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

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Abstract

[1] 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 equation imagecf*. Typically, clouds overlap between maximum and random with smallest equation imagecf* (medians → 0 km) associated with small total cloud amounts equation image, while the largest equation imagecf* (medians ∼3 km) tend to occur at equation image near 0.7. Global median equation imagecf* 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, equation imagecf* were reduced by typically <1 km. Median values of equation imagecf* 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 equation imagecf*. This suggests that sunup inferences of equation imagecf* 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 equation imagecf* are mildly sensitive to L thus suggesting the convenient possibility that a GCM parametrization of equation imagecf* might be resolution-independent over a wide range of resolutions. Simple parametrization of equation imagecf* might be possible if excessive random noise in equation image, 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 equation imagecf* was estimated at 2 to 3 W m−2 km−1.

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