A Case Study of Supercell Propagation

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.