Numerical Experiments of Tropical Cyclones on a Plane with Variable Coriolis Parameter

Abstract

Numerical experiments of tropical cyclones are performed using a moving variable grid scheme proposed by Kitade (1979). The formulation for parameterization of cumulus convection is based on the concept of penetrative convection. The physical processes, such as the surface friction, latent and sensible heat supplies from sea surface, eddy dissipation and diffusion, dry convective adjustment and large-scale condensation, are also contained with conventional ways in present model.
The initial vortex develops into a mature disturbance in about 2 days. Its structure is similar to that of the tropical cyclone in the real atmosphere and that in numerical experiments performed by many researchers. The simulated tropical cyclone has some asymmetric features due to the variable Coriolis parameter in spite of the adoption of an initial nearly symmetric vortex. Furthermore, the developed tropical cyclone has dynamically unstable regions in the upper layer, which may also contribute to the asymmetry of the simulated vortex.
The simulated vortex moves north-north-westward, which is due to the effect of variable Coriolis parameter pointed out by Rossby (1948). The moderate tropical cyclone moves at a speed of about 3km hour-1 for westward and at about 7km hour-1 for northward components. The meandering about the mean path occurs with a period of about 25 hours, which seems to be due to the Magnus effect suggested by Yeh (1950).
When the sea surface temperature is changed, the path of the vortex center is deviated a little from that in the basic experiment. The deviation is partly explained for the following reason. The strength of vortex is strongly influenced by sea surface temperature. According to Rossby (1948), on the other hand, the northward acceleration due to variable Coriolis parameter increases with the increase of strength of vortex. Therefore the sea surface temperature affects the path of vortex. The variation of speed of vortex movement through its life cycle is also understandable from such point of view. It is confirmed by an experiment with non-uniform sea surface temperature that the asymmetric heating of cumulus convection affects the path of the vortex center.