Polarization radar studies of precipitation development in convective storms

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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