The Diagnostic Analysis And Simulation On Intensity Change Of The Anomaly Weakened Typhoon Muifa(1109)

Super typhoon Muifa(1109) enhanced and weaken twice during its12days lifetime. The intensity was predicted stronger during3rd to7th August because the high SST of Kuroshio was over considered. In this paper, comprehensive and quantitative diagnostic analyses on main factors affecting typhoon intensity are performed. We found out that though the ocean thermal conditions are certainly favorable for the typhoon, the environmental field conditions, both the cold air advection and the environmental vertical wind shear, were unfavorable. The negative feedback which well matched the process was the main reason of the weakening.To further investigate the impact of various factors on the intensity change of Muifa, high-resolution numerical simulations using separate atmospheric model GRAPES and ocean-atmosphere coupling mode GRAPES_ECOM were performed during the Weakening period. The result show that the simulations of path were very close to the observation, and the coupling has little effect on the path; the strength of simulations did not recede during that period, and the coupled model is much closer to the observation. Because SST changed with the typhoon moving, the sensible heat and water vapor flux reduced accordingly, and it became clearly asymmetry in the northwest and southeast direction. The large-scale environmental fields of two tests were similar:cold and warm air involved into the outer circulation instead of the center of typhoon at the same time, and did not cause a wide range significantly affect; vertical wind shear which was small enough, was very favorable for typhoon development. Intensity simulation of coupled model was closer to the observation, the negative feedback of marine hinder Typhoon development, while the cold advection has little effect and the environmental wind shear is small enough.Based on the coupled model simulation as a control experiment, three groups of sensitive experiments on different factors were conducted. Because the diagnostic analysis showed the strong cold air involved into the lower center was the main reason of the weakening, a set of sensitive experiments with different temperature changes in the low-level center were conducted to simulate the cold air involved into the typhoon center on low-level. The test results showed that:the intensity change influenced by the temperature was immediate. The typhoon turned weaker because of the cooling, while it turned stronger because of the warming, and the typhoon intensity changes and temperature variations were proportional basically. Taking into account that the cold air came from the high-altitude westerly trough, and the presence of the trough impact on the vertical environmental wind shear, a series of sensitive experiments were conducted with upper troughs of different intensity as the initial fields. The test results show that:Intensity changed immediately at the start time of simulation. Deep troughs were favorable for typhoon intensity while the shallow trough was not because cold air behind the deep thoughts were much stronger. The vorticity advection and vertical environmental wind shear did not changed obviously because the trough was far away from the typhoon.