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 Meteorological Research Flight C-130 aircraft was used to make radiation and cloud microphysical measurements in situ on four different occasions. The sub-cloud aerosol concentrations ranged from 100 to 5000 cm-3, from fairly maritime to heavily polluted. Vertical profiles of microphysical variables indicate that small cumuli over the sea have some properties that are more similar to marine stratocumuli than to continental cumuli. Nonetheless, variables, such as liquid-water content and droplet-number 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-scattering cross-sections, and asymmetry parameters for all in-cloud samples. The average vertical profiles are in agreement with values computed by previous authors for trade-wind cumuli. Our results show that these properties depend upon the background aerosol concentrations. A simple two-stream radiative-transfer model, using the back-scattering cross-section, is presented and shows good agreement with run-averaged radiometric observations.