Cumulus clouds are important for the large-scale atmospheric dynamics because of their vertical transport of heat, moisture and momentum. From observations of cumulus fields we know that cumulus can be of many sizes, ranging from a few meters up to a few kilometers in diameter. The many small-sized cumuli outnumber the large-sized cumuli, but do not necessarily contribute the most to properties like cloud fraction, cloud cover, mass-flux and in-cloud buoyancy flux. Observational studies help to understand the dynamical processes of the cumulus topped boundary layer, which can lead to more sophisticated cumulus parameterization schemes in Global Circulation Models (GCM).

We present distributions of mass-flux, in-cloud buoyancy flux and cloud fraction obtained as a function of cloud size from several aircraft measurements in shallow cumulus over Florida during the Small Cumulus Microphysics Study (SCMS). Furthermore a Landsat satellite image near the flight area is used to extend the aircraft measurements over a larger area.

The satellite observations indicate that the cloud cover is dominated by intermediate sized clouds of about 700 m. The dynamics (mass-flux and in-cloud buoyancy flux) is dominated by larger clouds (with linear sizes of about 1 km). In the small cloud regime (up to approximately 500 m) the observed cloud cover density follows a power law with exponent 0.3. To describe the cloud cover density, we propose a functional form that contains a power law and a decaying exponential term.