Polarimetric radar analysis of rain drop size variability in clouds near Brisbane Australia

The Queensland Cloud Seeding Research program conducted near Brisbane Australia in 2007 - 2009 provided a unique opportunity to utilize polarimetric radar (CP-2) to observe rain drop size evolution in seeded clouds. The radar polarimetric variable differential radar reflectivity measures the flatness of falling raindrops which can then be related to rain drop size. However, it became apparent that there were significant variations in the rain drop size evolution from one storm cell to another, thus making it essential to first understand the natural variability in the drop size evolution before looking for possible cloud seeding effects.

Two primary types of rain drop size evolution were found. Type 1 was similar to that observed with the S-Pol polarimetric radar data from a field project in the Caribbean Sea called the Rain In Clouds over the Ocean (RICO). Type 2 was characterized by relatively large rain drops during the growth stage. Aerosol concentrations measured by aircraft at cloud base indicated that Type 1 evolutions were characterized by low aerosol concentrations in contrast to relatively high concentrations for the Type 2 evolutions. Analysis showed that the Type 1 (low aerosol concentrations) cases had air trajectories with long residence time over the ocean east of Brisbane in contrast to long residence time over land for the Type 2 cases. Based on the observed natural variability suggestions will be made for conducting future field programs to examine any possible effects cloud seeding might have on the evolution of rain drop sizes.