Wednesday, 13 September 2000: 2:15 PM
It has been known for a long time that supercells move in a direction to
the left or right of the mean wind. Through a series of numerical
simulations, Rotunno and Klemp found that for a clockwise turning
hodograph, cyclonically rotating supercells propagate rightward primarily
because of the upward-directed dynamic vertical pressure gradient produced
by storm rotation on the right flank of the updraft. Using linear theory,
they also demonstrated that the upward-directed pressure gradient is
associated with the linear effect in a curved hodograph case. They also
illustrated that the nonlinear term is dominant for a straight hodograph.
Unfortunately, there have been few attempts to observationally test Rotunno
and Klemp's supercell propagation theory, owing to coarse space and time
resolution of the data. During VORTEX95, a tornadic supercell near Garden
City, Kansas was intercepted by the VORTEX team. Detailed Doppler radar
data was collected over a 70-min period prior to tornadogenesis. The Garden
City storm provides us with a unique opportunity to perform a complete
perturbation pressure and buoyancy retrievals for a tornadic supercell. The
total perturbation pressure and buoyancy patterns were obtained for all the
analysis times, the perturbation pressure from the linear and nonlinear
effects were retrieved by using a modified retrieval technique. It is found
that the perturbation pressure from the nonlinear terms are responsible for
the rightward movement for the Garden City supercell, which is consistent
with Rotunno and Klemp's theory, since the Garden City storm has a
quasi-straight hodograph.
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