Principal Investigator: Lloyd J. Shapiro
Collaborating scientist(s):
James L. Franklin
Objective: To describe and understand how asymmetric interactions between a hurricane and its environment determine the hurricane's motion.
Rationale: Although the factors that contribute to hurricane motion in a two-dimensional barotropic context are reasonably well understood, the three-dimensional dynamics is not. The objective of this study is to describe and understand how three-dimensional asymmetric interactions between a hurricane and its environment determine the hurricane's motion. Specific questions to be addressed include: What atmospheric levels steer the storm? What spatial scales?
Method: Studies with simple barotropic and baroclinic models have established the importance of gradients of three-dimensional potential vorticity (PV) in hurricane vortex motion. PV is the natural context in which to understand the three-dimensional asymmetric interactions between a hurricane and its environment that determine the hurricane's track.

In the absence of diabatic and frictional effects, PV is conserved. Moreover, once a balance condition and appropriate boundary conditions have been specified, PV can be inverted to derive the entire three-dimensional momentum and mass distribution. These properties allow one, in principle, to isolate the physical factors that determine the three-dimensional wind field, and thus hurricane motion.

Multi-level, multi-nested analyses of hurricane Gloria of 1985, including Doppler winds near the storm's center and Omega dropwindsondes in its environment, are the most comprehensive yet developed for a single hurricane. Since three-dimensional height (pressure) fields are not available on the vortex scale, they are by necessity derived using the balance equation. Completed PV analyses resolve eyewall-scale features in the inner vortex core and embed analyses of these features within the larger--scale environment.

An inversion technique is being developed to deduce the three-dimensional distribution of PV that determined the steering flow that advected Gloria to the northwest. Advantage is being taken of the near-linearity of the weak asymmetric disturbances in Gloria near the core, and the linearity of PV in the environment.

Accomplishment: The large-scale PV distribution evidences asymmetries in the middle and upper troposphere that appear to be associated with Gloria's northwestward motion. The figure shows the contribution of the local upper-level PV anomalies to the deep-layer mean (150-850 mb) wind asymmetries [streamlines and isotachs (m/sec)] surrounding Gloria. Winds attributable only to PV anomalies confined to 100-400 mb and the region displayed (55-85 degrees West, 14-37 degrees North) are shown. The center of Gloria is shown by a hurricane symbol. Vectors indicate Gloria's observed motion (TC), and the "steering" wind at Gloria's center associated with the local upper-level potential vorticity anomalies (S_PV). The close agreement of the vectors implies that Gloria's motion was primarily attributable to PV anomalies near the hurricane, including those associated with a cold low over Cuba.
Key reference:
Shapiro, L. J., and J. L. Franklin, 1995: Potential vorticity in Hurricane Gloria. Mon. Wea. Rev., 123, 1465-1475.
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