Using the NWRT PAR to evaluate temporal sampling during two rapidly evolving tornado events

During severe weather events, a tornado may develop on the order of minutes or seconds. Operational radars such as the WSR-88D are capable of detecting tornadic vortex signatures (TVSs), but the WSR-88Ds are limited to using predefined volume coverage patterns with an update interval of 4.5 min or longer. Such update times are insufficient to track the rapid evolution of TVSs that persist for only a few minutes. Additionally, more frequent volumetric updates are needed to detect and monitor the rapid evolution of radar-based signatures that may indicate tornadic development.

This study uses data from the National Weather Radar Testbed Phased-Array Radar (NWRT PAR) to evaluate the impact of rapid sampling during two short-lived tornado events. In each event, volumetric updates were obtained with a maximum update time of 60 s; this scanning method provided frequent updates on the evolution of the observed circulations. On 19 August 2007, 45-s updates depicted the life cycle of a circulation associated with a tornado that formed between 0144 and 0147 UTC. Two minutes prior to tornado development, strong gate-to-gate shear of 40—50 m s^-1 was found over a depth of 2 km, and this shear persisted through a 10-min period including the tornado lifetime. A second circulation was sampled on 07 May 2008, when a mesoscale convective vortex (MCV) developed in the vicinity of a surface cyclone. A tornado developed at the western edge of the MCV and remained on the ground through the period 2221—2226 UTC. Strong gate-to-gate shear in excess of 30 m s^-1 was detected at 1.5 km AGL as early as 2217 UTC, providing indications that a strong circulation developed several minutes before the tornado reached the ground.

To examine the impact of sampling intervals on the evolution of these circulations, the original NWRT PAR data from both events are modified to produce temporal updates that are comparable with WSR-88D scanning strategies. Changes in gate-to-gate shear within the TVS are measured to compare the depiction and evolution of the tornadic vortex signatures. In addition, the positions of the TVSs are compared to evaluate the improvement that rapid sampling provides when tracking the location of a possible tornado.