Atmospheric conditions favorable for refraction of the radar beam can produce additional ground clutter return, called anomalously- propagated (AP) return. This AP return is a contaminant within the radar base data fields that causes erroneous estimates of rainfall accumulation, among other artifacts. The WSR-88D quality control system removes AP clutter return by manual application of additional clutter filters. Automation of clutter filter control is a desired goal of the AP Clutter Mitigation Scheme. The AP Clutter Mitigation Scheme is being implementation within the WSR-88D Open Radar Product Generator (ORPG) within Build 2 or 3. The AP Clutter Mitigation Scheme consists of four parts: the Radar Echo Classifier (REC), the Reflectivity Compensation Scheme (Z-Comp), by-pass map augmentation, and clutter filter control. Research efforts to date have concentrated on the REC and Z-Comp.

The Radar Echo Classifier (REC) is an expert system that uses fuzzy-logic techniques to estimate the type of scatterer measured by the WSR-88D. Currently, three algorithms have been designed and tested: the AP Detection Algorithm (APDA) determines anomalously-propagated (AP) ground clutter return, the Precipitation Detection Algorithm (PDA) determines convective and stratiform precipitation regions, and the Clear Air Detection Algorithm defines return from insects in the boundary layer. These algorithms have been developed using data from WSR-88D systems and from the NCAR S-Pol radar. Algorithms to detect sea clutter and to estimate confidence in REC algorithm output are under development. A technique for partioning stratiform and convective precipitation regions is being investigated. Results will be presented.

The Reflectivity Compensation Scheme (Z-Comp) uses a Gaussian approximation for precipitation spectra and a simulated WSR-88D clutter filter to estimate the correction necessary to offset the clutter-filter-induced bias in the reflectivity. Output from the REC PDA determines where the Z-Comp method is applied such that only regions of precipitation are compensated. The Z-Comp method has been tested using WSR-88D time-series data (Archive 1) collected at the Memphis (KNQA) WSR-88D and using NCAR S-Pol data. Results will be presented.

The Radar Echo Classifier has been implemented on the NCAR S-Pol radar and tested during the STEPS and IMPROVE field programs. The Z-Comp algorithm was partially implemented during the IMPROVE field program and is expected to be completed during the field operations of IMPROVE2 during November-December 2001 in Oregon.