Field Studies of Raindrop Axis Ratio and Orientation Distributions Using an Improved 2D-Video Disdrometer and Dual-Polarized Radar

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Polarimetric radar measurements of precipitation contain information about the shape of the precipitation particles. For raindrops, the shape may oscillate with time, but the average shape is approximately spheroidal, and the larger the drop the greater the deviation from spherical. Hence polarimetric measurements of rain contain information about drop size - information in addition to what is provided by the radar reflectivity alone. This is the basis for improved estimates of rainfall rate using polarimetry. This project contributes to radar polarimetry by directly measuring the sizes, shapes, and canting angles of individual drops at the ground. It employs an upgraded version of a video disdrometer manufactured by an Austrian firm, Joanneum Research. This instrument records the front and side view of each individual drop as it falls through a square, horizontal sampling area about 10 cm on the side. It measures the particle size, shape, canting angle, and fall speed. The data enable the calculation of a parameter relevant to polarimetric measurements, called "effective beta," which depends on the relation between size, shape, and canting angle. A recent theory shows that this parameter can also be inferred indirectly from radar observations of three polarimetric quantities, the reflectivity at horizontal polarization, the ratio of the reflectivities at horizontal and vertical polarization, and the differential propagation phase between the two polarization components. Furthermore, knowing the effective beta enables more accurate estimation of the rainfall rate. The purpose of the experiments is to test the theory by comparing radar estimates of beta with the values of this parameter determined directly from the disdrometer-measured properties of the drops. Experiments will be conducted at two sites: in Colorado, using the CSU-CHILL radar, and in Darwin, Australia, in collaboration with the Bureau of Meteorology Research Center. Using these sites will enable the evaluation of the theory in two climatic regimes with possibly different rainfall characteristics. The work should improve the accuracy of rainfall estimation by radar.