High Temporal Resolution Polarimetric Radar Observations of the 20 May 2013 Newcastle-Moore, Oklahoma EF-5 Tornado using the PX-1000

On 20 May 2013, the cities of Newcastle and Moore, Oklahoma were impacted by a long-track violent tornado that was rated as an EF-5 on the enhanced Fujita scale by the National Weather Service. A plethora of radar systems in and around the Norman/Oklahoma city area observed this tornado and its parent supercell at varying distances and with varying scanning strategies and capabilities. Despite a relatively sustained long track, damage surveys revealed a number of small-scale debris features that hint at storm-scale processes that occurred over very short time periods. The University of Oklahoma Advanced Radar Research Center's PX-1000 transportable, polarimetric, X-band weather radar was operating in a single-elevation PPI scanning strategy at the OU Westheimer airport throughout the duration of the tornado, collecting high spatial and temporal resolution polarimetric data every 20 seconds at ranges as close as 10 km and heights below 500 m AGL. This unique dataset contains the only known polarimetric radar observations of the Newcastle-Moore tornado at such high temporal resolution, providing the opportunity to analyze and study fine-scale phenomena occurring on very rapid time scales. This paper specifically analyzes a series of debris ejections and RFD surges that both preceded and followed a distinctive loop of the core tornadic vortex as it weakened over the Moore Medical Center before rapidly accelerating and re-strengthening to the east. The gust front structure, debris/hydrometeor microphysics, and ZDR arc breakdown are explored as strong evidence for a “failed occlusion.” Observations are supported by rigorous hand-analysis of critical storm features, including debris loading, debris volume, and the appearance of a large area of maximum correlation coefficient values during peak tornado intensity, indicative of very strong debris centrifuging. A conceptual description and illustration of the suspected failed occlusion process is provided, and its implications are discussed.