Verification of the Bragg Scatter Method on the WSR-88D

For the purpose of radar quantitative precipitation estimates, differential reflectivity (ZDR) plays a crucial role and must be accurately calibrated. Currently, some Weather Surveillance Radars – 1998 Doppler (WSR-88Ds) in the Next Generation Weather Radar (NEXRAD) fleet may have systematic ZDR biases in H and V channel measurements. 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 developed by the National Severe Storms Laboratory to detect Bragg scatter on the WSR-88D.

This study uses atmospheric sounding data as ground 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 detections between 00Z and 02Z. Sounding analyses reveal that the potential for Bragg scatter occurs in 95% of the radar/sounding pairs at vertical layers below 5 km in the continental United States. However, due to the method's volume coverage pattern (VCP) requirements and strict data filters, the method only detects Bragg scatter 4.03% of time (464 radar/sounding pairs). The primary cause of the low number of detections is the radar operates in a VCP not used by the radar based method in 43% of all radar/sounding pairs. Furthermore, precipitation and biota make potential Bragg scatter cases unusable for systematic ZDR bias estimation. Of the 464 pairs, Bragg scatter detection is verified on average 85% of the time at the same vertical layer indicated by the sounding. We expect to increase the number of Bragg scatter detections by improving data filters, improving scanning techniques, and performing 24 hour/day monitoring, as opposed to the current 2 hour/day monitoring.