Initial precipitation formation in warm Florida cumulus

The microphysical processes that lead to the development of precipitation in small, warm cumulus are examined using data from the Small Cumulus Microphysics Study near Cape Canaveral, Florida. Aircraft measurements are used to determine the concentration and size distribution of giant and ultragiant nuclei in clear air as a function of relative humidity, altitude, wind speed, and wind direction. The clear-air particle distributions show that ultragiant particles (radii extending from 10 to 150 μm) exist from the surface to cloud base in concentrations that correspond to the concentrations of raindrops observed during drizzle to moderate rainfall events. A shift of the spectra toward larger size with increasing relative humidity was observed, suggesting that the spectra are composed of deliquesced particles growing by condensation. The small cumulus clouds are shown to contain cores where the observed liquid water content was nearly adiabatic. The observed evolution of the cloud droplet distribution within the near-adiabatic cores as a function of height showed an increase in the small droplet mode associated with condensation and an increase in the concentration of larger droplets associated with growth by accretion. Droplets with radii extending to nearly 100 μm were present just above cloud base. These measurements were consistent with the clear-air measurements and provided evidence that the ultragiant nuclei can immediately act as embryos for raindrop growth by accretion upon entering cloud base. Comparisons of reflectivity computed from the cumulus core composite droplet distributions with the radar-observed reflectivity data provided independent evidence that the composite spectra reasonably represented the evolving microstructure of the cores of small cumulus clouds as they grew vertically. The analyses provide strong evidence of an efficient process for the initial development of precipitation in small Florida cumuli. This process consists of raindrop embryo formation on ultragiant nuclei followed by growth by accretion as the newly formed drops proceed upward through the adiabatic cores of the cumulus clouds. These data support the conceptual model of raindrop formation in marine clouds first proposed by Woodcock a half century ago.