The Diagnosis Of The Asymmetries And Their Influences On The Intensity Of Tropical Cyclones

The prediction of typhoon intensity is the key and difficult point in the develop-ment of numerical model.The complex interaction between asymmetric convective activities and typhoon circulation is an important reason for the short-term typhoon intensity change and path mutation.At present,there are still disputes about the asymmetric enhancement mechanism of typhoon.Among them,the diabatic heating of asymmetric convective activity is considered as an important energy source for the maintenance and development of typhoon intensity,while the dynamic mechanism of Asymmetric wind field circulation corresponding to convective activity to the change of typhoon intensity is the main focus of controversy.In this study,firstly,the influence of the central positioning deviation on the asymmetric characteristics of typhoon is studied by using the ideal typhoon model;secondly,a more accurate and reliable positioning algorithm is designed for the uncertainty of the typhoon central positioning algorithm,and a quantitative evaluation index is proposed to evaluate the capability of the algorithm;finally,based on the high-resolution numer-ical model,the intensity variation of typhoon is analyzed The evolution of typhoon's symmetric and asymmetric characteristics is discussed,and the dynamic effect of typhoon's asymmetric disturbance on the change of typhoon's intensity is discussed.Based on the ideal typhoon vortex,the influence of the center positioning de-viation on the asymmetric characteristic strength and structure is studied.In a symmetric typhoon vortex,the intensity of the asymmetric feature will increase rapidly with the increase of the offset distance when there is a center location shift.When there is a specific asymmetric vorticity disturbance in the typhoon circulation,the distance of the center shift will affect the intensity of the asymmetric vorticity,and the direction of the shift will significantly affect the spatial distribution of the asymmetric vorticity,especially the radial inclination and direction of the asym-metric disturbance.When the location of typhoon center is uncertain,under the same migration distance,the higher the wave number,the less the sensitivity of the asymmetric feature to the migration direction.An ideal typhoon vortex with multi-scale disturbance is constructed,and the influence of central positioning deviation on the understanding of asymmetric dynamic mechanism of typhoon is studied.The results show that when there is only 2km deviation in the center location,the different migration directions may get completely opposite conclusions.Based on the ideal symmetric typhoon model and the random disturbance scheme,the ideal typhoon dataset with multi-scale disturbance was constructed.In this paper,a two-dimensional Fourier transform based algorithm(FVC)for typhoon center location was proposed.The algorithm could automatically adjust its param-eters according to the scale of the target vortex,and could effectively eliminate the influence of asymmetric convection on the detection of the symmetric typhoon vortex center.Three quantitative indexes for evaluating center-finding algorithms are designed-the wind speed at center point,the smoothness of typhoon path and the smoothness of vertical tilt.The FVC algorithm and five commonly used center-finding algorithms were compared and tested by using the high-resolution typhoon numerical simulation data.The results show that the center of typhoon calculated by FVC algorithm can be basically located in the low value area of wind speed,and has a relatively smooth typhoon path and a small vertical tilt change.Its stability and reliability are better than other center positioning algorithms.The spectrum energy equation for studying the asymmetric intensification mechanism of typhoon was derived.Using the high-resolution numerical model,the evolution of asymmetric characteristics and the asymmetric energy spectrum during the intensity change of typhoon Soudelor(2015)were studied.Asymmetric convective activities were mainly concentrated near the typhoon eyewall in the mid-dle and lower troposphere,they were mainly manifested as the vortex Rossby waves where the disturbance of pressure and wind fields were close to the quasi-geostrophic equilibrium,and the disturbance of vorticity was about 1/4 phase behind the dis-turbance pressure.However,in the lower stratosphere,the asymmetric disturbances were mainly in the form of gravitational internal waves caused by inertial ascending motion,the phase of vertical velocity was opposite to the phase of virtual potential temperature.In the mature stage of the typhoon Soudelor(2015),the asymmetric disturbance energy spectrum of the inner core area in the lower troposphere was close to the exponential form of wave number-5/3 power,while the slope of the asymmetric kinetic energy spectrum increased when the typhoon weakened and in-creased in scales.The slope of asymmetric kinetic energy spectrum in the outer radius did not change obviously with the intensity and scale of typhoon,and the higher the height was,the closer the slope of energy spectrum was to-3.The symmetrical vertical circulation and the asymmetries of any scale could convert the available potential energy into the vertical kinetic energy,and the symmetric hor-izontal circulation of typhoon can obtain the energy from the symmetric vertical circulation.However,the asymmetric disturbance in the eyewall region was mainly reflected in the transformation of the horizontal kinetic energy into the vertical kinetic energy of the asymmetries.The effect of the asymmetric wind circulation is to enhance the symmetric tangential wind near the center of the typhoon,and cause the weakening of the tangential wind speed at outer radius.What's more,the asymmetries on the outside of the convective activity center would get energy from the symmetric horizontal circulation,while the asymmetries on the inner ra-dius will convert their kinetic energy into the symmetric circulation.As the energy of asymmetric wind field circulation gained from the symmetric vortex is larger than that returned back into the symmetric vortex,the dynamic effect of asym-metric asymmetries in typhoon would weaken the intensity of symmetric typhoon circulation.