Towards improving the Clutter Mitigation Decision algorithm in the WSR-88D radars

The Clutter Mitigation Detection (CMD) algorithm automatically detects ground clutter in real-time so that a clutter filter can be automatically applied as needed. The CMD is operational on the National Weather Service’s WSR-88D radar network. One of the biggest challenges for ground clutter mitigation is the detection and removal of clutter in mixed precipitation/clutter regions with a low clutter to signal ratio (CSR), i.e. the clutter echo is much weaker than the overlaying precipitation echo. The goal of this work is to improve the probability of detection (POD) of ground clutter with low CSR while not substantially increasing the false alarm rate (FAR) elsewhere.

Three strategies have been developed and tested that increase the POD of CMD in mixed clutter/precipitation cases. The first compares the clutter filtered RHOHV to the unfiltered RHOHV and identifies clutter where the filtered RHOHV is higher by a threshold. This ‘RHOHV-test’ was implemented on the WSR-88D radars starting in 2019. The second strategy improves the membership functions of the spatial standard deviations of Zdr and PHIDP. Adaptive membership functions were developed that account for the increase in measurement variance of Zdr and PHIDP as RHOHV decreases. Thus, appropriate membership functions are used for echoes with different RHOHV values. The third strategy is to apply a dynamic weight to the Clutter Phase Alignment (CPA) input to CMD. The CPA is an excellent indicator of pure clutter environments but is more ambiguous in clutter mixed with precipitation. The new dynamic weight utilizes CPA in pure clutter cases and phases out the weight in mixed cases. The addition of the dynamic membership functions for standard deviations of Zdr and PHIDP plus the dynamic weight for CPA result in an increase in POD of over 40% with no appreciable increase in the FAR on the cases tested.

This presentation will describe the challenge of low CSR clutter, the recent improvements made to CMD and show the results on several cases of WSR-88D data with widespread low CSR mixed clutter/precipitation regions.