Revisiting the Steering Principal of Tropical Cyclone Motion
Abstract. The steering principle of tropical cyclone motion has been applied to tropical cyclone forecast and research for nearly 100 years. Two fundamental questions remain unanswered. One is why the effect of steering plays a dominant role in tropical cyclone motion and the other is when tropical cyclone motion deviates considerably from the steering. A high-resolution numerical experiment was conducted with the tropical cyclone in a typical large-scale monsoon trough over the western North Pacific. The simulated tropical cyclone experiences two eyewall replacement processes. Based on the potential vorticity tendency (PVT) paradigm for tropical cyclone motion, this study demonstrates that the conventional steering, which is calculated over a certain radius from the tropical cyclone center in the horizontal and a deep pressure layer in the vertical, is not literally the steering or the advection of the symmetric potential vorticity component associated with a tropical cyclone by the asymmetric flow. The conventional steering also contains the contribution from the advection of the wavenumber-one potential vorticity component by the symmetric flow. The contributions from other processes are largely cancelled due to the coherent structure of tropical cyclone circulation and thus the conventional steering plays a dominant role. The trochoidal motion around the mean tropical cyclone track with amplitudes smaller than the eye radius and periods of several hours cannot be accounted for by the effect of the conventional steering and thus the instantaneous tropical cyclone motion can considerably derivate from the conventional steering.