Impact of Subgrid Variation of Water Vapor on Longwave Radiation in a General Circulation Model

Abstract

Most general circulation models compute radiation fluxes by assuming that water vapor is uniform within individual grid layers, which leads to an underestimation of satellite-observed longwave (LW) cloud radiative forcing (LWCF). To fix this problem, we calculated water vapor content separately for clear and cloudy portions and used them to compute LW radiation. The impacts of this modification were examined by comparing two global simulations with and without the modification (NEW and OLD, respectively). Global-annual mean LWCF from NEW was 1.8 W m−2 higher than that of OLD, thus remedying a long-standing negative bias of LWCF. This improvement is a combined result of more clear-sky and less all-sky upward LW flux at the top of the atmosphere than OLD. Large increases in LWCF and clear-sky LW flux occurred in the tropical deep convection and midlatitude storm track regions where upper- and middle-level clouds are abundant. Although only the LW radiation scheme was modified, global-annual mean shortwave cloud radiative forcing also increased, particularly in the vicinity of the eastern subtropical marine stratocumulus decks through radiative feedback processes. With this improved treatment, it may be possible to tune general circulation models in a more flexible and physical way without introducing compensating errors.