Modeling Study of the Global Distribution of Radiative Forcing by Dust Aerosol

To quantitatively understand the dust aerosol effects on climate change, we calculated the global distribution of direct radiative forcing due to dust aerosol under clear and cloudy skies in both winter and summer, by using an improved radiative transfer model and the global distribution of dust mass concentration given by GADS (Global Aerosol Data Set). The results show that the global means of the solar forcing due to dust aerosol at the tropopause for winter and summer are -0.48 and -0.50 W m-2, respectively ; the corresponding values for the longwave forcing due to dust are 0.11 and 0.09 W m-2, respectively. At the surface, the global means of the solar forcing due to dust are －1.36 W m－2 for winter and -1.56 W m-2 for summer, whereas the corresponding values for the longwave forcing are 0.27 and 0.23 W m-2, respectively.This work points out that the absolute values of the solar forcing due to dust aerosol at both the tropopause and surface increase linearly with the cosine of solar zenith angle and surface albedo. The solar zenith angle influences both the strength and distribution of the solar forcing greatly. Clouds exert great effects on the direct radiative forcing of dust, depending on many factors including cloud cover, cloud height, cloud water path, surface albedo, solar zenith angle, etc. The effects of low clouds and middle clouds are larger than those of high clouds. The existence of clouds reduces the longwave radiative forcing at the tropopause, where the influences of low clouds are the most obvious. Therefore, the impacts of clouds should not be ignored when estimating the direct radiative forcing due to dust aerosol.