The Theoretical Study On Tropical Cyclone Intensity Sudden Change Over The Coastal Waters Of China

In order to investigate the physical mechanism of tropical cyclone (TC) intensity sudden change over the coastal waters (ISCCOW) of China, a theoretical study is made with national typhoon year book and NCEP-NCAR reanalysis data. Some of the research results on it are valuable in application to the TC operational forecasting.The climatic characteristics of TC intensity change over the western North Pacific are analyzed. Theoretical diagnostic analysis on ISCCOW is carried out. The vertical structure, kinetic and thermodynamic features of the ISCCOW with composite data analysis are fulfilled. A super typhoon Saomai (0608) is chosen to study its characteristics and mechanism on its sudden intensification over the coastal water of China. The main results show that the ISCCOW of Saomai is closely related to the entrainment of vorticity belt into its circulation from the remnant of typhoon Bopha (0609) which is neighbouring to Saomai. On the other hand, the interaction between south Asia high, subtropical high and tropical cyclones also plays a role to its intensification. A weak environmental wind vertical shear attributes to the TC sudden intensification over the coastal water. Energy conversion process would enhance the rotational kinetic energy. The enhancement of the relative humidity near the TC centre and the strong vapor convergence in the lower troposphere would lead to TC intensification. The vapor transport to TC with sudden intensification is obviously stronger than those without intensification. Diabatic heating increases with the height in the mid-upper troposphere and the increase of vertical inhomogeneous heating in the lower troposphere near the TC center are favorable to TC sudden intensification. There are other favorable environment conditions for TC intensification such as strong divergence at the upper level, strong vorticity near the TC center, while the high potential vorticity (PV) and convergence or vorticity increase in the middle layer at troposphere would lead to TC weakening.The numerical simulation with the PSU-NCAR MM5 V3.5 demonstrates that the small terrain has a little impact on intensity change of TC, but a large one plays an important role on it. In addition, diabatic heating would affect TC intensity change. The merge of adjacent vorticity lumps is a favorable mechanism for TC intensification.There are some portents to the TC sudden intensification, for example, the response time of TC intensity change to the change of environmental wind vertical shear is around 18 h to 24 h, to the change of kinetic energy at 850 hPa is about 18 h and to the change of vapor flux convergence is about 24 h. All these pre-indications may give a helpful guidance to the forecasting of TC ISCCOW.Finally, a nonlinear dynamic-thermodynamic system of equations is employed to develop a TC model and qualitatively analyze the variation of physical parameters which may change with the model state and the critical condition and results in bifurcation when the steady state loses its balance. In the mean while, some physical parameters that can analyze and predict TC intensity change are derived. Based on the above research results, a theoretical model is formed to show the mechanism of the ISCCOW.