【作者】 刘海文；

【导师】 丁一汇；

【作者基本信息】 中国气象科学研究院 ， 气象学， 2009， 博士

【摘要】 论文利用1957-2006年中国740站逐日降水资料和NCEP/NCAR逐日再分析资料,采用EOF分析,小波分析等多种统计方法和天气诊断分析方法,从天气和气候学角度,对华北汛期降水的四种变化,即月内时间尺度变化,年际时间尺度变化,年代际变化以及长期趋势变化,进行了研究和分析,尤其从大气环流以及大尺度降水条件的角度,重点研究和分析了形成华北暴雨以及华北汛期降水年代际变化原因,得到以下结论:(1)从气候学意义上确定了华北汛期的开始和结束日期,即华北汛期大致始于6月30,止于8月18日,持续期大约为50天,其中7月19日-8月14日,为华北雨季。华北汛期的开始和结束,具有明确的天气学意义。在整个华北汛期期间,华北区域上空对流层以下基本为上升运动所控制,而在汛期前和后,华北区域上空的垂直运动并不具备这些特征。华北汛期开始时,中国大陆和中国东部海洋为“东高西低”的异常海平面气压场配置,异常偏南风已经到达了30°N附近,此时华北开始进入对流性天气多发期;而当华北汛期结束时,地面上中国大陆和其东部海洋地区出现“西高东低”的异常海平面气压场配置,异常偏北风控制整个中国东部地区,华北汛期结束。(2)华北汛期降水在1978年前后发生了年代际减少,这和华北地区雨带的年代际变化密不可分。从1978年前到1978年后,华北雨带降水强度由大变小,准纬向雨带由位置偏北变为位置偏南,准经向雨带由位置偏西转变为位置偏东。同时,中国东部雨带的移动也表现出显著的年代际变化特征。在华北汛期降水偏多阶段,准纬向雨带出现时间较早,存在明显北抬特征。准经向雨带,位置偏西,雨带基本能够接近华北西部边界。在华北汛期降水偏少阶段,准纬向雨带出现时间较迟,雨带北抬和西伸都不明显。西太平洋副热带高压的变动,华北上空以及渤海湾附近的大气稳定度,华北整个汛期期间东亚夏季风的年代际变化是导致雨带出现这种年代际变化的重要因素。华北汛期降水量、5类降水频率及其贡献率都有一定的下降趋势,华北汛期降水量的多寡主要受大雨的频率和暴雨的贡献率影响。华北暴雨贡献率的年代际突变是造成华北汛期降水量发生年代际突变的一个主要因素。(3)由于西太平洋副热带高压,东亚夏季风的月内时间尺度周期振荡存在显著的年代际变化,使得华北汛期降水在两个不同阶段,存在着显著不同的月内时间尺度的周期振荡。在华北汛期降水偏多阶段,整个汛期从开始到结束,都存在单一的、显著的3-8d天气时间尺度的周期振荡高频降水,有明显的“7下8上”之规律;在华北汛期偏少阶段,显著的天气时间尺度的高频降水,直到7月中旬才开始出现,出现时间较迟;而在7月22日以后,华北汛期日降水又存在着3-8d和10-20d两种时间尺度周期振荡的叠加,并且,在8月上中旬,10-20d低频降水以负位相开始,正位相结束,从而使华北汛期日峰值降水集中于“7下8上”的规律被打破。(4)根据华北暴雨日降水量落区的相似性原则,通过K-means聚类方法,可将华北暴雨分成3类。华北暴雨发生次数的年代际减少,主要是由“北部”型暴雨所造成。其在1978年以后减少的次数占1978年前暴雨总次数的1/3。分别对发生在1978年前和1978年后的“北部”型暴雨进行合成分析表明,在两个不同的阶段,形成暴雨的大尺度降水条件也发生了显著变化。发生在1978年后的暴雨和发生在其前的暴雨相比,发生日当天的高低空急流的强度在减弱,供应华北暴雨的水汽与湿度条件减弱,影响华北暴雨天气的气团也较干。但有意思的是,暴雨发生日当天的抬升凝结高度偏低,由于大气层结变得较为干冷,自由对流高度更高,但CAPE值较大,大气不稳定能量较大。K指数,A指数,全指数以及肖沃特指数等稳定度指数表明,偏少阶段的暴雨比偏多阶段的暴雨对流程度要强。(5)除了影响华北汛期降水的大气环流在1978年前后发生的年代际变化以外,大尺度降水条件也在1978年前后发生了年代际变化。即由南边界和西边界的异常水汽输送,改变为由北边界和东边界异常水汽输送;华北地区水汽由异常水汽辐合和盈余,改变为异常水汽辐散和亏损;东亚副热带高空西风急流位置由原来的偏西偏北,改变为偏东偏南,强度由强变弱;华北区域由低空异常上升、高空异常辐散运动改变为低空异常下沉、高空异常辐合运动;而其南部渤海至江淮流域由异常辐合下沉运动改变为异常辐散上升运动;由原来能够到达华北北部甚至华北最北边界的海洋暖湿气团,改变为只能抵达黄河南岸;高原上空大气热源由异常偏强改为异常偏弱,直接导致高原东部邻近地区包括华北在内的纬向垂直环流,由异常上升改变为异常下沉。高原及其邻近地区的大气视热源和华北上空视水汽汇的年代际减少,是华北汛期降水年代际减少的重要热力因素。更多还原

【Abstract】 The four kinds of variabilities of rainfall during rainy season over North China including submonthly variability,interannual variability,interdecadal variability and long-term trends, are studied in this paper based on 740 stations daily rainfall datasets in China and NCEP/NCAR reanalysis daily datasets, with EOF,wavelet analysis, other various statistical analysis and weather diagnostic analysis methods,especially in the viewpoint of atmosphere circulation and large-scale precipitation conditions. The major conclusions are as follows:(1) Rainy season for North China has an average duration of 50 days, starting from Jun 30 and ending on Aug 18 from climatology based on the understanding about the period of rainy season. Thereinto,it is the period of great rainfall for the rainy season of North China from Jul 19 to Aug 14.The initial dates and final dates of rainy season over North China have definite meaning of synoptic meteorology. It is controlled by the upward motion beneath the troposphere over North China region, but it has not these characteristics before and after rainy season of North China. When exits positive anomalies of geo-potential height over the west Pacific, meanwhile exits negative anomalies of geo-potential height in the 5oohpa filed over the North China, and the pattern is an anomalies of“east high and west low”between the Chinese mainland and west Pacific in the surface, that results the anomalies of south wind reaches 30N,the rainy season begins over North China; However, when exits smaller positive anomalies of geo-potential height in the 5oohpa filed over North China ,exits positive anomalies of geo-potential height in the 5oohpa filed over Japanese sea, and the pattern is anomalies of“west high and east low”between the Chinese mainland and west Pacific, that results the anomalies of north wind influence the Eastern China, the rainy season ends over North China.(2) There is close relationship between the interdecadal decrease of the rainfall over North China and interdecadal change of rain belt over North China. The intensity of rain belt over North China alters weaker from stronger; quasi-zonal rain belt locates from northward to the southward and quasi-meridian belt locates from westward to the eastward before 1978 and after 1978.Meanwhile, the shift of rain belt over Eastern China exits significant characteristics of interdacadal changes, that is, quasi-zonal rain belt appears earlier and shifts obviously northward; Quasi-meridian belt locates westward and can reach the borders in the west of North China during the flood period of rainfall over North China; on the contrary, quasi-zonal rain belt appears later and two rainfall does not shift obviously northward and weatward.The shift of west pacific subtropical high, the atmospheric stability over North China and nearby Bohai Gulf are important factors which result in these rain belt interdecadal change. The rainfall over North China during rainy season, five kinds of precipitation frequency and precipitation contribution over North China during rainy season have certain decreased trend with the different decreased rate. The rainfall of North China during rainy season is mainly influenced by heavy rain frequency and excessively heavy rainfall contribution. And excessively heavy rainfall contribution during rainy season results in the interdecadal abrupt change of rainfall over North China.(3) There exits significantly different submonthly timescale oscillation for the rainfall over North China during rainy season in the two different interdecadal periods due to interdecadal variability in west pacific subtropical high and submonthly timescale oscillation of east Asian monsoon. It exits an obviously singular high frequency rainfall with the 3~8days synoptic timescale periodic oscillation and has a distinct rule of“7 up and 8 down”in the flood period from the beginning and ending of the rainy season over North China;however,it does not take place until mid-July for the distinct synoptic timescale high frequency rainfall, and it appears little later, and there are two timescales with 3~8 days synoptic timescale superimposed 10~20days timescales periodic oscillation for the daily rainfall over North China.furthermore,it appears as negative phase and ends positive phase for the low frequency precipitation in early and mid Aug, which does dot obey the laws which the peak rainfall over North China during the rainy season takes place in the“7 up and 8 down”.(4) Heavy rainfall can be categorized into three types according to the similar distribution of its daily rainfall amount by means of K-means clustering algorithms. The interdecadal decreased days of heavy rainfall over North China are mainly caused by the North pattern. The decreased days before 1978 account for one-third of total days after 1978 for heavy rainfall over North China. The composite analyses show that there are distinct interdecadal changes for the heavy rainfall over North China occurring before 1978 and after 1978. The heavy rainfall over North China occurring after 1978 compare to that occurring before 1978,the intensity of the upper level jet stream and low level jet weaken, so do water vapor supply and humidity conditions over North China, and air mass that influences the heavy rainfall over North China is also drier in the heavy rainfall day.however,it is most interesting things that lift condensing level is lower while height of free convective is higher because of drier and colder atmospheric stratification, and atmospheric unstable energy is greater due to the bigger value of CAPE. The stable indexes, such as K-index,A-idex,Total Totals index and Showlter index indicate that heavy rainfall over North China occurring in the drought period is stronger convective than that occurring in the flood one.(5) The atmospheric circulation which influences the rainfall over North China during rainy season takes place the interdecadal change around 1978,so do the large-scale precipitation conditions. All the changes are as followings: Anomalous water vapor transport from south and west boundary changes into that from north and east one, while the water vapor from anomalous convergence and abundance converts into anomalous divergence and absence, and the location of east Asia west jet stream in high troposphere is from the westward and northward to eastward and southward, and intensity of east Asia west jet stream in high troposphere is from stronger to weaker, in addition, motion over North China is from anomalous divergent upward to anomalous convergent downward while it is from anomalous convergent downward to anomalous divergent upward between the Bohai Gulf and Yangtze River-Huaihe river valley. Warm and moisture air mass from ocean can reach in the north of North China and even can do the most north border before 1978, and they can only reach in the south bank after 1978.The atmospheric heat source becomes anomalous weaker from anomalous stronger over Qinghai-Xizang Plateau, which results in zonal vertical circulation convert into anomalous upward to anomalous downward in the east of Qinghai-Xizang Plateau and neighboring region including North China. The interdecadal change of the atmospheric heat source in the Qinghai-Xizang Plateau and apparent moisture sink over North China are important thermal factors for the interdecadal decrease of rainfall over North China during rainy season.更多还原