Validation of Gamma Raindrop Size Distribution Estimates Using Approximate Expressions with a Vertically Pointing Very-High-Frequency Radar

Characterizing the size distribution of raindrops is fundamental to a variety of applications, including radar-based quantitative precipitation estimation. Atmospheric radars or wind profilers can be used to measure the drop size distribution (DSD) by analyzing the Doppler spectrum, which is inherently linked to raindrop velocity. This is achieved by mapping the Doppler spectrum from velocity space into diameter space directly. Since the general Gamma distribution is extensively used to model the DSD characteristic by numerous researchers in the meteorological community, it can be retrieved from the Doppler spectrum by applying appropriate relationships between drop diameter and terminal velocity. In this study, a retrieval method based on an approximate analytical solution was validated with both simulated data and very-high-frequency (VHF) radar observations, where the DSD followed the Gamma distribution. The advantage of using analytical solutions is their computational efficiency for the real-time processing of large data sets. In order to verify the applicability of this method, the mass-weighted mean drop diameter Dm, which is associated with the parameters of the Gamma DSD, was used to present the results. Simulations showed that the retrieval method is effective for 0.7 mm <Dm< 4 mm, with errors decreasing as the signal-to-noise ratio (SNR) increases. Furthermore, comparisons between radar data and simultaneous disdrometer observations revealed that the precipitation parameters retrieved from the VHF radar at 1.65 km maintain moderate correlations with the ground-based in situ instrument measurements. Whether for stratiform or convective precipitation, this retrieval method produced reasonable estimates of aloft precipitation parameters.