Improving Representation of Tropical Cloud Overlap in GCMs Based on Cloud-Resolving Model Data

The decorrelation length (L-cf) has been widely used to describe the behavior of vertical overlap of clouds in general circulation models (GCMs); however, it has been a challenge to associate L-cf with the large-scale meteorological conditions during cloud evolution. This study explored the relationship between L-cf and the strength of atmospheric convection in the tropics based on output from a global cloud-resolving model. L-cf tends to increase with vertical velocity in the mid-troposphere (w(500)) at locations of ascent, but shows little or no dependency on w(500) at locations of descent. A representation of L-cf as a function of vertical velocity is obtained, with a linear regression in ascending regions and a constant value in descending regions. This simple and dynamic-related representation of L-cf leads to a significant improvement in simulation of both cloud cover and radiation fields compared with traditional overlap treatments. This work presents a physically justifiable approach to depicting cloud overlap in the tropics in GCMs.