Mobile, X-band, Polarimetric Doppler Radar Observations of the 4 May 2007 Greensburg, Kansas, Tornadic Supercell

Abstract

n 4 May 2007, a supercell produced an EF-5 tornado that severely damaged the town of Greensburg, Kansas. Volumetric data were collected in the ?Greensburg storm? by the University of Massachusetts X-band, mobile, polarimetric Doppler radar (UMass X-Pol) for 70 min; 10 tornadoes were detected. This mobile Doppler radar dataset is one of only a few documenting an EF-5 tornado and the supercell?s transition from short-track, cyclic tornado production (mode 1) to long-track tornado production (mode 2). Using bootstrap confidence intervals, it is determined that the mode-2 tornadoes moved in the same direction as the supercell vault. In contrast, the mode-1 tornadoes moved to the left with respect to the vault.From polarimetric data collected in this storm, the authors infer the presence of large, oblate drops (high ZDR, high ?hv) in the forward flank and surrounding some of the tornadoes. The authors speculate that the weak-echo column (WEC) in the Greensburg tornado, which extended above 10 km AGL, was caused primarily by the centrifuging of hydrometeors at low levels and rapid upward transport of relatively scatterer-free air at upper levels. This WEC was collocated at low levels with a low-ZDR, low-?hv column, indicating lofted debris.Dual-Doppler analyses, generated at ~10-min intervals using data from UMass X-Pol and the Dodge City, Kansas, Weather Surveillance Radar-1988 Doppler (WSR-88D), were used to locate updrafts and downdrafts near the hook echo. In the immediate vicinity of tornadoes, diminished ZDR values downstream of analyzed downdrafts may indicate the ingestion by tornadoes of relatively small drops, fallout of larger drops, or a combination of both.