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 (Shapiro and Franklin 1995) resolve eyewall-scale features in the inner vortex core and embed analyses of these features within the larger--scale environment. The resulting depiction of PV is the first presented for a real hurricane.
A piecewise inversion technique has been developed and implemented to deduce the three-dimensional distribution of PV that contributed to the deep-layer mean (DLM) flow that steered Gloria to the northwest. Advantage has been taken of the near-linearity of the weak asymmetric disturbances in Gloria near the core, and of PV in the environment. Thus, ad hoc aspects of the linearization required by other investigators are effectively eliminated. Removal of the hurricane vortex and the use of a climatological mean background state have been avoided as well. The present formulation links the highly rotational regime near the hurricane core with the outer environment in a uniform manner.
Wind anomalies attributable to pieces of anomalous PV restricted to cylinders of radii between 750 and 1500 km have been evaluated; the minimum vector difference between the associated DLM wind anomaly at the hurricane's center and the translation of Gloria is for a cylinder of 1000 km radius. The contribution of the PV anomaly at each mandatory level from 850 to 150 mb, as well as from the upper (U; 100 mb) and lower (L; 1000 mb) boundary conditions on potential temperature (FIGURE 2), indicates that the flow that steered Gloria to the northwest can be primarily attributed to PV anomalies within 1000 km of the center and 500 mb and above. This result implies that in order to establish short-term changes in the environmental flow field that steered Gloria, measurements of upper-level winds and heights are required at least out to 1000 km. The new Gulfstream IV aircraft is the best available for such measurements. A larger sample of cases is required to confirm these results.
Results of the present analyses have been accepted for formal publication (Shapiro 1996).
Shapiro, L. J., 1996: The motion of Hurricane Gloria: A potential vorticity diagnosis. Mon. Wea. Rev., 124, in press.
Last modified: 8/9/96
