Abstract
AbstractThis 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.
Highlights
We identify large‐scale meteorological drivers of shallow cumulus macrophysical properties from an extensive high‐resolution observational data set
We investigated the influence of large‐scale meteorological fields on shallow cumulus cloud fields as observed in 1,158 high‐resolution satellite images from ASTER, which were collected over the tropical Atlantic and Pacific oceans between 2000 and 2007
We characterize cloud fields observed in ASTER imagery by their macrophysical properties and go beyond most prior studies in covering cloud fraction and cloud top height (CTH), cloud size distribution, fractal dimension, and horizontal cloud distribution or spatial clustering
Summary
We identify large‐scale meteorological drivers of shallow cumulus macrophysical properties from an extensive high‐resolution observational data set. Previous literature demonstrates that data limitations impede the investigation of primary control factors on shallow cumulus or trade wind cumulus clouds within the tropical marine boundary layer, while at the same time, such clouds play a crucial role in the Earth's radiative balance. Due to their high‐albedo and ubiquitous nature, they reflect a substantial amount of shortwave radiation and cool the Earth system. Stratocumulus decks favor the cool sea surfaces of eastern boundary currents, which are associated with large LTS, a shallower boundary layer, a strong capping temperature inversion, and stronger subsidence. On the other hand, tend to form over warm surfaces deeper in the tropics, which are characterized by smaller LTS, a deeper boundary layer, a weaker inversion, and MIESLINGER ET AL
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