Boundary Layer Clouds in a Global Atmospheric Model: Simple Cloud Cover Parameterizations

Abstract

Subtropical boundary layer clouds have a fundamental role on the radiative budget of the atmosphere and on the modulation of the tropical circulations. The development of realistic parameterizations of these clouds in global atmospheric models is a major challenge. Unfortunately, this has been a difficult problem to solve in an acceptable way. In this paper, new and simple parameterization schemes for subtropical clouds are implemented in the U.S. Navy Operational Global Atmospheric Prediction System's (NOGAPS) forecast model. The parameterizations are partially based on large eddy simulation (LES) results and provide a substantial improvement when compared with observations and with the previous scheme. The global distribution of boundary layer clouds and of surface shortwave radiation is more realistic with this new scheme, particularly over the stratocumulus regions. The transition from stratocumulus to cumulus is well captured, the seasonal and diurnal cycles of stratocumulus are realistic, and the Arctic boundary layer temperature is improved with the new parameterizations.