A study of the aerosol direct forcing using ESSP/CALIPSO observation and GCM simulation

Shortwave direct aerosol radiative forcing (SWDARF) at the top-of the atmosphere (TOA) under clear-sky, cloudy-sky, and all-sky conditions are calculated using data of space-borne CALIOP lidar and MODIS sensor and simulation result of a global aerosol model, SPRINTARS. We investigate four scenarios for evaluating the SWDARF using both an observational and model approach: clear-sky, the case that aerosols exist above clouds, the case that aerosols exist below high-level clouds, and the case that aerosols are not detected by CALIOP in cloudy-sky condition. The cloudy-sky SWDARF is estimated by the latter three scenarios. The all-sky SWDARF is the combination of clearsky and cloudy-sky SWDARF weighted by the cloud occurrence. The results show that the TOA forcing over desert regions caused by dust with single scattering albedo (SSA) of 0.92 is positive regardless of cloud existence, due to high solar surface albedo. Off southern Africa, smoke aerosols with SSA of 0.84 above low-level clouds are observed and simulated and the annual mean cloudy-sky SWDARF is estimated at more than +2 Wm^-2, as consistent with past studies. Annual zonal averages of SWDARF from 60°S to 60°N under clear-sky, cloudy-sky, and all-sky are -3.72, -1.13, and -2.07 Wm^-2 from CALIOP, and -2.78, +1.07, and -0.58 Wm^-2 from SPRINTARS. The difference of aerosol loading and occurrence probability in the case that aerosols exist above clouds changes the sign of all-sky and cloudy-sky SWDARF.