3D Wind Field Retrieval Applied to Snow Events Using Doppler Radar

Abstract

The variational analysis method of Laroche and Zawadzki for the retrieval of the 3D wind field from single?Doppler radar data is adapted to analyze stratiform snow events. Modifications include prefiltering of noisy radial velocity data using a velocity azimuth display (VAD) analysis, the use of snow particle fall speeds derived from extended VAD analyses, and the usage of high-elevation angle data. Arctic cases were collected during the Beaufort and Arctic Storms Experiment with X-band radars located at Inuvik and Tuktoyaktuk, Northwest Territories, Canada. These were compared with C-band radar data acquired by the King City Doppler radar in southern Ontario, Canada, a midlatitude site. Retrieved vertical airspeeds were verified using data from a research aircraft. Mean horizontal speeds were verified using VAD analyses, and horizontal wind speed anomalies were checked using Mesonet station data and VAD residual analyses. The results indicate that retrieved vertical velocities are in relative agreement with the aircraft data, and that upward airspeeds tend to occur in higher reflectivity areas. Mean horizontal winds agree with the VAD analyses. The retrieval algorithm detects coarse (>1 km) horizontal wind anomalies due to mesoscale flow features. Regridding of radar data from spherical to Cartesian coordinates induces artifacts that require greater use of the smoothing constraint, especially in winter when stratiform storms are more prevalent. This smoothing precludes the detection of finescale (<1 km) features noted by the Mesonet data and the VAD residual analyses. The stratiform nature of the snow events also results in a greater sensitivity to the continuity constraint compared with the conservation of reflectivity constraint. The retrieval method shows promise in aiding forecasting and nowcasting efforts to form conceptual models of precipitation development and organization in winter situations, especially if data from more low elevation angles are available.