Numerical simulation of asymmetric hurricanes on a β-plane with vertical shear¹

1 Contribution No. 108 of the Geophysical Fluid Dynamics Institute, Florida State University.

Abstract

A three-layer, semi-implicit model was developed to simulate moving and asymmetric hurricanes on a β-plane, using Kuo's method of cumulus parametrization. Sensible and latent heat transfer from ocean to atmosphere was included implicitly in the model. In order to predict hurricane movement over a large area and yet resolve finer details near the eye, a multi-grid network was used with a movable finest grid of 20 km mesh size, surrounded by two coarse grid nets with mesh spacings of 60 km and 180 km, respectively. A comparison of the results with f-plane calculations shows that the vortex on the β-plane intensified at a slower rate before the storm stage, but at the same rate thereafter. The β-plane hurricane was asymmetric throughout its life cycle, and these asymmetrics looked similar to those observed in real hurricanes. The vortex moved on the β-plane with a phase velocity of 4.3 km hour−1 for the westerly and 3.3 km hour−1 for the northerly components. The model was also integrated for two cases where the initial vortex was superimposed on a vertically varying basic current. Results showed that the strength of the simulated hurricane depends very much upon the magnitude of the vertical shear of the basic current; for a large shear (≥ 15 m sec−1/12 km) the vortex failed to intensify into a hurricane. Computations showed that the pressure weighted mean of the basic current between the surface and 12 km level agreed very well with the magnitude of the steering current. As a result of the interaction between the hurricane circulation and the basic current, the hurricane moved in an oscillatory path with an amplitude of 30 km and a period of about 20 hours.

Notes

1 Contribution No. 108 of the Geophysical Fluid Dynamics Institute, Florida State University.