Evaluation of model clouds and radiation at 100 km scale using GOES data

Abstract

Model simulated cloud properties such as cloud fraction, cloud top height, and infrared (IR) radiance are compared to those from GOES-7 satellite observations over a monthly period from an ensemble of thirty-one forecasts launched each day of October 1993. The aim is to evaluate both the above mentioned parameters and the cloud optical properties (emissivity, optical depth) in a global model that carries cloud water (ice and water phases combined) as a prognostic variable. The results show that the model simulates well the cloud fraction, cloud top height, and IR radiance in the middle and high latitude regions, especially in the Northern Hemisphere. Most obvious deficiencies are found in the tropics and subtropics over the ocean: underestimation of cloud fraction and cloud top height in the inter-tropical convergence zone, and overestimation of low clouds in the subtropics. The causes of these deficiencies are discussed. The cloud emissivity parameterization is tested and its sensitivity to effective cloud fraction and outgoing radiance is assessed from comparisons with cases that assume an emissivity of unity. The resulting global infrared budget is realistic with a net loss at the top of the atmosphere of 243 W/m2 and 55 W/m2 at the surface. Although error compensation between cloud height and fraction occurs in some areas to yield good IR radiances, the outgoing radiation is excellent in regions where clouds are best, thus validating the chosen emissivity parameterization. After some adjustment of the parameterized visible cloud optical depth, the global shortwave budget is also reasonable compared to available climatology with a net surface flux of 157 W/m2 and a planetary albedo of 30%. The sensitivity to various parameters affecting the radiation budget is examined.