Multiple-time-scale Variability Analyses Of Rainfall During Rainy Season Over North China And Study On It's Evolution Of Large-scale Precipitation Conditions

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.