The rotation of 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 environmental (V_e) and asymmetric rotational (V_ψa) wind averaged over an area with a radius of 200 km from Khanun’s center. The results revealed that during Khanun’s intensification period, V_e barely changed, whereas the speed and direction of V_ψa exhibited significant changes as Khanun’s southwestward movement switched to northwestward movement. The streamfunction analysis revealed that the moving direction change was consistent with the ventilation flow of V_ψa across Khanun’s center associated with 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 rotation rate of asymmetric circulation was quantitatively estimated using the formulation based on the budget of asymmetric rotational kinetic energy (K_ψa). The calculation revealed that the rapid counterclockwise rotation resulted from the conversion of environmental kinetic energy (K_e) to K_ψa related to the horizontal advection of the environmental tangential flow. The rotation of asymmetric circulation displayed wandering behavior when the dissipation term became significant. The dissipation term plus the conversion from symmetric rotational kinetic energy (K_ψs) to K_ψa associated with the advection of symmetric tangential wind by environmental radial wind led to a slow clockwise rotation of the asymmetric cyclonic center to the north.