The Rotation of the Asymmetric Circulation Associated with the Track Direction Change of Tropical Cyclone Khanun (2017)

The dynamic processes responsible for the movement of tropical cyclone Khanun (2017) were studied by analyzing data from the mesoscale WRF model simulation. The simulated motion was induced by the ventilation flow of both the environmentaland asymmetric rotational wind averaged over an area within a radius of 200 km from Khanun’s center. The results revealed that during Khanun’s intensification period, environmental wind barely changed, whereas the speed and direction of asymmetric rotational wind exhibited significant changes as Khanun’s southwestward movement switched to a northwestward movement. The streamfunction analysis revealed that the change in the direction of movement was consistent with the ventilation flow of asymmetric rotational wind across Khanun’s center associated with the asymmetric circulation rotation. The cyclonic circulation center rotated counterclockwise, moving from the northeast to the north before and during the rapid intensification period, and exhibited wandering behavior during this period. The rotational rate of asymmetric circulation was quantitatively estimated using the formulation based on the budget of asymmetric rotational kinetic energy. This calculation revealed that the rapid counterclockwise rotation resulted from the conversion of environmental to asymmetric rotational kinetic energy and was related to the horizontal advection of environmental tangential flow. The rotation of the asymmetric circulation displayed a wandering behavior when the dissipation term became significant. The dissipation term plus the conversion from symmetric to asymmetric rotational kinetic energy associated with the advection of symmetric tangential wind by the environmental radial wind led to a slow clockwise rotation of the asymmetric cyclonic center to the north.