Characteristics of small tropical cumulus clouds and their impact on the environment

This study uses a number of data sets (Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator data, space shuttle photography, Radiation Measurement System data, aircraft data, and shipboard soundings) to investigate the characteristics of small tropical cumulus clouds and their impact on their environment. The goal is to uncover useful information with application to radiative transfer simulation and satellite remote sensing. In fields of small cumulus clouds, size distributions are found to decrease in number with increasing diameter according to a double power law relation, often with a clear break diameter. Fractal dimensions corresponding to the horizontal area and perimeter of the clouds are greater for the larger clouds th;tn for the smaller clouds, with the same break diameter as the size distributions, meaning that the larger clouds have more ragged perimeters. These two results suggest a characteristic horizontal length scale dividing larger and smaller boundary layer cumuli. Spatial distributions show a clear tendency toward clustering. Smaller cumuli appear to grow upward more quickly with increasing horizontal size than do larger cumuli. Albedo is found to increase with greater cloud fraction and higher solar zenith angle. Even sparse fields of small cumulus cause significant shortwave forcing at the ocean surface. Simulation suggests that small cumulus may introduce significant errors into sea surface temperature retrievals and that such clouds can be difficult to remove with operational cloud-filtering schemes. Clouds smaller than about 1 km in diameter are not seen to precipitate.