Validation of NEXRAD Radar Differential Reflectivity in Snowstorms with Airborne Microphysical Measurements: Evidence for Hexagonal Flat Plate Crystals

In situ measurements of winter storms with the Canadian National Research Council Convair 580 aircraft are being used to validate hydrometeor classification and differential reflectivity (ZDR) measurements made with NEXRAD dual-pol radars in Cleveland (KCLE) and Buffalo (KBUF) during the 2012 and 2013 winter seasons. The Convair is equipped with a full suite of thermodynamic, cloud microphysical, particle imaging, and radar (W-band and X-band) sensors. The Convair was flown within sensitive radar range to identify the presence of supercooled water to make comparisons to hydrometeor types based on NEXRAD observations. The main goal of these comparisons is to determine the quality of the dual-pol radar observations and assess their utility for the detection of icing hazards to aircraft.

In a Convair flight on February 28, 2013 near Buffalo, New York, an abundance of hexagonal flat plate crystals were detected by the PMS particle imaging 2DC probe, in regions characterized by NEXRAD ZDR values as large as +7 dB. This crystal type is of particular interest because it presents the most anisotropic response to dual-pol observations, amongst all hydrometeors. Though hexagonal flat plate crystals are commonly observed in laboratory cold box experiments, they are not ubiquitous in aircraft observations of natural clouds. The likelihood of detection in this instance may have been enhanced both by the radar observations that were used in real time to guide the aircraft tracks, and also by the occluded low that served to homogenize the microphysics in the region of detection. Characteristics of NEXRAD ZDR and hydrometeor classification associated with regions of in situ hexagonal plate observations will be discussed.

This work was sponsored by the Federal Aviation Administration under Air Force Contract No. FA8721-05-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Government.