A Study On Forecasting Methods Of Storm Surges Occurred At The West Coast Of The Bohai Sea

The west coast of Bohai Sea is an area frequently stricken by storm surges, with seven severe attacks after the establishment of the P.R.C, among which five occurred after 1990. With the increasingly occurring frequency, intensity and disaster losses, storm surge has become a direct threat to the security of people's life and property, which also hinders the economic growth along the coastal area. Thus, emphasis on the research about storm surge prediction and disaster reduction has become very important for the acceleration of local economy.Combining meteorological science and marine hydrological science, based on the Annual Typhoon Report 1951～2004 and materials acquired in tide stations such as Huanghua Harbour concerning the wind direction, wind force and hourly tide level from 1985 to 2004, this thesis carries on statistical analysis of the storm surges occurring on the west coast of Bohai Sea. As the result shows, typhoon or strong cold air, together with the influence of cyclone are the climate systems mainly responsible for the occurrence of storm surges in this area and the water increasing contributed by strong east wind, together with astronomical tide, the direct contributing factor. Both the storm surges and the astronomical tide have the form of semidiurnal tide. The average daily highest tide level and its relevant occurring period can be counted out through the statistics from daily highest tide levels. This thesis also devotes itself to researching on the relationship between water increasing and the typhoon that influences Bohai Sea and that between water increasing and cold air gales, based on perfect prediction method via climate data accumulated through many years and numerical forecasting products, establishing the prediction mode concerning the storm surges in the west coast of Bohai Sea, making the highest tide level prediction through the calculation of water increasing caused by typhoon or cold air together with the influence of cyclone, with the reference of astronomical tide materials, predicting the influenced areas at various tide levels, based on the natural, geological condition of the west coast of Bohai Sea, its economy layout and warning tide level, and coming up with protection methods.The main research results are as follows:(1) The storm surges lead disasters occurred every 5-7 years in the west coast of Bohai Sea, the longest interval is 13 years, and the shortest is 2 years,the occurring frequency and intensity have increased obviously since 1990s. Typhoon or strong cold air, which together with the influence of cyclone are the climate systems mainly responsible for the occurrence of storm surges in this area.(2) Seven storm surge disasters occurred during the 4th,5th,15th,16th and 17th days of the lunar calendar month, the time are also the periods of the astronomical tide which perform an important function. With the interaction of the synoptic system and astronomical tide, when the tidal levelbeyond warning water level, the storm surge disaster occurred.(3) Both the storm surge day and no storm surge day have the same tidal diurnal variation phase,which have the form of semidiurnal tide, that have two high-tide levels and two low-tide levels everyday. In the first half-day, the tidal level rise faster 3 hours than astronomical tide in the storm surge days, and the high tidal level keep longer which higher 2-3 meters than flood tide, while tide ebbs relatively slowly, the lowest tidal level is lower 1-2 meters than the highest tidal level. In the later half-day, the time it reaches the highest tidal level in both the storm surge day and no storm surge day is the same, the range of rise and descend is similar. In no storm surge day, the vertical distance form highest tidal level to lowest tidal level is about 3 meters, while in storm surge day, the vertical distance is about 2 meters. It is found from the hourly wind direction materials in the storm surge days that when the strong east wind keep long time, the water increasing contributed better, and vice versa.(4) Typhoon is the climate system mainly responsible for the occurrence of storm surges in the west coast of Bohai Sea. The shift, path, intensity, and duration of the typhoon play a critical role in the occurrence and various tide levels of storm surge. According to statistical analysis, there are three paths that typhoon enters the Bohai Sea. First, after entering the Bohai Sea. typhoon goes up directly and lands in The Northeast China Plain. Second, after entering the Bohai Sea, typhoon turns to the west and lands in The North China Plain. Third, after entering the Bohai Sea, typhoon moves to the northeast and lands in East Liaoning Peninsula.(5) The west coast of Bohai has normative north and south direction. The main air course causing tide rising is east wind, and the increasing tidewater value relates to the average wind speed in certain period and strong wind duration. The cold wind from Lake Baikal goes down to south along east-path, which confronts with the strong warm and dampness stream from south near 38°N, and forms the high baric fields in northeast and low in southwest. Because of warm and cold air masses are well matched in strength, which leads the cold front to maintain more than 20 hours, Bohai Straits undergoes the strong east wind for a long time. When astronomical tide occurred at this time, the storm surge disaster occurred. Combining the actual information and various prediction products, the coastal average wind speed is predicted, then the sea surface average wind speed and the average east wind speed is calculated and the extremum of rising water is calculated according to the average speed of east wind.(6) Forecast by the data of the storm surges occurred in the west coast of Bohai Sea since 1965. the forecast values and the practical measuring values are primarily the same, the maximum error is 18 cm, and the minimal error is 2 cm. It shows that this kind of method has fairly good forecasting capability to the storm surges occurred in the west coast of Bohai Sea. The materials for forecasting are tide materials of Huanghua Harbour and routine meteorological data which are easy to gain, and simple to operate. As the result, it is the pragmatic forecasting method for grass-roots stations which has the fairly good application prospect.(7) The forecasting wind field is detailed by applying the meso-scale numerical prediction model (MM5), results can be counted out once an hour. The spatial resolution can be accurate to less than 20 km. According to the centre pressure, centre wind power, shift speed and path of typhoon, wind direction, wind power and duration of the cold air, the rising water value caused by synoptic system can be counted out. After the method is improved, the forecasting interval, space density and accuracy are improved remarkably.