One of the classical unsolved problems in cloud physics is the observed short time between initial cloud formation and the onset of precipitation in warm clouds. Although a time interval of 30 minutes after cloud formation is often quoted for initiation of warm rainfall, few field observations carefully define the starting or ending times of trade-wind cumuli development.
Using data collected by the National Center for Atmospheric Research S-PolKa radar during the Rain In Cumulus over the Ocean (RICO) field campaign, the microphysical evolution of trade wind cumuli are characterized, with a focus on the potential role of giant nuclei in warm rain formation. The data set consists of 76 trade wind cumuli that were tracked from early echo development through rainout. To help determine the role of giant aerosols in precipitation formation, cloud analysis days were chosen so a sufficient range in giant sea salt aerosol concentrations existed. Clouds were then analyzed individually with a time-height section approach, which depicts the spatial and temporal evolution of the equivalent radar reflectivity factor (Ze), the differential reflectivity (ZDR), and the correlation between these two fields. Furthermore, this research attempts to define ‘time zero’ in trade wind cumuli development and determine a characteristic time for precipitation formation.
The results show that a great deal of temporal variability exists in warm rain development. While rain initiation occurred within the quoted time interval of 30 minutes on some days, the average time for precipitation formation increases to 80 minutes on other days. Additionally, our results imply that precipitation formation in warm clouds is influenced by cloud organization and dynamics rather than giant nuclei. Furthermore, it was found that giant nuclei do influence the rain drop size distributions as larger drops were present above cloud base when giant nuclei concentrations are higher. The DBZ and ZDR fields show better temporal and spatial correlation when the giant nuclei concentration is higher as well.