Multiple cyclic tornado production modes in the 5 May 2007 Greensburg, Kansas supercell storm

Multiple cyclic tornado production modes in the 5 May 2007 Greensburg, Kansas supercell storm

Robin L. Tanamachi

CIMMS / School of Meteorology

Monday, 6 December 2010, 11:30 a.m.

NWC 5930
A cyclic, tornadic supercell (“the Greensburg storm”) generated at least 22 tornadoes in southwest Kansas on 5 May 2007. One of these was the first documented EF-5 tornado (“the Greensburg tornado”), which destroyed 95% of the buildings in Greensburg, Kansas and caused 11 fatalities. The University of Massachusetts X-band, polarimetric, mobile Doppler radar (UMass X-Pol) collected data in the first 10 tornadoes produced by the Greensburg storm. During this time, a transition in the tornado production mode occurred, from short-track cyclic tornado production (mode I), to long-track, single (plus satellite) tornado production (mode II). This transition has only been documented a handful of times before by close-range Doppler radars.
In this study, the UMass X-Pol data (as well as contemporaneous data from the WSR-88D at Dodge City, Kansas, or KDDC) are examined with the aim of uncovering changes in the Greensburg storm and its tornadoes during the time that this transition occurred. A few UMass X-Pol data quality issues will be briefly addressed. In particular, it is found that:

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  Tornadoes produced via mode I tracked left with respect to the updraft motion, while the motion of the Greensburg tornado and its satellites (mode II) matched that of the updraft more closely.
  
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  The vortex signatures in the UMass X-Pol data are matched with the surveyed damage tracks. In addition, several non-tornadic vortices are documented.
  
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  A forward surge and retreat of the RFGF was documented a few minutes before the development of the Greensburg tornado.
  
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  Weak-echo holes are documented in several tornadoes, and found to be consistently collocated with corresponding vortex signatures in azimuth but biased slightly away from the radar in range.
  
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  A polarmetric tornadic debris signature is found near the surface in the mature Greensburg tornado. Other polarimetric supercell features (e.g., a Z_DR arc) are consistent with those found in previous studies.
  

Results of EnKF analyses, in which KDDC and UMass X-Pol data are assimilated, are also discussed. Both inclusion of the UMass X-Pol data in assimilation cycles and an initial environmental horizontal velocity profile containing a low-level jet result in near-surface vortices that are stronger, deeper, and longer-lived than in experiments without. Backward trajectory analyses are used to infer the likely origins of air being drawn into the tornadoes at low levels.