It is difficult for the algorithm to vertically associate two widely separated circulations as being part of one mesocyclone. It would be useful for the algorithms to adjust their vertical association techniques when the vertical shear is very strong, as in this case. The shear information could be obtained from the WSR-88D Velocity Azimuth Display (VAD) vetical wind profile, or from a Near-Storm Environment (NSE) algorithm reading in data from a mesoscale model.
Also, in many of the Minnesota storms, circulations were only visible on the lowest elevation scan. Having only one 2D circulation brings up an entirely new set of problems. Namely, the algorithm must have a criteria for determining when a 2D detection is strong enough to warrant classification as a 3D mesocyclone (Under non-low-top circumstances a 2D feature would not be considered a mesocyclone without vertical continuity).
Presented here are various WSR-88D radar images from the Minneapolis, MN radar (KMPX). Included in some of the WSR-88D images, there is output from NSSL's Mesocyclone Detection Algorithm (MDA). A yellow circle depicts a mesocyclone. The red-in-yellow circle depicts a mesocyclone whose base is at the lowest elevation scan(where it is a more likely tornado threat).
Judging from the Storm Relative Velocity Loop, it appears that this storm had a strong mid-level mesocyclone whose rotation was brought to the ground when the storm collapsed. This would be why the shear signature is much stronger preceeding the time of the tornado. The strongest shear had dropped below the view of the radar beam; in other words, Stearns Co. saw a descending mesocyclone versus the ascending mesocyclond seen near Cyrus.
The Closer Inspection images are especially interesting. The storm relative velocity portions of the two images illustrate the cyclonic flow surrounding this intense mesocyclone (red in yellow circle).
By matching damage reports to the radar data it appears that the tornado touched down around 2338 UTC and lasted to 2351 UTC.
The case was presented at the 28th Conf. on Radar Meteorology as the paper entitled Doppler radar algorithm performance during a highly sheared tornado outbreak by E. Howieson, B. Grant, G. J. Stumpf, and D. W. Effertz.
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