Statistical Characters And Typical Case Study Of Several Disastrous Weather In Weihai

Study on the formation mechanism of disaster weather and changes in the law, has become the base of our meteorological disaster forecast, disaster prevention and disaster reduction. This paper focus on the statistical analysis of disastrous weather such as cold air gales, heavy snowstorms, hot weather, fog, and rainstorms of typhoon. Moreover, synoptic analysis and physical field analysis are applied in typical cases. The results are as follows:(1) Statistical characteristics of cold air gales. During Dec28,2010and Jan12011, the cold air process of sustained gale and strong temperature decrease in Weihai was caused by the attenuation of the polar vortex, the trough west of Lack Baikal moving southward, and the combination of strong continental surface cold high and the and low pressure on the sea area. The area range and strength of temperature decreasing can well indicate the path and strength of the cold wave. It becomes more evident when positive vorticity replaced by negative vorticity before the cold wave, and change in to positive vorticity after, as well as the axis of jet stream in the500hPa level north further than in the300hPa level. Updraft gradually changes into downdraft from higher level downward to the lower level as the cold air moving southward and downdraft changes into updraft when the cold air process come to an end.(2) Statistical characteristics of snowstorm. The cold flow snowfall plays a major part in the heavy snowstorm process. With the establishment of Ural blocking high, development and movement of Baikal cyclone, transversal trough after the vortex with cold air moving southeastward intruding Shandong Peninsula, such path of the cold vortex is extremely beneficial to the severe convective weather in Weihai. Positive temperature anomaly in the Yellow and Bohai Sea in December provides ideal condition of temperature and humidity. Weihai, as the snowfall area, is in the position of the anterior part of the high energy area and the south of temperature advection front. Moreover, the concerning district also corresponds to the area of vast instability energy of the northwest flow. Positive vorticity area in the high level changes into negative vorticity, as well as convergence changed into divergence, meanwhile convergence and updraft sustained in the low level, all the above factors triggers energy release of the mid-scale systems, which corresponds to the concentrated snowfall in the sustaining snowfall process. The snowfall gradually terminates as the updrafts in the higher level in the concerning area change into downdraft.(3) The highest temperature in the year appears in a relatively long lasting time range. It appears from June to September, primary in the second and last ten days in June and July. The highest temperature in the year is about34℃. Deep continental warm high is the primary reason of the hot and dry weather. Downdraft in the high level is beneficial to the subsidence and temperature increasing. It is proved in GM (1,1) model that in sunny days with low humidity, surface temperature increases as a result of sun radiation; therefore the continental hot wind becomes the leading cause of extreme hot weather in Weihai. Prediction accuracy can still be increased by improving GM (1,1) model.(4) Fog is possible to happen all through the year in Weihai, especially from April to July, most in July and least in September. Advection fog shows a daily change, it happens mostly before dawn, as the time of sunrise changes in different season,. Fog most possibly happens when air temperature is higher than sea temperature by about1℃, and the SSE-S wind is4-5Beaufort scale. Fog happens mainly within25km in the coastal areas other than Chengshantou and rarely inland.(5) Statistical characteristics of typhoon rainstorm. Synoptic Diagnosis reveals that this heavy rainstorm process was caused by a typhoon landing and going northward. In the area of heavy rainstorm in typhoon periphery, the severe rainfall area was consistent with the front of θse. Development of LLJ provided sufficient water vapor, resulting in vapor convergence in east of Shandong Peninsula. Coupling of upper level jet in200hPa and LLJ in850hPa created strong updraft in the east of Shandong Peninsula. Seen from the vertical structure of the typhoon, location of heavy rainstorm matched along with areas of LLJ convergence, upper level divergence and strong updraft.