Dependence of rainfall interception on drop size - A comment

In a series of three articles Calder, Hall and colleagues have recently presented an extension of Calder's original stochastic model of rainfall interception. In their model, the dependence of rainfall interception on raindrop size is explicitly taken into account. All raindrops are assumed to have the same characteristic size, the so-called median-volume raindrop diameter D₀, which is taken to be a power law function of the rain rate R. However, the D₀-R relationship used in their model results from an erroneous interpretation of the general theory of the raindrop size distribution recently proposed by Sempere Torres and colleagues. As a result, this relationship is not consistent with the postulated underlying raindrop size distribution. Moreover, it seriously underestimates D₀ for a given R as compared to the consistently derived D₀-R relationships and to the D₀-R relationship reported in a classical article on the subject of raindrop size distributions. This may seriously affect the model results presented by Calder, Hall and colleagues.