Earth Radiation Budget and Cloudiness Simulations with a General Circulation Model

Abstract

The UCLA/GLA general circulation model has been endowed with new parameterizations of solar and terrestrial radiation, as well as new parameterized cloud optical properties. A simple representation of the cloud liquid water feedback is included. We have used the model and several observational datasets to analyze the effects of cloudiness on the Earth's radiation budget. Analysis of January and July results obtained with the full model shows that the simulated Earth radiation budget is in reasonable agreement with Nimbus 7 data. The globally averaged planetary albedo and outgoing longwave radiation am both slightly less than observed. A tropical minimum of the outgoing longwave radiation is simulated, but is weaker than observed. Comparisons of the simulated cloudiness with observations from ISCCP and HIRS2/MSU show that the model overpredicts subtropical and midlatitude cloudiness. The simulated cloud radiative forcings at the top of the atmosphere, at the Earth's surface, and across the atmosphere are discussed, and comparisons are made with the limited observations available. The simulated atmospheric cloud radiative forcing (ACRF) is comparable in magnitude to the latent heating. We have compared the clear-sky radiation fields obtained using Methods I and II of Cess and Potter; the results show significant differences between the two methods, primarily due to systematic variations of the cloudiness with time of day. An important feature of the new terrestrial radiation parameterization is its incorporation (for the first time in this GCM) of the effects of the water vapor continuum. To determine the effects of this change on the model results, we performed a numerical experiment in which the effects of the water vapor continuum were neglected. The troposphere warmed dramatically, and shallow convection weakened, and the radiative effects of the clouds were significantly enhanced.