Study of rain classification and the tendency of gamma DSD parameterizations in Mexico

Abstract

Precipitation is a key component of the hydrologic cycle, so the knowledge and comprehension of its processes of formation and evolution are of paramount importance. This study presents an analysis of microphysical data and drop size distributions (DSD) collected with PWS100 optical disdrometers at four sites in Mexico. Bulk parameters (accumulated water and rainfall intensity) estimated from microphysical data (drop size and fall speed) showed good agreement with observations obtained using rain gauges installed next to the optical devices. Estimates of accumulated water were used to evaluate the efficiency of four different methods of moments for the fitting of the DSD gamma parameters. The outcomes reveal that the second, fourth, and sixth DSD moments method has a better performance (by showing the lowest RMSE value with respect to the measurements) and yields the closest results to those for the Γ–normalized DSD parameterization. Also, a new method for rain classification based on the variation of accumulated water and median volume diameter (D0), which is used as an estimator of the DSD width, is proposed. The method allows for the characterization of rain into three types (convective, stratiform, and shallow), and results can be related to previous observations from DSD parameters (N0 jumps). The study shows that all three gamma DSD parameters (N0, Λ and μ) have large values when the spectra are dominated by large numbers of small and medium-sized drops (low R values), and smaller values when DSD shifts toward large diameters as R increases. Furthermore, the averages of the gamma parameters reveal lower, intermediate and greater values as rain is classified as convective (intense), stratiform or shallow (very light), respectively. The opposite behavior is observed for D0, which agrees with the previous results. These outcomes reveal that continuous research is necessary to refine the tendencies of DSD parameters, even at different places, with rainfall intensity or type of rain and for the improvement of our knowledge of rain processes.