Effect of rain on ground-based microwave radiometric measurements in the 20-90-GHz range

A discrete-ordinate radiative transfer algorithm for a multilayered plane-parallel atmosphere was used to evaluate the effect of raindrops on ground-based vertically pointing radiometric measurements in the 20-90-GHz range. The effect of rainfall rate and drop size distribution, the length of the rain column, the scattering phase function, and cloud liquid water content are examined. Errors in the estimation of columnar integrated vapor and liquid water are evaluated as functions of rainfall rate, length of rain column, and integrated cloud liquid water. Brightness temperatures calculated by the model, using a measured atmospheric state and measured raindrop size distributions, agreed with radiometric measurements at 20, 31, and 52 GHz with a standard error of estimate of 4.1, 7.4, and 6.8 K, respectively. Variances of model brightness temperatures calculated using 1-min-average raindrop size distributions over 10-min periods were also correlated with measured brightness temperature variances.