Rapid-scan observations of a bow echo storm with a dual-polarization WSR-88D

On 27 May 2008, supercellular convection across western Oklahoma congealed into a severe mesoscale convective system (MCS). The research polarimetric prototype WSR-88D in Norman (KOUN) collected data during this event. As the MCS approached Norman, KOUN began collecting volumetric 60° sector scans. This is the first time that a WSR-88D collected polarimetric data of a severe storm with such high temporal resolution. This rapid scan strategy allowed for 94-second volume updates within the sector, providing comparable temporal resolution to measurements from the Hazardous Weather Testbed Multifunction Phased Array Radar (MPAR), as well as higher spatial resolution, with polarization diversity. Of particular interest is the formation of a rear inflow jet (RIJ) that preceded the “bow” echo. An analysis of the evolution of the polarimetric variables provides microphysical insight about the effects of a developing RIJ. Evaporation and size sorting of precipitation particles falling into the subsaturated RIJ leads to a decrease in the observed reflectivity factor (Z_H) and specific differential phase (K_DP) with an increase in differential reflectivity (Z_DR). The negative buoyancy produced by evaporative cooling causes the RIJ to descend, as evident by the descending Doppler velocity extremum. The data suggest that erosion of K_DP at the rear of the storm may precede the erosion of Z_H, providing forecasters with earlier clues of a developing RIJ.