An Examination of the Polarimetric Structure of Tornadoes using a Rapid-scan, Mobile, Doppler Radar

Structural aspects of four tornadoes of varying intensity, including one large, violent EF-5 tornado (24 May 2011), one EF-3 tornado (24 May, 2011), and two weaker tornadoes (25 May 2012) are examined using a rapid-scanning, X-band, polarimetric, mobile Doppler radar (RaXPol). Polarimetric data are used to examine the characteristics of the tornado debris signatures (TDS) including the vertical extent and width of the TDSs, and the spatial and temporal correlation with the weak echo column evident in the reflectivity field. A period of weakening Doppler velocities with the EF-5 tornado corresponded with a reduction in the vertical extent and an increase in the horizontal diameter of the TDS, consistent with debris fall out. Debris shedding radially outward from the TDS was also observed. Constant-radius vertical cross sections of Doppler velocity, reflectivity and cross-correlation coefficient are compared for the various tornadoes and the onset of the TDS and WEH are related to the intensity of Doppler velocities.

Additionally, rapid-scan, near-surface data were collected for a period of 6 minutes over which time 2-second single-elevation PPI updates were available. During this period, a weak reflectivity band associated with a visually observed horizontal vortex developed east of the tornado as it was rapidly intensifying. The horizontal vortex was located along or just ahead of a rear flank downdraft surge and may have been associated with near-ground frictional processes. A correlation was noted between reduced radar-observed reflectivities and increasing radar-observed radial convergence/divergence in the vicinity of the vortex as is strengthened.