POTENTIAL VORTICITY INFLUENCES ON HURRICANE GLORIA'S MOTION
Lloyd J. Shapiro
James L. Franklin
To describe and understand how asymmetric interactions between a
hurricane and its environment determine the hurricane's motion.
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?
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.
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.
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
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.
Shapiro, L. J., and J. L. Franklin, 1995: Potential vorticity in
Hurricane Gloria. Mon. Wea. Rev., 123, 1465-1475.
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