May 22 - July 30
Using Bragg Scatter to Estimate Systematic Differential Reflectivity Biases on Operational WSR-88Ds
Nicole Hoban, Jeffrey Cunningham, and David Zittel
What is already known:
- Differential reflectivity (ZDR) plays an important role in rain rate estimations but is known to be vulnerable to radar calibration errors.
- At constant reflectivity, a 0.25 dB bias in ZDR will yield a 22% error in rain rate estimates for the rain rate equation currently implemented in the WSR-88D radar product generator.
- Bragg scattering typically has an inherent ZDR near zero dB and can be used to estimate systematic ZDR biases.
What this study adds:
- Comparisons of systematic ZDR biases between the weather method and Bragg scattering yields similar values.
- An automated method of estimating systematic ZDR biases from Bragg scatter has been developed for use on operational WSR-88Ds.
Abstract:
This study examines the feasibility of using Bragg scatter to estimate systematic differential reflectivity (ZDR) bias- es on operational WSR-88Ds. ZDR greatly impacts rain rate estimates. At constant reflectivity, a 0.25 dB bias in ZDR will yield a 22% error in rain rate estimates for the rain rate equation currently implemented in the WSR-88D radar product generator. Prior to this study, the Radar Operation Center (ROC) used plan position indicator scans of light rain (i.e. “scanning weather method”) to monitor systematic ZDR biases on a fleet of 159 operational WSR- 88Ds. While the scanning weather method is reliable for identifying radar calibration trends, it is too imprecise for absolute ZDR calibration because systematic ZDR biases estimates from the scanning weather method are subject to big drop contamination. Data filters based on single and dual polarization variables and two statistical filters were used to isolate Bragg scatter from clutter, biota, and precipitation. Six radars were examined in detail for May and June 2013 from 1400-2200 UTC each day. Systematic ZDR biases estimates from Bragg scatter were compared to reliable estimates from the scanning weather method. Bragg scatter derived systematic ZDR biases were compara- ble to those estimated by the weather method; most cases were within 0.20 dB. With these filters, Bragg scattering was found most frequently between 1400-2200 UTC. More cases of Bragg scattering were found in May than in June. This study demonstrates that Bragg scattering offers an alternative method for monitoring systematic ZDR biases on the WSR-88D fleet.