The Reflectance Distribution of Shallow Cumulus and Its Environs From High Spatial Resolution ASTER Images

Abstract We investigate the 1D reflectance distribution of shallow cumulus as a function of cloud size, using high‐spatial‐resolution observations from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). Reflectance transects through clouds are compared between Inner Mongolia and Tropical Ocean. The reflectance shows varying degrees of shadowing–illumination effects, depending on transect direction. Perpendicular to the solar azimuth (cross‐sun direction), where shadowing–illumination effects are minimal, the reflectance follows a symmetric, bell‐shaped decrease from the cloud center to the cloud edge and into the transition zone or radiative halo. The peak reflectance is located slightly off‐center for larger clouds, which can be explained by the competition between radiative smoothing and sharpening. Reflectance steadily increases with cloud size in both regions; however, Inner Mongolia clouds are significantly brighter and have larger cloud top height variations for the same chord length (cloud diameter). The size and height of continental clouds also systematically increase with surface temperature, indicating more vigorous convection over warm, dry land characterized by high Bowen ratio. In the halo region, reflectance increases by 20%–25% above its clear‐sky value within one chord length of the cloud edge, the brightness enhancement showing little variation with cloud size. The chord‐length normalized size of the halo steadily decreases with cloud size, as the absolute halo size does not scale linearly with cloud size. This observation, consistent with large‐eddy simulations of the moist buffer around shallow cumulus, suggests that the processes responsible for halo formation have size‐independent length scales.