A study of raindrop‐size distributions in the free atmosphere

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Abstract
Observations of frontal precipitation using a vertically‐looking 3.2 cm pulsed Doppler radar have been analysed to indicate the raindrop‐size distributions present below the melting band. The variations of size distribution with rainfall rate and with height are discussed. The distributions at 750 m above ground show both fewer small drops and fewer large drops than the corresponding Marshall and Palmer (1948) exponential distributions. At greater heights there are more small drops but even fewer large drops. The changes of rain‐flux and drop‐size distribution with height provide evidence in support of raindrop coalescence but, in general, indicate the absence of pronounced accretion or evaporation. Experimental relations between equivalent reflectivity factor Ze and rainfall rate R have been derived for two heights; extrapolated to ground level the relation is Ze = 285 R1.30 for continuous frontal rains of 0.1 to 5.0 mm hr−1 at Pershore. Instances of vertical air motion due to waves induced by hills have been diagnosed.

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