Application of a Maximum-Random Cloud Overlap Method for RRTM to General Circulation Models

An important consideration in the calculation of longwave radiation through multiple cloudy layers is the spatial relationship, or cloud overlap, among the cloud fractions in each layer. Many general circulation models (GCMs) apply random cloud overlap in which clouds in adjacent layers are assumed to be unrelated. However, this approximation is not appropriate for situations in which clouds are vertically correlated (e.g., deep convection), and recent studies have demonstrated the importance of cloud overlap treatment to GCM simulations (e.g., Jakob and Klein 1999). For this reason, a maximumrandom cloud overlap approach has been developed for the longwave model rapid radiative transfer model (RRTM) for use in GCMs and other radiative transfer applications. RRTM is a rapid and accurate, correlated-k, radiative transfer model (Mlawer et al. 1997) that has been developed for the Atmospheric Radiation Measurement (ARM) Program to address the ARM objective of improving radiation models in GCMs.