Radar Snowfall Estimates at NASA Wallops Flight Facility

A field study of falling snow was conducted through National Aeronautics and Space Administration (NASA) S-band dual-polarization scanning (NPOL) and K-band vertically pointing (MRR) radars, and a network of disdrometers and weighing bucket gauges at the NASA Wallops Flight Facility (WFF), Wallops Island, Virginia during the 2014 winter season. The study was part of the Global Precipitation Measurement (GPM) mission ground validation effort to provide a better parameterization of the microphysics of falling snow for GPM algorithm developers. Under the umbrella of physical validation, GPM algorithm developers use disdrometer based hydrometeor size distributions to test the parametric form of the size distribution from dual frequency radar measurements on board the GPM core satellite. In the presence of snow, the snow water equivalent (SWE) is estimated through derived snow size distributions with prescribed snow densities. This study utilizes collocated disdrometer and precipitation gauge measurements from eight snow events to derive snow density, snowfall velocity, and snow size distribution, from which reflectivity is calculated at S-band frequency. At WFF, there are three different types of disdrometer measurements that are analyzed and are comprised of an automated Parsivel2 unit (APU), a two-dimensional video disdrometer (2DVD), and a Precipitation Imaging Probe (PIP). It should be noted that the first two instruments were developed primarily for rain measurements while the latter is a snow disdrometer. Each disdrometer measures the size and fall velocity of individual flakes but the measurement principles are diferent between the disdrometers. The SWE, on the other hand, is directly measured through the precipitation gauges. The snow density, which is a critical parameter in calculation of reflectivity, can be estimated through joint disdrometer and precipitation gauge measurements. A comparative study between disdrometer derived, Zcal, and NPOL measured reflectivity, Zmea, helps to determine the accuracy of the retrieved snow density. Derived empirical relationships of Zcal-SWE will be applied to NPOL and operational dual-polarized radar for mapping the precipitation for direct comparison of satellite precipitation estimates.