Abstract:

For the purpose of radar quantitative precipitation estimates, differential reflectivity (ZDR) plays a crucial role and must be accurately calibrated. Currently, some WSR-88Ds in the Next Generation Weather Radar (NEXRAD) fleet may have systematic ZDR biases due to errors in the measurement of the H and V channels. The Radar Operations Center (ROC) monitors these systematic ZDR biases by measuring returns from external targets that should produce or can be adjusted to zero decibels (dB). One such target that has an intrinsic ZDR = 0 dB is Bragg scatter, a clear-air return caused by turbulent mixing in refractive index gradients. The ROC implemented a method the National Severe Storms Laboratory developed to detect Bragg scatter on the WSR-88D. This study uses atmospheric sounding data as truth to verify the radar based Bragg scatter detection method from January to June 2014 (11,521 radar/sounding pairs). Measurements of refractivity gradients and Richardson number from the 00Z sounding (indicators of conditions conducive to Bragg scatter) are compared to radar-based method detections between 00Z and 02Z. Sounding analyses reveal that the potential for Bragg scatter occurs 95.43% of radar/sounding pairs at vertical layers below 5 km in the continental United States (CONUS). However, due to the method’s strict data filters and volume coverage pattern (VCP) requirements, the method only detects Bragg scatter 4.03% of the time (464 radar/sounding pairs). Of the 464 pairs, Bragg scatter detection is verified 84.7% of the time at the same height indicated by the sounding. Climate region characteristics influence variability of verification statistics. We expect that improvements to the data filters for Bragg scatter detection, better use of available VCPs, and improved scanning techniques will increase frequency of Bragg scatter detection.

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