TOA over the Saharan Desert, West Asia, and northern China, with an approximate maximum increase of 3.0 and decrease of 0.5 W m(-2). The global annual means of shortwave (longwave) AFs due to spherical and non-spherical dust aerosols are -0.55 (0.052) W m(-2) and -0.48 (0.049) W m(-2) at the TOA for all sky, respectively, and the corresponding values for clear sky are -1.07 (0.066) W m(-2) and -0.95 (0.062) W m(-2). All AFs of dust become much weaker than their corresponding IRFs. The absolute values of annual mean AF for non-spherical dust are approximately 13% (11.2%) and 6% (6%) less than those of spherical dust for the shortwave and longwave for all sky (clear sky), respectively. The results indicate that the non-spherical effect of dust can reduce their AFs more obviously than do their IRFs. (C) 2012 Elsevier B.V. All rights reserved.