Mechanisms for the Rapid Generation of Precipitation in Warm Clouds

Some of the earliest cloud physics work on warm clouds considered why marine clouds have a much larger propensity for the production of precipitation than continental clouds. This propensity (susceptibility) has been largely attributed to differences in the sub-cloud CCN that feed warm cumulus clouds. Although the ease at which marine clouds can produce precipitation is apparent from visual observations, these observations also indicate how quickly cloud matter can become precipitation. Although it has been shown that giant and ultra-giant CCN (GCCN and UGCCN) can accelerate precipitation production, models of this process still produce precipitation on time scales that are smaller than observed. Further, there is an indication that GCCN may have a smaller effect in clean air masses than those in areas with higher CCN concentrations. Another possible mechanism for the rapid production of precipitation that has been proposed is cloud electrification processes. But to date there has been insufficient in-cloud observational evidence to support this idea. In this study we examine photographic evidence of rapid cloud precipitization to estimate a rough time scale associated with this process. To quantify the properties of rapidly precipitating clouds (RPCs) we use observations made from the CIRPAS Twin Otter (TO) research aircraft made near Key West Florida in the summer of 2010 under conditions where numerous RPCs were observed. The aircraft observations are used to characterize the sub-cloud CCN and the cloud and precipitation structure of clouds and precipitation streamers in various phases of development. An upward-facing FMCW Doppler radar mounted on the TO is used to characterize precipitation in and around the clouds sampled. Field mills mounted on the aircraft are used to examine the electric fields near the cloud sides and tops. Analyses of these data will be used to examine the mechanisms that may be responsible for the rapid production of precipitation observed in this case.