Drop shape studies in rain using 2-D video disdrometer and dual-wavelength, polarimetric CP-2 radar measurements in south-east Queensland, Australia

In November 2008, a well-calibrated, low profile 2D video disdrometer (2DVD) was installed in south-east Queensland near Brisbane, Australia, at a location which not only has polarimetric radar coverage but also dual-wavelength radar observations at S and X bands. Since the installation of the 2DVD, several events have been recorded, many of which has been analyzed to determine the drop shapes and the drop axis ratio distributions for various drop diameter intervals. The modes of the various axis ratio distributions are similar to those from the outdoor, artificial rain experiment conducted earlier under calm wind conditions where drops were allowed to fall a distance of 80 m. However, the natural rain data shows considerably higher skewness, with the distributions being skewed towards more spherical axis ratios for some events. Moreover, in the 1.6 – 1.8 mm drop diameter range, there seems an unusually wide axis ratio distribution, possibly indicating oscillations due to the resonance with eddy-shedding frequency. 2DVD measurements from other locations such as Alabama, USA and Sumatra, Indonesia, have also shown these unusually wide distributions in the same diameter interval.

Most of the events recorded by the 2DVD in SE Queensland were accompanied by the S-band (dual polarization) and X band radar observations, with precisely aligned and matched radar-beams. One example was on 19 Feb 2009, a squall-line event with a leading edge of deep convection, part of which passed over the 2DVD site between 07:40 – 08:00 UTC. An analysis technique has been developed to infer drop shapes from the ratio between the X-band specific attenuation and the differential propagation phase at S-band, and its variation with the S-band differential reflectivity. The event analysis has indicated that there could be significant shape variations, but the most probable variation inferred from the radar data agree with the most probable shape measurements from the 2DVD. The synergistic approach also provides an insight into drop oscillations and their impact on drop size distribution and rain retrieval algorithms for polarimetric weather radar.