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Abstract

This study identifies meteorological variables that control the macrophysical properties of shallow cumulus cloud fields over the tropical ocean. We use 1,158 high-resolution Advanced Spaceborn Thermal Emission and Reflection Radiometer (ASTER) images to derive properties of shallow cumuli, such as their size distribution, cloud top heights, fractal dimensions, and spatial organization, as well as cloud amount. The large-scale meteorology is characterized by the lower-tropospheric stability, subsidence rate, sea surface temperature, total column water vapor, wind speed, wind shear, and Bowen ratio. The surface wind speed emerges as the most powerful control factor. With increasing wind speed the cloud amount and cloud top heights show a robust increase accompanied by a marked shift in the cloud size distribution toward larger clouds with smoother shapes. These results lend observational support to the deepening response of a wind-driven marine boundary layer as simulated by large-eddy models. The other control factors cause smaller changes in the cloud field properties. We find a robust increase in cloud amount with increasing stability and decreasing sea surface temperature, respectively, which confirms a well-known behavior of marine stratocumulus also for shallow cumulus clouds. Due to the high resolution of cloud images, we are able to study the lower end of the cloud size distribution and find a robust double power law behavior with a scale break at 590 m. We find a variation in the shape of the cloud size distribution with Bowen ratio, qualitatively consistent with modeling results and suggesting the Bowen ratio as a new potential control factor on shallow cumulus clouds. © 2019. The Authors.