During the Queensland Cloud Seeding Research Program the CP2 polarimetric radar parameter differential radar reflectivity (Zdr) was used to examine the rain drop size evolution in both maritime and continental clouds. The focus of this paper is to examine the natural variability of the drop size distribution. The primary finding is that there are two basic rain drop size evolutions, one associated with continental air masses characterized by relatively high aerosol concentrations and long air trajectories over land and the other with maritime air masses with lower aerosol concentrations. The size evolution difference is during the growth stage of the radar echoes. The differential radar reflectivity in the growing continental clouds is dominated by large rain drops, whereas in the maritime clouds differential reflectivity is dominated by small rain drops and drizzle. The drop size evolution in many of the maritime air masses was very similar to those observed in the maritime air of the Caribbean observed with the NCAR S-band polarimetric radar (S-Pol) during the Rain In Cumulus over the Ocean (RICO) experiment. Since the tops of the Queensland continental clouds ascended almost twice as fast as the maritime ones in their growth stage, both dynamical and aerosol factors may be important for the systematic difference in drop size evolution. Recommendations are advanced for future field programs to better understand the causes for the observed variability in drop size evolution. Also considering the natural variability in drop size evolution comments are provided on conducting and evaluating cloud seeding experiments.
