青藏高原东北部强降水天气过程的气候特征分析

根据青藏高原东北部地区降水特点,定义青藏高原强降水概念,利用该区域内各测站自建站以来的气象资料,分析青藏高原强降水的时空分布特征和相对强度。结果表明:青藏高原东北部地区强降水的分布明显受到地形影响,年降水量和强降水次数自东向西呈阶梯性递减趋势,分别在青藏高原东北部的外流河谷地区和东南部四川北部地区存在大值中心;外流河谷地区两侧山脉的年降雨量较大,年均强降水日数较多,河源处相对较小,具有河谷地形的特点;青藏高原强降水的时段集中,雨强大,局地性强,且具有夜发性的特点;强降水日数和站数具有明显的年代际变化特征,近10年来出现区域性强降水的次数增加;青藏高原东北部外流河谷地区强降水的相对强度较大,同长江以南地区暴雨相对强度差不多。

Climatic Characteristics of Heavy Rainfall in the Northeast Tibetan Plateau

The northeast Tibetan Plateau is the margin area with the topographic altitude down sharply,and which has the highly frequent interaction between synoptic systems over the plateau and the westerlies systems.The heavy rainfall in this area exhibits the features of the Plateau's special geographic position,topography and underlying surface.The concept of heavy rainfall in northeast Tibetan Plateau is defined first according to the characteristics of the precipitation and the need of the operational forecast in this region.The features of spatial and temporal distribution of rainfall and relative intensity of heavy rainfall are analyzed using the data of meteorological stations there from which the stations are built up to the year of 2000,the results show that the distribution of heavy rain is obviously affected by topography.The distribution of annual mean heavy rain frequencies in target area is very asymmetry and descending from the east to the west.There are two obvious big value centers,one is located in the valley region in the northeast of the plateau,the other lies in the north of Sichuan Province.The characteristic of annual mean precipitation is asymmetry,the precipitation in the southeast is more than in the northwest.The annual mean precipitation of Qaidam Basin is the least and the southeast is the largest.There is a much bigger value center in the valley region of the northeast Plateau,corresponding with two big value centers of the frequencies of annual mean heavy rain. The distribution of the rainstorm and its frequency in the valley area are analyzed,it is found that the distribution of its valley topography characteristics is consistent with the frequencies and the annual mean precipitation in the two sides of the mountain which are more than those in the rivers origins.The characteristics are the same as those in the north which concentrate on the period from July to August and these two months account for more than 60% of the total time.There is a distinct decadal variability in numbers of stations,days and frequencies of heavy rainfalls increase in recent decade.The heavy rainfall usually occurs at night with evident local variability and great relative intensity by which the largest precipitation of every process and annual mean rainfall are calculated.The annual mean precipitation in the valley area is much less than that in the south of the Yangtze River in China,but the relative intensity of rainfall in this region is as much as that in the south of the Yangtze River.