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 D-0, which is taken to be a power law function of the rain rate R. However, the D-0-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 D-0 for a given R as compared to the consistently derived D-0-R relationships and to the D-0-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. (C) 1999 Elsevier Science B.V. All rights reserved.