Observed Southern Ocean Cloud Properties and Shortwave Reflection: Calculations of SW Flux From Observed Cloud Properties

Cloud properties derived from several remote sensing instruments are combined to investigate their impact on the top of atmosphere upwards shortwave radiative flux (SWUP) over the Southern Ocean, where climate models indicate a strong negative cloud shortwave feedback. The SWUP and reflectivity calculated by this method is verified against CERES data from 2007-2008 and shows values of, respectively, R>0.95 and R>0.67. The relative significance of seasonally varying cloud properties to the upwelling SW is tested. Low cloud fraction reaches a maximum and droplet effective radius reaches a minimum in summer, which combine to increase SWUP during this season. Effective radius decreases account for 4-8Wm-2 of enhanced reflected shortwave during summer and summertime increases in low cloud fraction account for 9-11Wm2. However, summertime SWUP is decreased by 1-4Wm-2 owing to decreases in the liquid water path of low clouds. The increase in SWUP due to suppression of the liquid to ice transition projected from a 1K warming throughout the low and midlle cloud in the Southern Ocean is calculated. Depending upon how the microphysics are assumed to change with warming, the estimate of the feedback effect of such transitions varies by a factor of 2-5 between two simple assumptions, highlighting the importance of a more comprehensive understanding of aerosol and cloud processes in this region.