An Investigation on Wind-induced Evolution of Raindrop Size Distribution

In this study, we investigated the impact of horizontal wind on the evolution of raindrop size distribution (DSD) and elucidated the governing microphysics. The DSD information plays an important role in a variety of meteorological and hydrological applications (e.g. remote sensing, hydrologic modeling, and climate studies). In these applications, the DSD is typically modeled using static mathematical distributions (e.g. gamma) without considering the DSD evolution throughout the air column. The DSD, however, is shaped from cloud level to the ground by a number of physical processes. These processes include breakup/coalescence of raindrops, local updraft/downdraft, and evaporation/condensation.

We examined potential wind effects on the DSD shape modifications with a goal to shed light on the driving microphysical process. In our analysis, we employed the datasets from the Midlatitude Continental Convective Clouds Experiment (MC3E). The datasets include quality-controlled DSD spectra from Parsivel and 2DVD disdrometers, and rainfall amount and wind velocity measurements. Our analysis revealed a statistically significant effect of wind on the DSD shapes, which will be discussed in our presentation in detail. Very briefly, increased wind speeds modified the DSD shapes by increasing the number of smaller drops and decreasing the number of larger drops. The microphysical process responsible for this phenomenon is the raindrop breakup. There are 3 potential scenarios: (i) spontaneous breakup alone, (ii) collisional breakup alone, or (iii) both spontaneous and collisional breakups together are responsible for the observed phenomenon. Our analysis, details of which will be discussed in our presentation, revealed that the collisional breakup alone was the governing microphysical process for wind-induced DSD modifications.

This material is based upon work supported by the National Science Foundation under Grant No. AGS-1144846.