The Statistical Characteristics And Physical Mechanics Of Strong Winds In Bohai Sea

Bohai Sea, which is located in the North China Sea, is a semi-closed shallow-water sea. There are strong winds all through the year because of its particular position. Strong winds caused by cyclones, cold wave and clod air have a serious effect on ship-activity, industry and agriculture.Based on surface synoptic chart from 1991 to 2000 and meteorological records made by surface observational stations in Bohai Strait and North Yellow Sea, a statistical analysis of strong winds is made, the dynamic mechanisms of strong winds are put forward. The conclusion is that cold front, extratropical cyclone and tropical cyclone are the main systems which cause strong winds in Bohai Sea.Results show that wind directions in Bohai Sea have obvious seasonal variation, northwest wind prevails in winter and southeast wind in summer. The daily variation of wind directions is opposite between sea and land. The change of strong wind directions are relevant to temperature contrast between land and sea, temperature difference between air and sea, and topography.Cold airs from the northwest-path most frequently influence the Bohai Sea. From synoptic weather process view, the developing of small trough is the most frequent reason of the cold air. However the transversal trough is the most strongest one which brings cold air to the Bohai Sea, although its happening frequency is the lowest among all the cold air synoptic processes. It plays an importance role in strong winds caused by cold air that the upper level jet stream at SOOhPa, the positive vorticity advection in front of upper trough, the cold advection in vertical direction andthe variation of ground pressure.The extratropical cyclones that influence Bohai Sea often occur in spring and autumn. The upper divergence trough, the warm temperature ridge and the diabatic heating of underlying surface are advantageous to engender surface cyclone. The development of upper level jet stream and the radial strengthen of north jet stream make the unusual variation of intensity and path of the cyclone. The deep and strong positive vorticity advection in front of the upper trough, the strong convergence in the lower layer and divergence in the upper layer of cyclone center are the main dynamic factors which dominate the outbreak and development of the surface cyclones.The tropical cyclones that influence Bohai Sea are usually occurred in July and August. The tropical cyclones are easy to move northward and influence Bohai Sea, when the location of subtropical high is fairly north and west, and the westerly trough is fairly weak. It is found that the spatial distribution of some physical variables influences the moving direction of tropical cyclones. The divergence in the troposphere upper layer and the positive vorticity advection are the main dynamic factors that control the development of the tropical cyclones after it lands.