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  • Author or Editor: I. J. Caylor x
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A. J. Illingworth and I. J. Caylor

Abstract

The differential reflectivity (Z DR) measures the mean shape of hydrometeors and provides an estimate of the mean size of raindrops Observations of Z DR for rain may be combined with the conventional radar reflectivity factor (Z) and fitted to any two-parameter raindrop size distribution and this information used to derive more accurate rainfall rates. In such work the precise shape of raindrops is a critical parameter. Recently available data suggest that large raindrops are more oblate than previously believed. These new shapes support the idea that Z DR values above 3.5 dB can be attributed to rain. Average values of Z DR as a function of Z obtained in heavy rain by the Chilbolton radar agree very closely with those predicted using the new shapes. Statistics are also presented of the natural variability of raindrop spectra in heavy rain. Analytic expressions are proposed for computing rainfall rate from Z and Z DR.

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A. R. Jameson and I. J. Caylor

Abstract

As microwaves propagate through rain, the rate of phase change with increasing distance is different depending upon whether the transmissions are polarized horizontally or vertically. This rate of change is the so-called specific propagation differential phase shift ΦDP. This paper demonstrates that at several frequencies and over a wide domain the ratio of ΦDP to the rainwater content W is nearly linearly related to D m, the mass-weighted mean drop size. An investigation of errors indicates that this new approach is likely to yield more accurate estimates of W than the other classical reflectivity factor Z, attenuation, or polarization techniques. The most accurate estimates of W are most likely at the highest frequency considered, 13.80 GHz.

In lieu of such high-frequency measurements, these somewhat esoteric results are made more concrete through an analysis of 3-GHz radar measurements collected during the Convection and Precipitation Experiment in a tropical rainstorm in Florida. Among the principle advantages of using ΦDP to measure rain are that an absolute calibration of the radar is no longer required and the estimates are decoupled from measurements of the radar reflectivity factor. Consequently, temporal and spatial structures of rain estimates do not simply mimic those of the reflectivity factor, as happens for classical estimation techniques using Z.

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