Drop shape and orientation angle distributions in rain from 2-D video disdrometer

Drop shapes and orientation angles are two factors which govern the relationships between various parameters involved in the rainfall estimation from polarimetric radar data. They also govern the relationships required for attenuation correction schemes which may need to be applied, particularly for C-band and higher frequencies.

In an earlier study, the drop shapes were investigated using the 2-D video disdrometer (2DVD) in artificial rain. Contoured shapes (which filters the quantization noise of the instrument) were derived for over 115,000 drops and were shown to be consistent with the Beard-Chuang (non-oblate) shape model. Drops with equivalent diameter (Deq) greater than 4 mm were shown to deviate more and more from oblate shapes. A fitted equation for the mean shapes was derived based only on Deq.

In this study, we compare the fitted equation with the contoured shapes in natural rain, derived once again from 2DVD. Data from two different sites in the west Pacific regions were used in the comparisons. Our fitted equation for the mean shapes agree well with the contours obtained in natural rain. Also derived from the 2DVD based contoured shapes are the ‘limits' of the drop shapes for a given Deq interval. For the 4-4.25 mm Deq interval for example, the vertical limit ranged from 2.6 to 4.1 mm whilst the horizontal limit ranged from 3.8 to 4.9 mm. Compared with the mean dimensions of 3.35 mm vertical and 4.4 mm horizontal, the limits suggest that shape variation occurs more in the vertical than horizontal.

Also derived from the 2DVD data are the canting angle distributions from the two cameras using a procedure developed by Joanneum Research. From these two distributions the zenith angle and the azimuth angle of the orientation of the symmetry axis of the oblate drops can be estimated. For modeling the Zdr, LDR and Kdp, a Gaussian shape is generally assumed for the zenith angle with mean of 0 and standard deviation around 5-10 deg, while the azimuth angle is assumed uniform between 0-180 deg. This model is used to account for turbulence-induced orientation fluctuations. We derive the distributions of the zenith and azimuth angles from the 80 m bridge data consisting of ‘artificial' rain under calm winds. We also apply the methodology for natural rain under light wind situations from a variety of locations with 2D video disdrometers.