A Preliminary Study on Activities of Tropical Cyclones over the Western North Pacific during the Summer in 2006

The characteristics of tropical cyclones over the western North Pacific during summer in 2006 are as follow:1) The intensity of tropical cyclones over the Northwest Pacific in the summer 2006 is stronger than that in the normal years.These tropical cyclones tend to cluster temporally around some period(27 June—31 August).Therefore more tropical cyclones landed on China to make heavier disasters during this period.2) The most damaging landing typhoons are Bilis and Saomai,the former influenced a great area,brought heavy precipitation,and lasted for a long time;the latter brought strong wind,and decayed very soon.Some primary researches are performed in terms of the two characteristics.The circulation features are compared between tropical cyclone clustering period and no cyclogenesis period.The results show that:during the first period(1—26 June),the Western Pacific Subtropical High was more south than the climate average one,and the Intertropical Convergence Zone(ITCZ) is also more south.Near 5°N,the vertical shear of zonal wind(200—850 hPa) is small.These circulation conditions resulted in the few formation of tropical cyclones.On the contrary,during the second period(27 June—31 August),both the Western Pacific Subtropical High and the ITCZ moved north.The weak vertical shear of zonal wind is just over the ITCZ.The cyclogenesis favors these atmospheric circulation conditions.The Indian Monsoon extends east or not,and the intensity of the cross-equatorial flow may also play a very important role on cyclogenesis.The reason why Bilis and Saomai have different features is also researched.The results show that the difference between Bilis and Saomai are derived from different vapor channels and origin.