A COMPARISON BETWEEN GEOSTROPHIC AND NONGEOSTROPHIC NUMERICAL FORECASTS OF HURRICANE MOVEMENT WITH THE BAROTROPIC STEERING MODEL

Akira Kasahara The University of Chicago

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Abstract

The steering flow of a hurricane is obtained by eliminating the vortex pattern from the total flow field. The evolution of the steering flow is predicted by solving the barotropic nondivergent vorticity equation. Based upon the steering-flow prediction, a forecast of the hurricane movement is obtained with the use of an equation which provides for interaction between the hurricane and the steering flow.

In the geostrophic model, the geostrophic-wind assumption is used in solving the vorticity equation. In the nongeostrophic model, the stream function governed by the ‘balance equation’ is adopted. With the above two prediction models, 45 pairs of predictions of the 24-hr and 48-hr movement of hurricanes Diane and Connie (August 1955) and Betsy (August 1956) at the 500- and 700-mb levels were prepared by the use of an electronic computer.

A detailed comparison between performances of the two prediction models is presented. Generally speaking, there is a remarkable similarity between the nongeostrophic and geostrophic forecasts of hurricane movement. The accuracy of the 700-mb hurricane forecasts appears to be comparable with that obtained from the 500-mb forecasts. However, for both prediction models, the ‘resultant’ forecast, which is the vector mean of the 700- and 500-mb predicted displacements, seems to provide a significant improvement over the accuracy of a single-level barotropic forecast. These analyses suggest that further improvements are possible by advancing the prediction model from the barotropic model to the equivalent barotropic or the baroclinic model.

Abstract

The steering flow of a hurricane is obtained by eliminating the vortex pattern from the total flow field. The evolution of the steering flow is predicted by solving the barotropic nondivergent vorticity equation. Based upon the steering-flow prediction, a forecast of the hurricane movement is obtained with the use of an equation which provides for interaction between the hurricane and the steering flow.

In the geostrophic model, the geostrophic-wind assumption is used in solving the vorticity equation. In the nongeostrophic model, the stream function governed by the ‘balance equation’ is adopted. With the above two prediction models, 45 pairs of predictions of the 24-hr and 48-hr movement of hurricanes Diane and Connie (August 1955) and Betsy (August 1956) at the 500- and 700-mb levels were prepared by the use of an electronic computer.

A detailed comparison between performances of the two prediction models is presented. Generally speaking, there is a remarkable similarity between the nongeostrophic and geostrophic forecasts of hurricane movement. The accuracy of the 700-mb hurricane forecasts appears to be comparable with that obtained from the 500-mb forecasts. However, for both prediction models, the ‘resultant’ forecast, which is the vector mean of the 700- and 500-mb predicted displacements, seems to provide a significant improvement over the accuracy of a single-level barotropic forecast. These analyses suggest that further improvements are possible by advancing the prediction model from the barotropic model to the equivalent barotropic or the baroclinic model.

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