Overlap of fractional cloud for radiation calculations in GCMs: A global analysis using CloudSat and CALIPSO data

Abstract

Assumptions made by global climate models (GCMs) regarding vertical overlap of fractional amounts of clouds have significant impacts on simulated radiation budgets. A global survey of fractional cloud overlap properties was performed using 2 months of cloud mask data derived from CloudSat-CALIPSO satellite measurements. Cloud overlap was diagnosed as a combination of maximum and random overlap and characterized by vertically constant decorrelation length ℒ cf *. Typically, clouds overlap between maximum and random with smallest ℒ cf * (medians →; 0 km) associated with small total cloud amounts Ĉ, while the largest ℒ cf * (medians∼3 km) tend to occur at Ĉ near 0.7. Global median ℒ cf * is ∼2 km with a slight tendency for largest values in the tropics and polar regions during winter. By crudely excising near-surface precipitation from cloud mask data, ℒ cf * were reduced by typically <1 km. Median values of ℒ cf * when Sun is down exceed those when Sun is up by almost 1 km when cloud masks are based on radar and lidar data; use of radar only shows minimal diurnal variation but significantly larger ℒ cf *. This suggests that sunup inferences of ℒ cf * might be biased low by solar noise in lidar data. Cloud mask cross-section lengths L of 50, 100, 200, 500, and 1000 km were considered. Distributions of ℒ cf * are mildly sensitive to L thus suggesting the convenient possibility that a GCM parametrization of ℒ cf * might be resolution-independent over a wide range of resolutions. Simple parametrization of ℒ cf * might be possible if excessive random noise in Ĉ, and hence radiative fluxes, can be tolerated. Using just cloud mask data and assuming a global mean shortwave cloud radiative effect of -45 W m -2, top of atmosphere shortwave radiative sensitivity to Cp was estimated at 2 to 3 W m -2 km -1.