A description is given of the effects of variations in the shape or breadth of the drop size distribution (DSD) on rainfall parameters deduced from a measurement technique which employs the differential reflectivity factor ZDR and the reflectivity factor at horizontal polarization ZH. The mathematical form of the DSD used is a gamma distribution. Justification for such a form is given through consideration of varying DSD shape in nature as implied by the results of empirical analyses of other workers. Theoretical expressions are derived for rainfall rate R, liquid water content W, and median volume diameter D0 in terms of ZDR, ZH, and size distribution dependent factors. The latter calculations assume backscattering cross sections for oblate, nonoscillating raindrops falling in still air with equilibrium shapes. These expressions are used to assess quantitatively the effects of changes in DSD breadth on values of R, W, and D0 deduced from ZDR and ZH. They are also used to show the effects of measurement errors in ZDR and ZH on R, W, and D0. The potential improvement in accuracy which is possible when account is taken of DSD shape variations is shown by simulating a (ZDR, ZH) dual-measurement method using experimental raindrop size spectra. Methods by which DSD shape variations could be detected through the use of a third remote measurable are discussed.
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