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M. C. Hodson

Abstract

This paper reviews some of the published results relating to raindrop-size distributions and couples this with some of the authors' results in order to show that the mathematical description of the distribution can be divided into three categories. At rainfall rates greater than 25 mm h−1, the distribution is exponential with a fixed slope Λ lying between 2.1 and 2.3 mm−1. At rates leer than 2 mm h−1, the distribution is exponential and Marshall-Palmer. Rainfall rates falling in between these two categories give rise to an exponential distribution that is a modified Marshall-Palmer with drops less than 1.25 mm in diameter being modified by coalescence and the shape of the distribution for larger drops being changed by spontaneous drop breakup. The effect of this on the measurement of rainfall is discussed, leading to the conclusion that for rainfall rates greater than 25 mm h−1, attenuation is proportional to reflectivity and, provided the vertical wind velocity is negligible, the rainfall rate is proportional to reflectivity.

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Peter W. Baker and M. C. Hodson

Abstract

In 1964, R.R. Rogers proposed a method for estimating vertical air velocity in rainfall which is based on the Marshall-Palmer raindrop size distribution. Many investigators have shown that there are deviations from the Marshall-Palmer distribution such that the intercept N 0, and the shape may vary. Essential to Rogers' method is the relationship between reflectivity Z and Doppler fall velocity . A numerical method has been used to obtain versus Z relationships for raindrop size distributions different from those of Marshall and Palmer and to compare these with results obtained using Rogers's method. Thus errors that are likely to occur in estimating vertical air motions have been evaluated and are shown to be a maximum of 1.4 m s−1.

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