Extended Abstract (64K)7C.2
Understanding the dynamics of vertically sheared hurricanes
Paul D. Reasor, NOAA/AOML/HRD, Miami, FL; and M. T. Montgomery
This work investigates the role of vorticity axisymmetrization in the vertical alignment of tropical cyclones forced by vertically-sheared flow. Recent theoretical studies of tilted geostrophic vortices have identified a linear free-alignment mechanism in which the decay of vortex tilt occurs through the redistribution of vorticity by sheared vortex Rossby waves. The theory is extended here to hurricane-strength vortices using both the Asymmetric Balance and Primitive Equation models. To the extent that one can represent the hurricane tilt as a small but finite amplitude perturbation to an otherwise aligned vortex, the dynamics of hurricane alignment in the absence of diabatic sources is accurately explained by the vortex Rossby wave mechanism. How one determines the free-alignment rate from the linear theory will be our focus here.
A problem perhaps more relevant to tropical cyclone forecasting is how vortex tilt evolves in the presence of sustained forcing by environmental vertical shear. A simple forcing is included in the linear model to mimic a uni-directional vertical shear flow. Vertical shear is shown to act as a vortex Rossby wave generator. The accuracy of the linear, vortex Rossby wave model is demonstrated through comparison with fully nonlinear results. In cases where the free-alignment rate of the vortex is large, the vorticity asymmetry generated by differential advection of the vortex by vertical shear rapidly disperses as sheared vortex Rossby waves. Consequently, the vortex remains vertically coherent. How the linear model might shed light on the boundary between alignment and irreversible tilting of vortices in shear will be discussed.
Session 7C, Tropical Cyclones: Vertical Shear (Parallel with Sessions 7B and 7D)
Tuesday, 30 April 2002, 4:00 PM-5:30 PM
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