Interaction of a hurricane with the steering flow and its effect upon the hurricane trajectory

Abstract

If the steering flow is defined at all times as the residual after subtraction of an axially symmetric vortex that moves with variable speed but invariant shape, then interaction terms appear in the prediction equation that governs the steering flow. The vortex is taken as the one that minimizes the kinetic energy of the steering flow in the initial data. The propagation velocity of the vortex is defined in such a way as to minimize interactions between vortex and steering flow, and is found to have a second-order dynamic part proportional to the gradient of steering-flow absolute vorticity. It also is shown that in a uniform steering flow, interaction terms give rise to an acceleration of cyclonic vortices in the direction of increasing steering-flow absolute potential vorticity. Results are presented for numerical computations made from 600-mb data on hurricane Betsy 1966 with the proposed prediction model with and without the interaction terms. The interaction terms remove the major part of a systematic rightward bias found in earlier hurricane-trajectory forecasts based upon steering-flow models.