Drop axis ratio and fall velocity distributions from 2-D video disdrometer

Accurate estimates of drop axis ratios in rain are needed for radar retrievals of rainfall rates and for developing attenuation correction schemes for dual polarization radars operating at microwave frequencies. The ratios also govern the differential attenuation as well as cross-polar effects in rain needed for satellite communication systems operating with orthogonal polarizations at microwave and millimeter wave frequencies.

To obtain accurate estimates of the drop axis ratios in terms of the equivalent drop diameter, an experiment was conducted using a 2-dimensional Video Disdrometer (2DVD), developed by Joanneum Research in Austria. Under calm conditions, drops were generated from a hose located on a bridge 80 m above ground (Jauntal bridge in Carinthia, Austria), this height being sufficient to allow drop oscillations to reach steady state. The disdrometer data had to be carefully processed so as to eliminate the drops mismatched by the instrument and to remove the system spreading function. The total number of drops analyzed was around 115,000. Their axis ratio distributions were obtained for diameters ranging from 1.5 to 9 mm. The mean axis ratio decreases with increasing drop diameter, in agreement with the upper bound of the Beard and Chuang (1987) equilibrium shape model (BC). The inferred mode of oscillation appeared to be dominated by the oblate-prolate axisymmetric mode for the diameter range 1.5 to 9 mm. The drop terminal velocities were also measured and these showed very good agreement with Gunn and Kinzer (1949) data as well as the Beard and Pruppacher (1969) data.

Under light-wind conditions, it is also possible to measure the drop axis ratio distributions with the 2DVD in natural rain. A number of different types of rain events have been examined from a sub-tropical oceanic regime (Okinawa, Japan), a mid-latitude location (Tokyo) and an equatorial region (Sumatra, Indonesia). The events ranged from shallow rain and ‘Baiu' frontal rain in Okinawa to stratiform and convective rain in Tokyo and Sumatra. On average, the mean drop axis ratios lie within the upper bound numerical model of Beard and Chuang and agree reasonably well with the results from the bridge experiment. The axis ratio distributions will be compared with the bridge data to identify the dominant mode of oscillation in natural rain. Also compared will be the fall velocity distributions.