Initial precipitation formation in warm Florida cumulus

The microphysical processes that lead to the development of precipitation i n small, warm cumulus are examined using data From the Small Cumulus Microp hysics Study near Cape Canaveral, Florida. Aircraft measurements are used t o determine the concentration and size distribution of giant and ultragiant nuclei in clear air as a function of relative humidity, altitude, wind spe ed, and wind direction. The clear-air particle distributions show that ultr agiant particles (radii extending from 10 to 150 mum) exist from the surfac e to cloud base in concentrations that correspond to the concentrations of raindrops observed during drizzle to moderate rainfall events. A shift of t he spectra toward larger size with increasing relative humidity was observe d, suggesting that the spectra are composed of deliquesced particles growin g by condensation. The small cumulus clouds are shown to contain cores wher e the observed liquid water content was nearly adiabatic. The observed evol ution of the cloud droplet distribution within the near-adiabatic cores as a function of height showed an increase in the small droplet mode associate d with condensation and an increase in the concentration of larger droplets associated with growth by accretion. Droplets with radii extending to near ly 100 mum were present just above cloud base. These measurements were cons istent with the clear-air measurements and provided evidence that the ultra giant nuclei can immediately act as embryos for raindrop growth by accretio n upon entering cloud base. Comparisons of reflectivity computed from the c umulus core composite droplet distributions with the radar-observed reflect ivity data provided independent evidence that the composite spectra reasona bly represented the evolving microstructure of the cores of small cumulus c louds as they grew vertically. The analyses provide strong evidence of an efficient process for the initia l development of precipitation in small Florida cumuli. This process consis ts of raindrop embryo formation on ultragiant nuclei followed by growth by accretion as the newly formed drops proceed upward through the adiabatic co res of the cumulus clouds. These data support the conceptual model of raind rop formation in marine clouds first proposed by Woodcock a half century ag o.