A Radiation Modeling Study of Cloud Fraction and Overlapping Variations in Numerical Weather Prediction

Cloud radiation interactions play an important role in atmospheric energy budget balance and in modulation of various scales of atmospheric circulations. The accuracy of cloud radiation interactions not only depend on modeled cloud water contents, but also on cloud fraction parameterizations and cloud layer overlapping applications. These factors affect cloud water profiles into a radiative transfer model and result in modified cloud optical properties in radiation modeling. US Navy's Coupled Ocean Atmosphere Mesoscale Prediction System (COAMPS) has explicit cloud physics, 4-stream radiation transfer, and high-resolution land surface model. In this study, some diagnostic cloud fraction and overlapping schemes are experimented in COAMPS through modeling verifications against observational radiative fluxes from the Atmosphere Radiation Measurement (ARM) and sensitivities to vertical grid resolutions. The impacts of them on weather forecasts are investigated in Navy's operational regions through the statistical error studies with conventional RAOB sounding and surface station data. It has been preliminarily found that vertical resolution is another important factor for effectiveness of cloud fraction and overlapping applications.