A consideration on the relative strengths of aerosol direct and indirect forcings on the earth's climate system

This study compares the radiative forcings caused by aerosol direct and indirect forcings to discuss which one is more effective to change the cloud and precipitation fields. We obtained the direct forcing in clear and cloudy conditions by combined analysis of CALIPSO and MODIS data and found the TOA aerosol direct forcing is much smaller than that of model-evaluated value by GCMs, e.g., the CCSR/NIES/FRCGC/MIROC-GCM coupled with the SPRINTARS aerosol model. This suggests that the TOA aerosol direct forcing may be much smaller than the estimate by IPCC-AR4, because the aerosol direct forcing by the MIROC+SPRINTARS model is in the group of smallest forcing among the world's models. On the other hand, the surface aerosol direct forcing can effectively change the cloud and precipitation field through 1st and 2nd indirect forcing as well as through changing the sea surface temperature as found also MIROC simulation with a coupled ocean model. On the other hand we found from model simulation of the high resolution global non-hydrostatic model, NICAM, shows the 2nd indirect effect may not be significant in the global mean as also found by the old AVHRR analysis by Nakajima et al. (2001). We will discuss this NICAM result is relevant or not by comparing the model result with the cloud optical thickness (COT) and effective droplet radius (CDR) from MODIS along with cloud radar echo data from CLOUDSAT.