Trade wind cloud evolution observed by polarization radar—relationship to aerosol characteristics

One of the classical unsolved problems in cloud physics is the explanation of the observed short time between initial cloud formation and the onset of precipitation in warm clouds. Although a time interval of about 20 minutes is often quoted for initiation of warm rainfall, few field observations have carefully defined the starting or ending times of trade-wind cumuli development. The goals of this research are: 1) define a characteristic time and height for warm rain formation in trade-wind cumulus & 2) examine the sensitivity of precipitation development in shallow maritime cumuli to variations in giant (sea salt) condensation nuclei (GCN) and cloud organization.

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 is characterized focusing on the potential role of giant nuclei in influencing the ZDR signal. The data set consists of 76 trade wind cumuli that have been tracked from early echo development through rainout over six days of the field project. Analysis days were chosen based on the results of Colόn-Robles et al. (2006) which show GCN concentrations are a strong function of near-surface wind speed. Each cloud was analyzed with the time-height section approach which displays the spatial and temporal evolution of the equivalent radar reflectivity factor (Ze), the differential reflectivity (ZDR), and the correlation between these two fields. Other measures used for the time-height analysis include ‘averaged' Ze and ZDR values which are calculated for entire constant-elevation angle sweeps through the cloud. A statistical analysis of the radar observables for the ensemble of trade wind clouds has been performed in order to determine if there is a statistically significant difference in the precipitation evolution that can be related to the initial giant aerosol concentration.

Our results show that a great deal of temporal variability exists in warm rain development. Warm rainfall was observed within 30 minutes of cloud formation; however, the average time for precipitation formation was also observed at 80 minutes. The data showed a large amount of spread in both the characteristic time and height for warm rain formation, indicating giant nuclei have a minimal influence on warm rain development. Rather, our results and the statistics imply that precipitation formation in warm clouds is heavily influenced by cloud organization and dynamics. It was found that giant nuclei do influence the rain drop size distributions as larger drops were present above cloud base when the giant nuclei concentrations below cloud base are greater. The DBZ and ZDR fields show better temporal and spatial correlation when the giant nuclei concentrations are greater as well.