Monday, 5 November 2012: 10:30 AM
Symphony I and II (Loews Vanderbilt Hotel)
Tilting of horizontal vortex tubes by the updraft at a gust front has been proposed as a means of getting near-ground rotation and hence a tornado. A simple numerical simulation of a strong density current in a neutrally stratified environment with extreme shear is used as demonstration that this mechanism will not work. Warm air parcels approaching the density current are decelerated by a strong adverse pressure gradient. Thus, the near-surface horizontal vorticity available for upward tilting is greatly reduced by horizontal compression before it is tilted. Consequently, uplifting of vortex tubes produces little vertical vorticity near the ground. One cannot argue that because there is large amount of horizontal vorticity in a surface-based layer in the environment, abrupt tilting of it at an "obstacle" (such as a gust front or topographical barrier) will produce similar strength vertical vorticity very close to the surface. Linear thinking (i.e., assuming that horizontal vorticity is unmodified from environmental values) is misleading in this case because the abrupt tilting is unavoidably associated locally with a stagnation flow that greatly compresses the horizontal vortex tubes prior to tilting.
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