Characteristic Analysis Of Path Swing And Eyewall Extreme Low Pressure Of Super Typhoon Wilmason (2014)

The fine-scale structure in the inner-core of tropical cyclones(TCs)and corresponding internal dynamics play a vital role in changes of tack and intensity.With the development of observations and numerical simulation,it is possible to investigate the small-scale process in TC inner-core,which also provides an important information for the fine-scale forecast as well as disaster prevention and mitigation.Based on the conventional meteorological observation data,doppler radar data and high-resolution numerical experiments with large-eddy simulation of Super Typhoon Rammasun(2014),this study analyses the small-scale track oscillation,including the trochoidal motion around the mean path and the precession of the upper-level vortex center relative to the low-level center,as well as small-scale pressure perturbations,which may contribute to false track oscillation in previous studies.The main results of this study are as follows:(1)The trochoidal oscillation in the tracks of Super Typhoon Rammasun can be retrieved with the 6-minute radar data.The detected track oscillation always exhibits cyclonic rotations around a mean track,with the amplitude of 1?3 km and the period of?4 h.The numerical simulation of Typhoon Rammasun indicates that the smallamplitude track oscillation can be simulated in terms of the period.Based on two different center-detecting methods,it is shown that the simulated track oscillation represents the wobble of the whole inner-core circulation.(2)The vertical vortex tilt reduction,accompanied by the precession of the upperlevel vortex center relative to the low-level center is exhibited before the Rapid Intensification(RI),and the onset of RI is consistent with the timing of the vertical alignment.The further diagnosis by Potential Vorticity Tendency(PVT)method indicates that,the TC motion at low-level is dominated by horizontal advection and vertical advection,whereas the contribution at upper-level mainly results from the of horizontal advection and development of the asymmetric circulation.The difference of the development of asymmetric circulation of TC inner-core between upper and low plays important role in maintaining the vertical coherent structure.(3)The extremely minimum pressure of 899.2 h Pa with dropping by 9.1 h Pa in 1min,was observed in the eyewall of Super Typhoon Rammasun,which was lower than the pressure observed in TC eye area.This study also demonstrated that small scale,negative pressure perturbations in the tropical cyclone eyewall are associated with the tornado-scale vortex(TSV)simulated with the large-eddy simulation technique in a semi-idealized numerical experiment with horizontal resolution of 37 m.The negative pressure perturbation associated with TSVs is nonhydrostatic,dynamically resulting from the shearing effect.In general,the stronger the TSV,the lower the negative pressure perturbation and the stronger the instantaneous wind speed.