Rapid-Scan, Polarimetric, Doppler Radar Observations of Supercell Tornadogenesis and Intensification

Rapid-Scan, Polarimetric, Doppler Radar Observations of Supercell Tornadogenesis and Intensification Jana Houser, Howard Bluestein and Jeff Snyder

Despite the availability of more than 4 decades of radar-based tornado observations, the exact sequence of events leading up to and during tornadogenesis remains nebulous and incomplete. One major factor limiting scientific understanding of these processes is the lack of observational data available on time scales comparable to tornado formation. It is generally accepted that tornadoes form over timescales on the order of 10 s (Bluestein et al. 2003). Therefore, observations must be acquired at least this rapidly to enable adequate sampling during tornadogenesis. Unfortunately, technology has not been available to meet the demands of such rapid temporal sampling without sacrificing spatial resolution or data quality until recent years. On 24 May 2011 in Central Oklahoma, a new, state-of-the-art mobile Rapid-Scan X-band Polarimetric (RaXPol) radar successfully collected a dataset of sufficient temporal (15-17 sec volumes) and spatial resolution (1° beam width, 75 m gate spacing at a range < 10 km) to observe tornadogenesis and intensification of the tornado to EF-5 strength. This study examines rapidly-evolving storm-scale processes prior to and during tornadogenesis, and analyzes the development of tornadic rotation. The evolution of the strength and size of the velocity couplet (maximum outbound – maximum inbound Doppler velocity) associated with the tornado and parent mesocyclone is investigated to determine how the rotation associated with the tornado reaches the surface, and how the rotation behaves over time and height while the tornado is intensifying. It specifically answers the question of whether the tornado formed via top-down processes (i.e. the dynamic pipe effect) or from the bottom-up. A weak reflectivity band associated with a horizontal vortex is identified during intensification and is hypothesized to play a role in the rapid intensification of the tornado.

Bluestein, H. B., W.C. Lee, M. Bell, C. C. Weiss, A. L. Pazmany, 2003: Mobile Doppler radar observations of a tornado in a supercell near Bassett, Nebraska, on 5 June 1999. Part II: Tornado-vortex structure. Mon. Wea. Rev., 131, 2968–2984.