IMPACTS OF MODEL IMPROVEMENTS ON GENERAL-CIRCULATION MODEL SENSITIVITY TO SEA-SURFACE TEMPERATURE FORCING

The sensitivity of the 4 degrees x 5 degrees resolution general circul ation model of the Goddard Institute for Space Studies to some basic c hanges in model formulation was studied by comparing parallel simulati ons forced by global sea-surface temperature of June August 1987 and 1 988. The new modelling schemes were substituted far the control model' s parameterizations of moist convection, planetary boundary layer, gro und hydrology, and cloud optical thickness in a series of sensitivity experiments. In addition, linear and quadratic upstream schemes for ad vecting tracers were tried in place of second-order differencing. Elim ination of the vertical mixing of horizontal momentum by moist convect ion was also tested. Impacts of the new modelling schemes on simulated circulation, temperature, and precipitation rates were inferred from pairs of simulations made by model versions that differed with respect to a single change. No discernible positive impacts were found for th e new ground hydrology scheme or for changes in the determination of c loud optical thickness. Profiles of zonal wind speeds and lower tropos pheric circulation patterns, both in the tropics, were more realistic when the vertical mixing of horizontal momentum was included. Major im provements in modelling the interannual variability of the planetary c irculation, mid-tropospheric temperature, and precipitation can be att ributed to the salutory effects of the new moist convection, new plane tary boundary layer, and the quadratic upstream scheme for advecting t racers.