Radar analysis of precipitation development in Florida cumulus clouds

The initiation and rapid acceleration of the precipitation process through collision-coalescence in warm cumulus clouds requires a mechanism for droplet spectral broadening, specifically the growth of the large end tail of the droplet spectrum. Several mechanisms that lead to droplet spectral broadening have been proposed and one of the simplest hypotheses, developed more than a half century ago, is that giant and ultragiant aerosols, such as sea-salt particles in marine environments, form large droplets upon entering cloud base. Warm cumulus clouds in their early stages were studied during the Small Cumulus Microphysics Study (SCMS), which was conducted in the summer of 1995 near Cape Canaveral Florida, with the goal of understanding the factors that control the time to onset of precipitation. The purpose of our study is to investigate the radar reflectivity evolution of SCMS clouds with the goal of determining whether any significant differences are detectable in the time to onset of precipitation that can be associated with the clouds' continental or maritime characteristics. If precipitation development is dominated by accretional growth on giant and ultragiant aerosols then there is likely to be little difference in the early evolution of the radar signatures. On the other hand, if growth of precipitation is dependent on the spectral broadening associated with other processes, then the evolution of the radar reflectivity in these two cloud populations should be distinct since the continental clouds, with their narrow initial droplet spectra, require greater time for spectral broadening. For our study we have examined the entire SCMS radar data set and identified those clouds for which a sufficient radar history from their earliest detection through precipitation was clearly documented. Our results support the hypothesis that the relevance of giant and ultragiant aerosols for precipitation formation is diminished in clouds with low cloud condensation nuclei concentrations. We further found that there was a quite abrupt transition in the early evolution of the X-band radar reflectivity of SCMS clouds that formed over the ocean versus clouds that formed over the Florida land mass.