| The Bohai straits is an important sea transportation in China and the geography position is very important. However, because of the complicated sea conditions and the weather, here is one of the most shipwrecks happened in China. Among them, strong wind weather frequently results in the shipwreck.In this paper we firstly investigate the wind data of 6 stations in the coastal area of Yantai and Dalian from 1975 to 2005 total 31 years, the basic climatic characteristics such as the distributions of the gale days, wind direction and power, lasting days etc. have been preliminary analyzed. On this foundation, time and space distribution of gale characters over Bohai straits has been studied. On one side time distributes, the variation of the gale days in annual is studied, on the other side space distributes, area distribution compared each other. Using data in Nanhuangcheng, further studies are carried to the gales over Bohai straits. Weather conditions bring gales are analyzed. Two kinds of weather systems such as cold wave gales and Mongolia cyclone gales are deeply researched. Finally, thunder gale's Doppler radar characters are analyzed over Bohai straits using the NEXRAD (CINRAD/SA) data in YantaiResults show that average gale days over Bohai straits are 73.3d, and fresh gales are 4.9d, obviously larger than the inland region, different from the data 150.4d statisticed by Caogangfeng etc. The reason was studied from statistics and considered that conclusions are more realizable in this paper.The gale's speed over Bohai straits mainly varies from 6th to 7th grade, over 8th grade merely 6.7%. The infinite and maximum gale's speed all reaches 12th grade. The infinite gale's speed is stronger 1 or 2 grades than the maximum. But strong convection weather result in short-time gust often larger than 5 grades. The gales over Bohai straits appears not only frequently but also last a long time. The longest average last days are 11.3d in April.From time distribution characters, the gale days over Bohai straits have a trend to reduce year after year. Gradually decrease with the trend -2.4d annually, but also exist break. The Bohai straits has the typical feature of monsoon, which the gale's direction is northern in winter and is southern in spring and autumn mainly, annual variation tendency configuration has 2 apexes and 1 valley.By means of maximum entropy spectral the series of each station annual gale days are analyzed, detect that gale over Bohai straits have 3 main periods in 15-16 years, 5-6 years and 2-3 years. By means of wavelet transform(mexh wavelet) the wavelet coefficient distribution diagram of each station annual gale days series are studied, results show that series variation appears in 10-16 years in large scale, scale in 3-5 years too. By means of wavelet analysis, it is confirmed that gale days have a break. The series has a reduce trend in long term using scatter wavelet transform(db3 wavelet) to study the results of each station monthly gale days.From space distribution characters, island stations have more gale days and strong wind power and represent gale situation perfectly over Bohai straits. The main gale's direction over Bohai straits is NW-NNE. The gale's direction is northern in Dalian coastal, and the south gale days increase obviously in Yantai coastal in spring and autumn; north and south gale days is more than east and west.By studying the data of Nanhuangcheng station, it is discoverd that the gale days in Nanhuangcheng station are more than those in Changdao station, and wind power are stronger than those in Changdao station. The begining time difference when wind power become large is evident between both stations. So, forecasting focuses should be put on these island stations, and further monitoring ability should be strengthened in Bohai straits, then it can provide important reference and analysis foundation to enhance "safe weather" service ability.The weather conditions which can bring about gales over Bohai straits, can be summarized to 7 patterns, including northern gale behind the cold front, northern gale caused by the pattern of high in north (west) and low in south (east), cyclone gale, southern gale caused by the pattern of low in north (west) and high in south (east), southern gale caused by the Huabei topography trough, typhoon gale and the brief gale cause by severe and convective system. Typical systems, which bring strong gale and serious disaster, are cold wave gales and Mongolian cyclone gales.Based on the radial velocity, reflectivity and educed other products of NEXRAD (CINRAD/SA) data in Yantai, mesoscale and microscale weather gales are analyzed. The results show that thunder gale often happened under strong wind region pattern and divergence area near the surface in radial velocity, the more close the strong wind region, divergence area near ground, the more thunder-storm and disaster strong wind in ground occur. The radial velocity is as big as the ground wind. Adverse wind area is also an index of the strong wind. Although mesoscale cyclone doesn't often appear, it is an important sign of the strong weather. Once it appears, strong weathers, such as short time rain-storm, hail and disaster strong wind etc. usually appear, too. Thunder-storm gales are often caused by line form. Bow form, hook form and V form etc. often closely related to the occurrence of strong weather. When thunder-storm gale appears, obvious vertical wind shear appears too. The thunder-storm gale has a strong echoes, the VIL is also high. These forecast signs will enhance the ability in the analysis of Doppler radar characters and the detection of strong convection.
|
PDF Full Text Request