Potential vorticity Diagnostics of Hurricane Movement. Part 1: A Case Study of Hurricane Bob (1991)

Abstract

Potential vorticity (PV) diagnostics are applied to evaluate the control by the large-scale environment of hurricane movement and, more importantly, to assess the storm's influence on its own track. As a first application of these diagnostics, an observational case study of Hurricane Bob (1991) is presented using the twice-daily National Meteorological Center Northern Hemisphere final analyses gridded datasets. Defining the seasonal climatology as the mean reference state, piecewise potential 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 flows at the central position of the hurricane, one can identify the influence of each PV perturbation on hurricane movement. The hurricane advection flow is also defined as the balanced flow at the storm center associated with the whole PV distribution, excluding the positive PV anomaly of the hurricane itself. The results from the observational study of Bob show that the hurricane advection flow is a good approximation to the real storm motion. The results also show that the balanced flows associated with the climatological mean PV and perturbation PV distribution in both the lower and upper troposphere are both important in contributing to Bob's movement. However, it is difficult to separate PV anomalies directly or indirectly attributable to the storm from ambient PV anomalies. Results from other cases will be presented in a companion paper.