Observations of microphysics pertaining to the development of drizzle in warm, shallow cumulus clouds

The evolution of drizzle in small cumulus clouds in a subtropical environment was studied. Observations were made using three instrumented aircraft, a ground-based radar, and a 95 GHz airborne Doppler radar during the Small Cumulus Microphysics Study in east central Florida in summer 1995. Data from six clouds, from two days, are examined in this paper. Both sets of clouds were less than 2 km in depth at observation times; in-cloud temperatures were > 10 degrees C, Droplet spectra observed in these clouds were generally bi-modal. The sizes of the droplets within the large mode are consistent with growth through condensation. It remains unclear how droplets within the small mode formed, but their existence appears to be tied to entrainment and mixing. Drizzle drops (drops with diameters > 50 mu m) were found at all levels in the clouds. The penetration-averaged concentrations of drizzle drops were less than 15 L-1, except in the uppermost regions of clouds on one of the days where drizzle drop concentrations exceeded 100 L-1. In general, the presence of drizzle drops in the upper regions of the clouds is consistent with models of droplet growth through condensation and stochastic collection. The existence of drizzle at low and mid-levels may be due to collection initiated by ultra-giant aerosols, or to the redistribution of drizzle drops from neighbouring or earlier cloud elements.