Thursday, 30 October 2008
Madison Ballroom (Hilton DeSoto)
Boundaries likely have a significant role in tornadogenesis. The rear flank downdraft (RFD) of supercells introduces a storm-scale surface boundary beneath the updraft. Though supercells are prolific tornado producers, elevated rotation is not necessary for tornadogenesis in many cases. Kinematic and thermodynamic boundaries with horizontal shear can develop vertically oriented vorticity that can be stretched into a tornado-like vortex. Vortices of this nature have been observed (e.g., Wilson 1986; Pietrycha and Rasmussen 2004; Murphey et al. 2006) and simulated (e.g., Lee and Wilhemson 1997a, b). They have also been observed along the rear flank gust front (RFGF) (e.g., Bluestein et al. 2003). Small changes in the thermodynamic and kinematic gradients along the RFGF may alter the size and depth of the vorticity generated. It is hypothesized that the magnitude of RFGF gradients alters vertical vorticity production thereby affecting the low-level mesocyclone vorticity budget and could determine tornadogenesis.
Until recently, fine-scale resolution in-situ data sets were difficult to attain due to the safety concerns that the near-mesocyclone environment poses to observers. During the spring seasons of 2007 and 2008, the Texas Tech StickNet probes provided multiple high resolution data sets in the vicinity of supercell mesocyclones and tornadoes. These data sets are complimented by mobile mesonets (MMs) that, in one 2008 case, were able to measure the environment near a non-mesocyclone tornado that developed along the RFGF of a supercell. The MM data set and the numerical studies of non-supercell tornadoes will be used as a springboard to compare the near-tornado thermodynamic and kinematic gradients sampled by StickNet. Using a standard objective analysis technique, thermal and kinematic gradients for each case will be graphically compared to ascertain correlations between these gradients and vertical vorticity. Radar radial velocity will be used for verification where possible.
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