Anlysis On Summer Extreme Precipitation Events And Their Circulations In The Hetao Area

Based on the daily precipitation data collected from740meteorologic stations inChina during1960-2009and the NCEP/NCAR daily circulation reanalysisdata(including height fields, wind fields, specific humidity),according percentilemethod, the extreme precipitation threshold values were defined in different stations,the summer(JJA) extreme precipitation events were analyzed,it is found that extremeprecipitation events in the Hetao has significant features: the variability of the EPE inthe Hetao area is large,the total precipitation of EPE accounted for an high ratio of thetotal precipitation. Chosing the rainfall data in the summer seasons of40stations inthe Hetao area,temporal and spatial characteristics of extreme precipitation events(EPE) in recent50years were studied. Chosing the representative of regional extremeprecipitation days, analyzed the water vapor transport features and the circulationevolution,finally the energy propagation in regional extreme precipitation process inHetao is analyzed as well.The main results as follow:(1)The trend of change of summer extreme precipitation events exist clear differencein every sub-regions in China,the EPE has decreased in North China and NortheastChina,while in the Yangtze River basin shows an increasing trend. It is found thatextreme precipitation events in east of Northeast China,North China,the Hetaoarea,South China and Southeast coast of China showing reverse change with otherregions is the primary mode by using empirical orthogonal function (EOF).(2)There is a good correlation between summer EPE and summer precipition in theHetao area,their spatial pattern both showed consistency in the wholebasin,northwest-southeast distribution,southwest-northeast distribution.The frequencyof summer extreme precipitation events had an decrease trend in the Hetao area inrecent50years; extreme precipitation events exist obvious internnual and interdecadalvariations:on decadal scales,it has quasi-15periodical; on interannual scales, there areperiodical variations of2.5-3years before1980. From intraseasonal,in summer heavy precipitation mainly occurred in July and August.(3)The moisture transport from the Indian Ocean or the Northwest Pacific Oceanplays an important role in the occurrence of regional extreme precipitation in theHetao area,the abnormal moisture transport from the Indian Ocean or the NorthwestPacific Ocean brought a sufficient water vapor supply for the heavy rain in the Hetaoarea, in addition the abnormal moisture transport from the Northwest China enhancesthe moisture to some extent.(4) Circulation evolution and wave-activity flux showed that from the Ural Mountainsto the Sea of Japan there is a “positive—negative—positive” anomaly wave train, thewave-activity flux dispersion energy downstream through the wave train, theexistence of the positive anomaly circulation upstream near the Urals and the positiveanomaly circulation downstream near the Sea of Japan enhanced and westward playsa key role when the regional extreme precipitation occurs.