The refraction characteristics of weather radar signals are dependent on the vertical profiles of temperature and humidity in the lowest few kilometers of the atmosphere. Under certain atmospheric conditions, the signal is refracted into the ground, causing ground clutter echoes that contaminate weather radar base data fields. This anomalous propagation (AP) clutter contamination causes serious problems for automated operational algorithms such as precipitation estimation and microburst detection. Ground clutter filters are able to remove the clutter contamination in most situations. However, the filters also introduce biases into the base data with undesirable consequences within precipitation echo. It is therefore not optimal to apply clutter filters globally. Currently in the Weather Surveillance Radar - 1988 Doppler (WSR-88D) system, expert operators must identify AP ground clutter and manually turn on the clutter filters in the appropriate regions. No effort is made to correct for the clutter filter bias. The AP Clutter Mitigation Scheme has been developed with the goal of optimal, automated clutter filter control and bias correction. The result will be improved weather radar base data quality benefiting a wide range of end users including, hydrology, numerical model initiation and air traffic control. Radar operators will no longer have the time consuming and tedious responsibility of clutter filter control.

The AP Clutter Mitigation Scheme consists of four parts: the Radar Echo Classifier (REC), the Reflectivity and Radial Velocity Compensation Scheme (ZV-Comp), Clutter Filter Control and Clutter Map Augmentation. The Radar Echo Classifier (REC) is an expert system that uses fuzzy-logic techniques to estimate scatterer type from WSR-88D measurements. Currently, three algorithms have been designed and tested: the AP Detection Algorithm (APDA) identifies anomalously-propagated (AP) ground clutter return, the Precipitation Detection Algorithm (PDA) identifies convective and stratiform precipitation regions, and the Clear Air Detection Algorithm identifies return from insects in the boundary layer. These algorithms have been developed using data from various WSR-88D systems and from the NCAR S-Pol radar. Algorithms to detect sea clutter and partition stratiform and convective precipitation regions are currently under development. The ZV-Comp algorithm uses a lookup table of corrections produced using a series of Gaussian approximations to precipitation spectra and a simulated WSR-88D ground clutter filter. The ZV-Comp algorithm has been verified using WSR-88D time series data and S-Pol base data. The output of the REC is used to apply ZV-Comp only to regions identified as precipitation, thereby avoiding compensating clutter filter residue. Clutter Filter Control and Clutter Map Augmentation are topics for future investigation.

The AP Clutter Mitigation Scheme is a planned implementation within the WSR-88D Open Radar Product Generator (ORPG). A brief overview of the techniques as well as updated results will be presented.