Observed failure modes of the WSR-88D velocity dealiasing algorithm during severe weather outbreaks

The current WSR-88D Velocity Dealiasing Algorithm (VDA) has been in place without significant changes for a number of years. It has proved itself to be a robust and reliable algorithm without a significant number of failures when the PRF used for velocity data collection is high enough to provide a reasonably large Nyquist co-interval (~+/- 25 m/s or larger). However, during some severe convective weather outbreaks, VDA failures have been noted. A study has been performed to find examples of VDA failure during severe weather and identify the causes. Two primary failure modes were detected: 1) harmful effects of low-level residual ground clutter with accompanying near 0 velocities, and 2) high-speed mid-level winds along storm right flanks where the rotating antenna first encounters a large region where all velocities are aliased. Examples of the failure during recent tornado outbreaks will be shown and suggestions for mitigation, some already in the process of being fielded, will be discussed.