NEAR-GLOBAL SURVEY OF EFFECTIVE DROPLET RADII IN LIQUID WATER CLOUDS USING ISCCP DATA

A global survey of cloud particle size variations can provide crucial constraints on how cloud processes determine cloud liquid water conten ts and their variation with temperature, and further, may indicate the magnitude of aerosol effects on clouds. A method, based on a complete radiative transfer model for AVHRR-measured radiances, is described f or retrieving cloud particle radii in liquid water clouds from satelli te data currently available from the International Satellite Cloud Cli matology Project. Results of sensitivity tests and validation studies provide error estimates. AVHRR data from NOAA-9 and NOAA-10 have been analyzed for January, April, July, and October in 1987 and 1988. The r esults of this first survey reveal systematic continental and maritime differences and hemispheric contrasts that are indicative of the effe cts of associated aerosol concentration differences: cloud droplet rad ii in continental water clouds are about 2-3 mum smaller than in marin e clouds, and droplet radii are about 1 mum smaller in marine clouds o f the Northern Hemisphere than in the Southern Hemisphere. The height dependencies of cloud droplet radii in continental and marine clouds a re also consistent with differences in the vertical profiles of aeroso l concentration. Significant seasonal and diurnal variations of effect ive droplet radii are also observed, particularly at lower latitudes. Variations of the relationship between cloud optical thickness and dro plet radii may indicate variations in cloud microphysical regimes.