RESPONSE OF A GENERAL-CIRCULATION MODEL TO A CHANGE IN-CLOUD SOLAR FORCING - MODEL FEEDBACKS AND COMPARISON WITH SATELLITE DATA

The response of a general circulation model to a change in its treatme nt of cloud solar forcing is investigated. Radiation field data from t he forecast model of the Navy Operational Global Atmospheric Predictio n System for five Julys (1979-1983) are presented in an investigation of the effect of a change from grid cell averaged clouds to maximally overlapping clouds in the model's solar radiation scheme. The model re sults are compared with Nimbus 7 Earth Radiation Budget top of the atm osphere (TOA) solar and longwave irradiances and with derived surface solar irradiance data. Although the maximal overlap scheme performs co nsiderably better than the grid cell averaging scheme (reducing maximu m deficiencies in TOA and surface solar irradiance by over 100 W m(-2) ) significant errors remain. The simulated correlation between TOA net solar and longwave irradiance improves at low latitudes in the northe rn hemisphere, with little change at higher latitudes. This improved c orrelation is consistent with the greater consistency between the trea tments of solar and longwave cloud radiative forcing brought to the mo del by the new solar radiation scheme. The change in the radiation tre atment is shown to have the greatest direct effect on solar radiation over convective regions, a consequence of the scarcity of optically th ick clouds produced by the model's cloud parameterization in other reg ions. The model responds with an increase in convective activity over land and an increase in the flux of moisture from sea to land. Planeta ry cooling over the oceans increases because of a decrease in cloud co ver. From mid to high latitudes in the northern hemisphere, there are scattered regions of increased cloud water content associated with inc reased tropospheric temperatures. Over land the model response in term s of TOA downwelling solar irradiance tends to counter the increase in solar irradiance caused by the model change in all latitudinal zones in the northern hemisphere. This response is caused primarily by chang es in the cloud fields, which thus act as a negative feedback followin g the change in cloud solar forcing. The significance of this response is examined with respect to the perturbation in solar irradiance repr esented by the model change. An estimate of this perturbation is obtai ned by taking the difference in solar irradiance diagnosed by the two cloud solar forcing treatments for Simulations employing the grid cell averaging scheme. The response is significantly greater in magnitude in the tropics than at midlatitudes, both in an absolute sense and as a percentage of this perturbation. Because TOA longwave irradiance exh ibits a positive response in the tropics, and a negative response at m idlatitudes, however, the percentage response in net TOA downwelling i rradiance is actually greater in magnitude at midlatitudes. In a numbe r of regions the cloud feedback is very large, showing the importance for cloud field prediction of improvements in the treatment of cloud s olar forcing. Such cloud feedback also explains the small improvement seen here in the prediction of TOA solar irradiance in certain regions . increases in surface sensible heating and longwave Cooling are gener ally considerably less than increases in surface latent heating, thoug h a notable exception occurs in arid central Asia. A large ground temp erature increase in that region is strongly correlated at low levels w ith the atmospheric temperature increase observed at midlatitudes in t he northern hemisphere.