Implementation of subgrid cloud vertical structure inside a GCM and its effect on the radiation budget

The GISS (Goddard Institute for Space Studies) GCM predicts stratiform and convective cloud cover and optical thickness at nine atmospheric levels in horizontal grid boxes of 4°C lat × 5° long. Until now, the radiative fluxes were calculated once per grid box, assuming clear sky or a complete cloud cover. Here, a refinement of the radiative flux calculation is explored by introducing a horizontal subgrid cloud overlap scheme in which cloud blocks are formed by adjacent cloud layers using maximum overlap. The horizontal subgrid cloud overlap scheme leads to significant geographical and seasonal changes of the global mean cloud effects on top-of-atmosphere radiative fluxes that are in slightly better agreement with satellite observations. Two extreme assumptions of horizontal cloud size distributions within the cloud blocks are also tested, leading to different column occurrence probaiblitites. Whereas the global and zonal mean cloud effects of radiative fluxes stay the same, regional differences between the two assumptions can be as large as 15% in cloud cover and 25 (50) w M^-2 in LW (SW) net fluxes. The implemented cloud overlap scheme is necesaary to study radiative effects of different cloud types separately so that one can better understand the discrepancies in cloud radiative effects between observations and model.