Estimating the path-average rainwater content and updraft speed along a microwave link

Abstract

A recently proposed technique uses the difference between the observed rates of attenuation A with increasing distance at 38 and 25 GHz (A38-25) to estimate the rainwater content W. Unfortunately, this approach is still somewhat sensitive to the form of the drop size distribution. An alternative proposed here uses the ratio A38/A25 to estimate the mass-weighted average raindrop size Dm, itself a useful parameter. Rainwater content is then estimated from measurements of polarization propagation differential phase shift (ΦDP) divided by (1 - R) where R is the mass-weighted mean axis ratio of the raindrops computed from Dm. This paper investigates these two water-content estimators using results from a numerical simulation of observations along a microwave link. It appears that the combination (R, ΦDP) produces more accurate estmates of W than does A38-25. In addition, by combining microwave estimates of W and Ro with the mass-weighted mean terminal fall speed derived using A38/A25, it appears possible to detect the potential influence of vertical air motion on raingage-microwave rainfall comparisons. -from Author