A Satellite View of the Radiative Impact of Clouds on Surface Downward Fluxes in the Tibetan Plateau

Abstract

sing 13 yr of satellite observations for the Tibetan Plateau, the sensitivities (or partial derivatives) of daytime surface downward shortwave and longwave fluxes with respect to changes in cloud cover and cloud optical thickness are investigated and quantified. Coincident cloud and surface flux retrievals from the NASA Moderate Resolution Imaging Spectroradiometer and the Clouds and the Earth?s Radiant Energy System, respectively, as well as ground-based observations at 11 stations across the plateau are used to examine the spatial and seasonal variability of this sensitivity over the entire plateau. The downward shortwave flux is found to be modulated primarily by changes in cloud cover, but changes in optical thickness also have an impact, as revealed by a multiple regression fit. The coefficient of determination of the regression increases by more than 15% when optical thickness is added. There is significant seasonal and regional variability in the cloud radiative impact. On average, at all stations, the sensitivity of surface shortwave flux to changes in cloud cover is about ?0.5 ± 0.1 W m?2 %?1 in winter according to both ground-based and satellite observations but in summer reaches ?1.5 ± 0.3 and ?1.8 ± 0.2 W m?2 %?1 according to ground-based and satellite observations, respectively. Cloud cover itself has little impact on the sensitivity when clouds are optically thin, but above an optical thickness of 12, sensitivities increase with both cloud cover and cloud optical thickness. The daytime longwave flux response to changes in cloud properties is also examined. The radiative impact of a decrease in cloud cover on the surface net flux can be offset or even canceled if cloud opacity increases by 5%?10%.