| Tropical cyclones with high temperature and low pressure center of the strong vortex formed in the tropical oceanic atmosphere, it is the most destructive weather events and natural disasters in the world. Currently, more progress of research on a single typhoon and forecast has been made, but the strength and genesis relatively poor. Especially study on two or more typhoons is more a lack in all aspects. This paper NCEP/NCAR 1°×1°analysis data are applied in the non-hydrostatic mesoscale model WRF (Weather Research and Forecasting Model) to simulate a process of three tropical cyclones in 2009, and with Davis piece PV inversion technology and simulation results, the trigger mechanism of the formation of Etau, heavy precipitation reasons of Morakot, the three interactions and movement of typhoons are analyzed, the following conclusions:First of all, the high-level large value of potential vorticity play a key role in the genesis of Etau, but it works only when the middle-lower large value PV come across. The PV inversion shows that, single high-level or low level large value PV couldn't make a strong development Etau. When the two meet, with the interaction of them, in 800hPa the inversion minus the climate-averaged height field of Etau embryo is large than the linear superposition of single large value of high-level inversion minus climate-averaged height field and single large value of middle-lower-level inversion minus climate-averaged height field of fields about 36%(other fields having the same phenomenon). Through non-geostrophic Q-vector diagnosis it is found that upward motion happens in the front of direction of forward motion of the large value of high-level potential vorticity and sinking are in the rear of it, the same to the lower-level large value of potential vorticity, with the distinction just that the large value of high-level PV has main influence on high-level and the large value of lower-level PV on lower-level. When the two close, the upward motion areas of high-and lower-level are turned on, between them the middle-lower-level convergence strengthening, resulting in movement upward increased, triggering a strong development of low-level disturbance. Because the interaction between multiple typhoons, research of Etau will have greater significance, for formulation at the sudden of Etau, Morakot moving slowly northwards near Taiwan.Secondly, this process before and after Morakot landing Taiwan, the typhoon Morakot path changes are in relation to interactions with Morakot and Goni, Morakot and Etau. The sensitive experiments indicate that by the principle of conservation of angular momentum, the radius of the monsoon depression changes cause enhancement of wind speed by the side near the monsoon low pressure of the typhoon, and changes in the typhoon path. In the case of double-typhoon, when the radius of the monsoon low pressure shorten, the typhoon in the western of monsoon low pressure, for the strengthening of the north wind, is hard to move northwards; in the eastern of the typhoon, for the increasing of the south wind, is apt to go northwards, typhoon intensity while also strengthening. This result is confirmed by deep steering flow in control and sensitive experiments. Therefore, in many typhoons appears, it is important that pay attention to changes in monsoon depression.Thirdly, the main reason in heavy precipitation Morakot is that sufficient water vapor transport sustains in the southern Morakot, which comes down to the effective support of Goni and the monsoon depression; and Morakot moving slowly near Taiwan makes the strong precipitation area more concentrated, and the reasons for the slow movement near Taiwan of Morakot is interaction between multiple typhoons. The system brought heavy rainfall is mainly eye wall of Morakot, forced by the terrain enhancing the upward motion and bringing a rare heavy rainfall.
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