14.8 Do Tropical Cyclones Intensify by WISHE?

Wednesday, 19 August 2009: 5:45 PM
The Canyons (Sheraton Salt Lake City Hotel)
Michael T. Montgomery, NPS, Monterey, CA; and R. K. Smith, S. V. Nguyen, and J. Persing

In this study we seek a basic understanding of tropical cyclone intensification in three dimensions when precipitation and evaporative-cooling (warm rain) processes are included. Intensification with warm rain physics included is found to be dominated by highly-localized deep convective structures possessing strong cyclonic vorticity in their cores - dubbed 'Vortical Hot Towers' (VHTs). Unlike previous studies, the findings herein suggest an intensification pathway that is distinct from the wind-induced-surface-heat-exchange (WISHE) intensification mechanism, which requires a positive feedback between the azimuthal-mean boundary layer equivalent potential temperature and the azimuthal-mean surface wind speed underneath the eyewall of the storm. Intensification from a finite-amplitude initial vortex is shown to not require the 'evaporation-wind' feedback process. Indeed, when the surface wind speed in the sea-to-air vapour fluxes are capped at a nominal (trade-wind) value the vortex still intensifies by the same pathway identified in the main experiments via the generation of locally buoyant VHTs and the near-surface convergence that the VHTs induce within the boundary layer.

The present findings and interpretations challenge the prevailing view that tropical cyclones are premier examples of vortical systems arising from WISHE. Given the potential significance on our understanding of the dynamics of hurricanes, and given the limitations of the present modeling framework, further tests of these predictions are advocated.

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