A Study On The Relationship Of The Summer Asian Blocking To Strong Rain Events In China And Its Activity Mechanism

Using NCEP/NCAR daily reanalysis data and precipitation data from NMC to study the relationship between the Asian Blocking and summer rain belt in China, the Blocking events which occurred during period of Yangtze river flood in 1998 and Huaihe flood in Jun 2003 are Analyzed, and further, the effects of the Blocking process act to heavy rain and its establishing/maintaining mechanism are diagnosed. At last, combined with numerical model, a possible mechanism for complexity and inter-annual difference cause of the Blocking is advanced. The main results as follow:(l)The summer blocking activities are frequent in Eurasian high-mid latitudes. The lifetimes of most processes are less than 10 days, 3-day and 8-day events are prominent. They mainly occur in some latitude from 40°N to 70°N, five preponderant regions are divided in zonal band. Particularly, the Blocking frequency is most in Ural and east part of Baikal. There is more active part in every region. The intra-season, inter-annual and decadal scale vary features of the Blocking are very clear. Dual-blocking pattern is outstanding in Jun and the mid-blocking pattern raise contrasting to dual-blocking descending in July and the mid-blocking is prominent as a result of blocking activity increase in Baikal region. There exists longer cycle period except 1-year timescale and the period is different in the five region, such as the long period is 5. 8 year in east part of Baikal, which is shorter than Eurasia average. Most blockings are stable, the moving event is only 6.6% in total in Eurasia.(2)Strong rain events have characteristic of space-time variation, inter-annual and decadal variation are clear. The frequency of the continuous strong rain event raises with heavy rain's. As a rule high heavy rain year is commonly disaster. Persisting rain of more than 3 days occur most frequently in south china, south to Yangtze is also sensitive area, the boundary space of south Sichuan Guizhou, Yunnan province and Chongqing is a center. Rainfall that comes from persisting rain is less than 10mm except south of Liaoning province and eastpart of Xinjiang in the north of China. Intra-seasonal variation of the rain is notable, its location moves orderly from east to west and from south to north from Jun to Aug. comparatively, it is invariable in south of china .The circulation background of three kinds of strong rain event have common characteristic. The circulation is usually stable, long wave system anomalies lead to ceaselessly rain, especially, the Blocking is vital in some typical year. The background of three events in south China is clearly different, the large scale system is not entire reason, the meso-scale systems may cause heavy rain that occurs everyday.(3)The summer rain belt in china is closely related to the Blocking pattern inEurasia high-mid latitudes. Rain distribution is distinct with various blocking pattern. Rainfall is most severe when dual-blocking exist, the south to the Yangtze will occur frequently heavy rain, mid of the south is a center. East blocking pattern cause rain center shift to low-mid reaches of the Yangtze. Rainfall belt from east mid-blocking pattern is differ from that west mid-blocking causes. the rain belt locate in Huaihe valley when a blocking exist in the west of Baikal, but it takes usually place in south more when a blocking maintains in east of Baikal.The diagnostic analysis of Blocking affect rain in Jun 1998 and 2003 shows that the Blocking forces 500hPa westerly front to move south, leads cold air flow to low-mid latitude, results in exchange of air and energy at mid-latitude. An atmosphere energy front and matter front maintain in north of rain belt when blocking strengthen. The front locates in north to Huaihe, 850hPa subtropical front settles chronically near 35°N in Jun 2003, whereas, the front locates in south to 35°N in Jun 1998. Thus, heavy rain occurs ceaselessly with momentum and heat transporting.The location and intensity of Blocking is closely related to other westerly systems and subtropical High, the Blocking inf...