P2G.5 Tropical cyclone steering as a potential vorticity advection process: the role of cumulus parameterizations in defining an optimum steering column

Thursday, 1 May 2008
Palms ABCD (Wyndham Orlando Resort)
Brett Thomas Hoover, University of Wisconsin, Madison, WI; and M. C. Morgan

In a purely barotropic framework, tropical cyclone (TC) steering can be thought of as advection of the TC's vortex by the environmental flow. Typically, a 'steering column' is defined as a horizontal and vertical range over which an average of the horizontal winds in the vicinity of the TC can be performed to separate the symmetric TC flow from the asymmetric environmental flow. Special attention has been paid to the vertical range of this steering column in previous research.

Wu and Wang (2000) and Chan et al (2002) relate TC motion to potential vorticity (PV) tendency, with horizontal advection of PV by environmental flow being the dominant term in PV tendency for steady TC motion. Since the PV structure of a TC is dictated by the distribution of latent heat release, the choice of convective parameterization scheme could potentially play a large role in the steering of a modeled TC.

A set of numerical modeling experiments is performed to simulate the growth of a TC from tropical depression to hurricane strength. Two simulations are performed with identical physics and initial conditions but differing convective parameterization schemes. The steering of these two simulations is then analyzed from the perspective of TC steering as a PV advection process. Attention is paid to the time surrounding the track-split between the two simulations, and the roles of steering versus non-steering contributions to PV tendency are investigated. Differences in the PV structure of the two simulations are observed, and a relationship between TC PV structure and optimum steering column structure is presented, with observations of how that relationship changes as the TC intensifies and its PV structure changes.

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