POTENTIAL VORTICITY DIAGNOSTICS OF HURRICANE MOVEMENT .1. A CASE-STUDY OF HURRICANE-BOB (1991)

Potential vorticity (PV) diagnostics are applied to evaluate the contr ol by the large-scale environment of hurricane movement and, more impo rtantly, to assess the storm's influence on its own track. As a first application of these diagnostics, an observational case study of Hurri cane Bob (1991) is presented using the twice-daily National Meteorolog ical Center Northern Hemisphere final analyses gridded datasets. Defin ing the seasonal climatology as the mean reference state, piecewise po tential vorticity inversions are performed under the nonlinear balance condition. This allows one to determine the balanced flows associated with any individual perturbation of PV. By examining the balanced flo ws at the central position of the hurricane, one can identify the infl uence of each PV perturbation on hurricane movement. The hurricane adv ection flow is also defined as the balanced flow at the storm center a ssociated with the whole PV distribution, excluding the positive PV an omaly of the hurricane itself. The results from the observational stud y of Bob show that the hurricane advection flow is a good approximatio n to the real storm motion. The results also show that the balanced fl ows associated with the climatological mean PV and perturbation PV dis tribution in both the lower and upper troposphere are both important i n contributing to Bob's movement. However, it is difficult to separate PV anomalies directly or indirectly attributable to the storm from am bient PV anomalies. Results from other cases will be presented in a co mpanion paper.