Geographical and Seasonal Availability of Light Rain, Dry Snow, and Bragg Scatter to Estimate WSR-88D ZDR System Bias

In 2013, the United States' National Weather Service completed upgrading its fleet of Weather Surveillance Radars – 1988 Doppler (WSR-88D) to dual polarization.  In addition to the standard base data moments reflectivity (Z), velocity (V), and spectrum width (SW), dual polarization provides estimates of differential reflectivity (Z_DR), differential phase (PHI_DP), and correlation coefficient (RHO_HV).  Of the three new parameters, accurate measurement of Z_DR is especially important because it, in combination with Z, is used by the WSR-88D's Quantitative Precipitation Estimate (QPE) algorithm to estimate rain rate.  While the WSR-88D requires a calibration of Z to within 1 dBZ, the required accuracy for Z_DR is 0.1 dB for the QPE algorithm to obtain optimal precipitation estimates.  To arrive at a true Z_DR value, the WSR-88D calculates and applies an offset to the measured Z_DR based on measurements of receiver, transmitter, and antenna path biases in the horizontal and vertical channels. The Z_DR offset obtained from the “engineering-derived” method cannot be easily verified.  As an independent means of estimating system Z_DR bias, the authors evaluate radar returns from light rain, dry snow, and Bragg scatter. Bragg scatter should inherently have a Z_DR bias of 0 dB; light rain and dry snow can be empirically adjusted to have a Z_DR bias ~0 dB.  These conditions being mutually exclusive provide three independent methods for estimating system Z_DR biases.

The Radar Operations Center (ROC) recently added the capability to store base data moments and dual polarization fields for a full year for the WSR-88D fleet.  This allows us to examine seasonal and regional availability of each of the external methods to determine system Z_DR bias.  Furthermore, the three methods may be compared with each other at individual radar sites to check for consistency between them.  Preliminary results from a partial year for the lower 48 states indicate the dry snow method is available in both warm and cool seasons from nearly all sites; the Bragg scatter method is generally available from most sites in the warm and cool seasons but may be unavailable in south and central US due to biota contamination.  The light rain method is most commonly detected in the eastern US regardless of season.