Impact of Interactive Radiative Transfer on the Macroscopic Behavior of Cumulus Ensembles. Part I: Radiation Parameterization and Sensitivity Tests

Abstract Implementation of a broadband radiation parameterization in the UCLA cumulus ensemble model (CEM) is discussed in this study, with emphasis on the specific problems associated with adequate calculation of radiative transfer processes in the CEM. The radiation parameterization is based on the Harshvardhan et al. broadband radiative transfer model with cloud optical properties as formulated by Stephens et al. The lowest CEM layer is divided into a thick layer and a very thin layer near the surface for the longwave radiation calculation. Diagnostic tests have been performed to compare the accuracy of this parameterization with results from a more complicated radiative transfer model. Simulations with fully interactive radiation are performed to compare two methods for invoking the radiation module. The time interval for calling the radiation module has to be very small for the conventional method, which keeps the radiative heating rate constant until the radiation module is called again. It can be l...