Structural Characteristics and Radiative Properties of Tropical Cloud Clusters

Abstract

By identifying individual tropical cloud clusters in eight months of the International Satellite Cloud Climatology Project data, the size distribution, average cloud properties, and their variation with system size in tropical convective systems (CS) is examined. The geographic distribution of CS shows a concentration over land areas in the summer hemisphere with little seasonal variation except for the major shift of location into the summer hemisphere. When the tropics are considered as a whole or a region is considered over a whole season, CS of all sizes (from individual convective towers at 2?20 km to the largest mesoscale systems at 200?2000 km) form a continuous size distribution where the area covered by the clouds in each size range is approximately the same. Land CS show a small excess of the smallest CS and a small deficit of the largest CS in comparison to ocean CS. Average CS cloud properties suggest two major cloud types: one with lower cloud-top pressures and much higher optical thicknesses associated with deep convection, and one with higher cloud-top pressures and lower optical thicknesses associated with the mesoscale stratiform anvil clouds. The anvil cloud properties show some evidence of a further division into optically thicker and thinner parts. The avenge properties of these clouds vary in a correlated fashion such that a larger horizontal extent of the convective system cloud is accompanied by a lower convective cloud-top pressure, larger anvil cloud size, and larger anvil cloud optical thickness. These structural properties and their diurnal variation also suggest that the smallest CS may represent a mixture of the formative and dissipating stages of CS, while the medium and large sizes are, principally, the mature stage. A radiative transfer model is used to evaluate the local radiative effects of CS with average cloud properties. The results imply that the mesoscale anvil cloud reinforces the diabatic heating of the atmosphere by the convection and may help sustain these systems at night. The radiative effects of the convective clouds, while unimportant to the total effect of the CS at the top of the atmosphere, may reinforce the diurnal variation of convection. Evaluating the radiative feedback of tropical cloudiness on climate is shown to be very difficult because of the significant diurnal and geographic variations of convective system cloud properties.