IMPLEMENTATION OF SUBGRID CLOUD VERTICAL STRUCTURE INSIDE A GCM AND ITS EFFECT ON THE RADIATION BUDGET

The GISS (Goddard Institute for Space Studies) GCM (general circulatio n model) predicts stratiform and convective cloud cover and optical th ickness at nine atmospheric levels in horizontal grid boxes of 4 degre es lat x 5 degrees long. Until now, the radiative fluxes were calculat ed once per grid box, assuming clear sky or a complete cloud cover. He re, a refinement of the radiative flux calculation is explored by intr oducing a horizontal subgrid cloud overlap scheme in which cloud block s are formed by adjacent cloud layers using maximum overlap. Different cloud blocks are separated by an atmospheric level of clear sky and a re assumed to overlap randomly inside the grid box. This subgrid cloud structure allows determination of the occurrence probabilities of col umns with different vertical structures inside each horizontal grid bo x. Then, radiative fluxes are calculated for each of these columns. Th e radiative fluxes of each horizontal grid box are obtained as the occ urrence probability weighted sum of the column fluxes. Compared with t he standard GCM version, the horizontal subgrid cloud overlap scheme l eads to significant geographical and seasonal changes of the global me an cloud effects on top-of-atmosphere radiative fluxes that are in sli ghtly better agreement with satellite observations. Two extreme assump tions of horizontal cloud size distributions (very small cloud element s or one horizontally continuous cloud) within the cloud blocks are al so tested, leading to different column occurrence probabilities. Where as the global and zonal mean cloud effects on radiative fluxes stay th e same, regional differences between the two assumptions (i.e., uncert ainties ib GCM cloud cover and radiative fluxes produced by a lack of knowledge of, subgrid cloud size distributions) can be as large as 15% in cloud cover and 25 (50) W m(-2) in LW (SW) net fluxes. The impleme nted cloud overlap scheme is necessary to study radiative effects of d ifferent cloud types separately so that one can better understand the discrepancies in cloud radiative effects between observations and mode l. This study is not possible with the standard version of the GCM bec ause the instantaneous fluxes do not correspond to realistic cloud str uctures. But by comparing in more detail the radiative effects of high opaque, cirrus, midlevel, and low clouds with help of the new scheme in GCM and in simultaneous Earth Radiation Budget Experiment and Inter national Satellite Cloud Climatology Project observations, one finds o ut that high opaque clouds in the GCM have a cloud cover that is too s mall and are too thin over winter hemisphere ocean, whereas cirrus clo uds appear with a cloud cover that is too high. Low clouds in the GCM seem to be too low by about 100 hPa.