Contributions to Tropical Cyclone Motion by Small, Medium and Large Scales in the Initial Vortex

Abstract

A two-dimensional Fourier decomposition procedure is used to isolate small (≤500 km), medium (500< ? ≤ 1500 km) and large (>1500 km) scale components of some typical tangential wind profiles used in theoretical studies of tropical cyclone motion. The contribution of these scales to the vortex motion is studied in a nondivergent barotropic model with no initial basic flow by selectively retaining or deleting different scales. Transfer of energy between wave groups due to nonlinear scale interaction occurs slowly in this model so that a scale group that is removed in the initial conditions is not restored by 72 h. The largest scales, which account for a significant fraction of the vortex structure, primarily determine the speed of motion. That is, the speed of motion is proportional to the percentage of the total vortex that projects onto the largest scales. The medium and small males that contain less energy (because of the assumed vortex structure parameters) have a significant effect on the direction of motion by influencing the rotation of the asymmetric gyres that are induced primarily by the largest scales. The most important implication of these tests for dynamical tropical cyclone forecast models is that the initial vortex specification will project energy onto longer wavelengths that significantly affect track prediction. The agreement between the scales in the initial structure of the vortex and in the environmental analysis needs to be carefully considered.