MICROPHYSICAL AND RADIATIVE PROPERTIES OF SMALL CUMULUS CLOUDS OVER THE SEA

Microphysical and radiative data obtained in fields of cumulus clouds over the sea around the United Kingdom are presented. The Meteorologic al Research Flight C-130 aircraft was used to make radiation and cloud microphysical measurements in situ on four different occasions. The s ub-cloud aerosol concentrations ranged from 100 to 5000 cm-3, from fai rly maritime to heavily polluted. Vertical profiles of microphysical v ariables indicate that small cumuli over the sea have some properties that are more similar to marine stratocumuli than to continental cumul i. Nonetheless, variables, such as liquid-water content and droplet-nu mber concentration, exhibit a larger variability than in stratocumuli, owing to more vigorous entrainment in cumuli. Mie theory was used to compute extinction cross-sections, single scattering albedo, back-scat tering cross-sections, and asymmetry parameters for all in-cloud sampl es. The average vertical profiles are in agreement with values compute d by previous authors for trade-wind cumuli. Our results show that the se properties depend upon the background aerosol concentrations. A sim ple two-stream radiative-transfer model, using the back-scattering cro ss-section, is presented and shows good agreement with run-averaged ra diometric observations.