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Morris A. Bender


Asymmetric structure of tropical cyclones simulated by the Geophysical Fluid Dynamics Laboratory high-resolution triply nested movable-mesh hurricane model was analyzed. Emphasis was placed on the quasi-steady component of the asymmetric structure in the region of the eyewall. It was found that the asymmetry was primarily caused by the relative wind, that is, the flow entering and leaving the storm region relative to the moving storm. A set of idealized numerical experiments was first performed both with a constant and a variable Coriolis parameter ( f ) and the addition of basic flows that were either constant or sheared with height. Analysis was then made for one case of Hurricane Gilbert (1988) to demonstrate that the quasi-steady asymmetric structure analyzed in the idealized studies could be identified in this real data case.

Vorticity analysis in the variable f experiment indicated that quasi-steady asymmetries resulted in the eyewall region through the effect of vorticity advection due to differences between the beta gyre flow in the lower free atmosphere and the storm motion. This was roughly matched with a persistent area of divergence and vorticity compression in the lower free atmosphere ahead of the storm and enhanced convergence and vorticity stretching to the rear. An asymmetric structure in the upward motion and accumulated precipitation, when averaged over a sufficiently long period of time, exhibited a corresponding maximum in the eyewall’s rear quadrant.

With the addition of an easterly basic flow, a pronounced change in the asymmetry of the time-averaged boundary layer convergence resulted, with maximum convergence located ahead of the storm. However, the asymmetries in the average vertical motion in the middle troposphere and accumulated precipitation were more affected by the convergence field in the lower free atmosphere produced by the relative flow there. The relative flow depended on both the basic and beta gyre flow. With the addition of an easterly vertical shear to the easterly basic flow, the storm moved faster than the lower-level winds, and strong relative wind was from the front to the rear in the lower free atmosphere and from the opposite direction in the outflow layer aloft. As a result, the upward motion was significantly increased in the front of the storm and reduced in the rear, and the precipitation maximum shifted to the left front quadrant.

Overall, analysis results suggest that the flow relative to the storm motion is an important factor contributing to the formation of quasi-steady asymmetries in the convergence and vertical motion fields, as well as in the mean precipitation pattern of tropical cyclones.

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