CHARACTERISTICS OF SMALL TROPICAL CUMULUS CLOUDS AND THEIR IMPACT ON THE ENVIRONMENT

This study uses a number of data sets (Moderate Resolution Imaging Spe ctroradiometer (MODIS) Airborne Simulator data, space shuttle photogra phy, Radiation Measurement System data, aircraft data, and shipboard s oundings) 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 a nd satellite remote sensing. In fields of small cumulus clouds, size d istributions are found to decrease in number with increasing diameter according to a double power law relation, often with a clear break dia meter. Fractal dimensions corresponding to the horizontal area and per imeter of the clouds are greater for the larger clouds th;tn for the s maller 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 lar ger and smaller boundary layer cumuli. Spatial distributions show a cl ear tendency toward clustering. Smaller cumuli appear to grow upward m ore quickly with increasing horizontal size than do larger cumuli. Alb edo is found to increase with greater cloud fraction and higher solar zenith angle. Even sparse fields of small cumulus cause significant sh ortwave forcing at the ocean surface. Simulation suggests that small c umulus may introduce significant errors into sea surface temperature r etrievals and that such clouds can be difficult to remove with operati onal cloud-filtering schemes. Clouds smaller than about 1 km in diamet er are not seen to precipitate.