Investigating the Effect of Size Sorting on the Vertical Variation of Rain Drop Size Distributions Using A Collocated FMCW Profiling Radar and Parsivel Disdrometer During VORTEX-SE

Abstract:

Raindrop size distributions (DSDs) vary with height as a result of several processes, including collision, coalescence, evaporation, spontaneous breakup, and size sorting. Storm-relative winds can induce sustained size sorting owing to the different residence times of drops of different sizes as they fall toward the ground, resulting in variable horizontal advection leading to a distinct horizontal spread of drops of different sizes at a given height. Numerous recent studies based on scanning radar observations and numerical simulations have shown that this process can explain certain low-level microphysical features in convective storms. Vertically pointing radars provide an additional source of information about the detailed vertical “footprint” of this process that complements scanning radar observations and can be used to verify simulations. The goal of this study is to analyze observations from a vertically pointing radar and collocated disdrometer.

For this study, we utilized a Frequency Modulated Continuous Wave (FMCW) radar that yields vertical profiles of reflectivity and Doppler velocity from which DSDs can be derived if the terminal velocity-diameter relationship as well as the air motion are known. A collocated instrumented package (PIPS) that includes a parsivel laser disdrometer, as well as instruments to measure relative humidity, reflectivity, temperature, pressure, rain rate, wind speed and direction was used to validate the DSDs derived from the FMCW. These two instruments were deployed during the Verification of the Origin of Tornadoes Experiment-Southeast (VORTEX-SE) field program and were located at the Scottsboro Airport, Scottsboro, Alabama. We examine two convective storm events that passed over the FMCW site on 27 MAR 17 and 30 APR 17, respectively. The 27 MAR case featured a nontornadic supercell thunderstorm whose forward flank passed over the site from south to north, while the 30 APR case featured a weakening QLCS with embedded circulations. We will present preliminary results characterizing the vertical and temporal variation of DSDs in these two events and compare with predictions of idealized models of size sorting owing to the storm-relative winds using near-storm wind profiles derived from observations and model analyses and discuss differences between the two cases.