Doppler Circulation and Areal Contraction Rate as Detected and Measured by a Phased-Array Radar: The El Reno, Oklahoma, Tornado of 31 May 2013

Abstract A single-Doppler technique is developed to utilize data acquired by a phased-array radar that derives observed circulations and areal contraction rates through analyses of circuits at constant radii around the circulation center of a powerful tornadic supercell near El Reno, Oklahoma, on 31 May 2013. Since the rapid-scan, phased-array radar measures only the component of 3D flow in the radar viewing direction, velocity normal to the beam is set to zero to enable the calculation of circulations and areal contraction rates of circles within a surface of constant elevation angle. To obtain reliable standard measures of vortex strength, mean Doppler velocities are bilinearly interpolated to points on circles of specified radii concentric with circulation centers. The Doppler circulations around and material contractions of these circles are then calculated. Circulation is large and flow is convergent just prior to tornado formation. At its maximum size and intensity, the circulation of the tornado was very large, 9 × 10 5 m 2 s −1 at a height of 1.5 km. As the tornado’s size increased, the divergence within it increased. During the tornado’s mature stage, areal rates of vortex core change from expansion to contraction to expansion over a period of 12 min. The use of rapid volumetric updates of approximately 1 min (or faster) available with phased-array radar (PAR) technology to track significant Doppler circulations and areal contraction rates may provide indications of imminent tornadogenesis and important updates of tornadoes in progress, though much future work remains to quantify any potential warning implications. Significance Statement This paper uses phased-array radar data that can fully scan a thunderstorm every 60–90 s and radar-estimated kinematic parameters to examine the rapidly evolving characteristics of a large tornado near El Reno, Oklahoma, on 31 May 2013. This tornado is the widest on record. We observed large circulation and convergent flow just prior to the tornado and more divergent flow in the tornado as its size increased. This investigation indicates that phased-array radar and circulation estimates could become useful operational tools.