| With the rapid development of observational techniques and numerical models,it is possible to provide prediction of Tropical Cyclones(TCs)at a finer scale,in which the TC track is the primary concern in the detailed prediction.Numerical and observational studies document that the track of TCs often accompanied by trochoidal movement.Such oscillation or wobble,not only affects the work of disaster prevention and mitigation in the affected areas,but also have an impact on the strength and structural changes in TC,is critical in the short-range forecasting.At present,there are little studies on small-scale trochoidal tracks,especially the lack of understanding of the track in high spatial and temporal resolution numerical data.In this study,the high-resolution WRF(Weather Research Forecast)numerical simulation data of Hurricane Wilma(2005)is used to discuss the small-scale wobbling characteristics and mechanism of the track.The research of the small-scale trochoidal track needs to be diagnosed by the Potential Vorticity(PVT)method.In high-spatial resolution data,there will be errors of the TC speed that are calculated by the PVT method and TC center.Those errors can be largely reduced by either shorten the time interval of the model output,or perform spatial smoothing on the original field that is required for PV calculation.Since the structural change of potential vortex also affects the trochoidal track,the potential vorticity budget is performed.The result shows that the advection effects reshape the spatial distribution of PV.The heating effect generates the PV in the lower and middle layers inside the eyewall,and the generated PV is then transported upward the eyewall by vertical advection.The effect of precipitation decreases the PV near the eyewall,but its contribution is one order of magnitude smaller than the other effects.In addition,the track is more sensitive to the TC center detecting methods.A reasonable positioning of the center not only affects the accuracy of the path,but also affects the strength and structural changes of the TC,especially the structural changes of the wind field and potential vorticity in the inner-core area.We evaluated the four commonly used center detecting methods: MVC(Minimum pressure Variance Center),MTC(Maximum Tangential wind Center),PCC(Pressure Centroid Center),and PVC(Potential Vorticity centroid Center).Result shows that the differences in the detected center position and vertical tilt are generally small during the rapid intensification and eyewall replacement due to the strong TC intensity,and the methods all lead to similar small-scale trochoidal track that rotate cyclonically around the mean track.While the MVC lead to a relatively smooth rotation,abrupt changes exist in the track oscillation of the MTC,the track oscillation of the PCC and PVC contains amplified embedded rotations that are associated with the pressure and potential vorticity field in the eye region.The MVC leads to the strongest symmetric structure in the tangential wind,PV,and radial PV gradient in the eyewall region,while the PCC tend to enlarge the asymmetric component.In this high-resolution numerical data,the four center detecting methods all objectively give the small-scale trochoidal characteristics of the track,with its amplitude from a few hundred meters to ten kilometers,and its period ranging from tens of minutes to several hours.Further analysis found that wavenumber-1 instability in the inner-core region is the main reason,and the structural change of potential vorticity in the inner-core region may lead to the shortterm oscillation of the track.In our paper,the reliability of the methods is verified and improved,and the possible mechanism of small-scale trochoidal track are given from the perspective of TC structure changes.It is of great scientific significance and practical reference value to lay a solid foundation and put forward ideas for further studying the relationship between the trochoidal tracks and structural changes. |
PDF Full Text Request