Estimating the Path-Average Rainwater Content and Updraft Speed along a Microwave Link

Abstract

While there are many microwave techniques proposed for measuring the rate of rainfall in still air (Ro) there is a scarcity of methods for accurately estimating the mass of rainwater rather than its flux. 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??) where ? is the mass-weighted mean axis ratio of the raindrops computed from Dm. In lieu of anticipated future experiments, this paper investigates these two water-content estimators using results from a numerical simulation of observations along a microwave link. From thew calculations it appears that the combination (?, ΦDP) produces more accurate estimates 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.