Shortwave cloud forcing of marine stratocumulus clouds

Abstract

Abstract Shortware cloud forcing (SWCF) is one measure of the impact of clouds on radiation and climate. This quantity is determined by insolation, surface albedo, cloud thickness and height, cloud cover and the size distribution of the cloud droplets. We have carried out controlled experiments with a single column model in order to facilitate interpretation of climate model predictions of cloud parameters and radiation budget components. For this purpose we have used the radiation code of NCAR CCM2. The sensitivity of SWCF to the above parameters is investigated for marine stratocumulus clouds. We find that clouds with low liquid water path. (LWP) are most sensitive to a change in LWP whereas clouds with high LWP are most sensitive to a change in cloud cover. Both effective radius and latitude have a modest effect on the sensitivity. To check the validity of the results we have used measurements of SWCF and LWP taken between 20 and 30°N, and 120 and 130°W in July. The comparison of the shortware cloud forcing shows that the model highly overestimates the monthly averaged forcing. We find that the overestimation can largely be explained by the combined effects of a fractal cloud structure, day-to-day variations in cloud cover and LWP, and the model's treatment of a partly cloudy sky.