Whereas different hydrometeor types cannot be readily distinguished using radar reflectivity alone, this paper shows how the differential reflectivity available from dual‐polarization radars can be used as an indicator of hydrometeor type (when supported by reflectivity factor also), and that the spatial variability of differential reflectivity is a useful addition. Various hydrometeor types, ground targets, and burnt chaff from straw fires may be identified, whereas conventional radar reflectivity measurement alone would require considerable pattern recognition to make the same distinctions.

Dual‐polarization radar measurements of rain (using horizontal and vertical polarizations with copolar transmit and receive) are compared with simultaneous measurements using a Joss‐Waldvogel distrometer and a rapid response rain gauge. The radar pulse volume was situated 200 m above the ground‐based instruments, at a range of 8 km. The parameters compared are absolute and differential radar reflectivity factors and rainfall rate. A correlation of 0.95 was found between the radar and distrometer measurements of absolute and differential reflectivity factor, with the radar estimate of absolute reflectivity exceeding the distrometer estimate by 1.6 dB on average, and the distrometer estimate of differential reflectivity exceeding the radar estimate (assuming Pruppacher and Pitter drop shapes) by 0.1 dB on average. The latter difference is small but significant and is possibly attributable to drop canting or oscillation. A new relationship between drop shape and drop size is proposed, reducing the mean difference in differential reflectivity to ±0.02 dB. Radar estimates of rainfall rate, assuming an exponential drop‐size distribution, tend to produce significant overestimates compared with the direct measurements of the two ground‐based devices. This is shown to be largely attributable to departure from a strictly exponential distribution of drop sizes, and a correction factor is derived to compensate for this.

Radar observations of the differential reflectivity (ZDR) of developing convective clouds are presented. This parameter provides a measure of a mean hydrometeor shape, and when analysed in conjunction with the conventional reflectivity ( Z ) , it gives an indication of whether the precipitation is ice or liquid water; for rain it enables an estimate of raindrop size and concentration to be made. The results suggest that some isolated echoes which subsequently develop into large cumulonimbus clouds initially have very small concentrations of large (diameter greater than 4 mm) supercooled raindrops, a very different size distribution from that normally observed in mature convective clouds. An alternative explanation in terms of an even lower population of much larger ice particles in wet growth is technically possible but does not seem physically realistic.

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