The Influence And Mechanism Of Several Large-Scale Patterns On Tropical Cyclogenesis Over The Western North Pacific

The variation of large-scale low-level circulation is one of the key factors in controlling the genesis of tropical cyclones over the western North Pacific in summer. Thus, it is one of the most important isuses that the influence and mechanism of low-level circulation patterns on tropical cyclogenesis. Using the best track and the reanalysis data, several typical large-scale patterns are identified during the period of tropical cyclogenesis. The influence of large-scale circulation patterns on tropical cyclogenesis and its mechanism is systematically explored in this dissertation by using data analysis, energy diagnosis, and numerical simulation. The follows are the main results:(1) Present study classifies five large-scale circulation patterns in association with the tropical cyclone (TC) formation over the western North Pacific (WNP) for the period1991-2010:the monsoon shear (MS), the monsoon confluence (MC), the reverse-oriented monsoon trough (RMT), the monsoon gyre (MG), and the trade wind easterlies (TE). The first three patterns account for about80%of tropical cyclogeneses. Through a diagnosis of energetics, it is found that the tropical cyclogenesis in the MS, MC, and RMT patterns is highly associated with the barotropic energy conversion. Analysis shows that the horizontal shear of basic zonal flow provides a favorable condition for the eddy kinetic energy (EKE) growth in the MS pattern. When a TC forms in the MC pattern, the horizontal shear and convergence of basic zonal flow are both important for the EKE growth. When the basic flow is the RMT pattern, in addition to the horizontal shear of basic flow, zonal and meridional wind convergence play an important role for tropical cyclogenesis over the WNP and the South China Sea, respectively. However, the barotropic energy conversion appears not to be a main mechanism for the EKE growth in the MG and TE patterns.(2) The influence of the interannual variation of cross-equatorial flow (CEF) on tropical cyclogenesis over the western North Pacific is examined The results show that the number of TC formed over the southeastern part of the western North Pacific is well correlated with the variation of the cross-equatorial flows near125°E and150°E, and there is a significant negative correlation for the TC geneses over the northern part of the western North Pacific. The composite analyses show that during the strong-CEF-years, the variation of OLR, vertical wind shear between200-850hPa,850hPa relative vorticity and200hPa divergence are favorable for tropical cyclogenesis over the southeastern part of the western North Pacific, vice versa. Moreover, it is also discussed from the barotropic energy conversion view that during the strong-CEF-years, the eastward of the monsoon trough leads to the rapid growth of the eddy kinetic energy over the western North Pacific, which is favorable for tropical cyclogenesis.(3) The influence of large-scale circulations on tropical cyclogenesis during the year of2004and2006is analyzed. The result shows:the main large-scale pattern controls tropical cyclogenesis over the WNP is MC in July to September,2004, and is MS in the year of2006. The convergence of zonal wind, low-level vorticity, high-level divergence, vertical wind shear located at certain areas where tropical cyclone events occers. Moreover, the barotropic energy conversions during tropical cyclogenesis period in the year of2004and2006are calculated to analyze the3-8day wave train over the western North Pacific and the variation of eddy kinetic tendency. The result shows the large eddy kinetic growth areas are consistent with the tropical cyclone geneses in both2004and2006.(4) By using WRF model, the tropical cyclogeneses during July,2004to September,2004are well simulated. Although the number of TC formation is smaller than those in the JTWC dataset, the features of large-scale circulation,3-8day bandpass filtered perturbation are well captured, which are favorable for tropical cyclogenesis.