Mechanism Of Summer Rainfall Anomaly In Huanghuai Area And The Using Of Downscaling Method

Huanghuai area lies in the transition band of plentiful rainfall area and arid area.Cause of climate anomaly is complicated. Accuracy rate of summer rainfall predictionis low. In this paper, using400stations rainfall data，evenly distributed all overChina, using orthogonal REOF technology, the rainfall in China is divided into13areas. The summer precipitation in Huang Huai area is selected as the researchobject.Under the background of El Nino and La Nina, the cause of the regionalsummer precipitation anomaly, external forcing and its influence mechanism arestudied. Based on the detrended data, a downscaling interpretation and applicationprediction method is developed. The main results of this research can be summarrizedin the following items:(1)The result of13regions division makes the partition results in the Midwestdivision more clear and complete. Huanghuai area include The area(30oN-36.5oN,105oE-125oE) is defined as Huanghuai,including suoth of Shanxi,Henan, Shandong, Jiangsu, Anhui, North of Hubei. It is the studying object in thispaper.(2)Under the background of El Nino, when the precipitation is more inHuanghuai Area, shear line between North and Southwest wind appears on theHuanghuai Area. The Subtropical High is stronger.The key areas which influencesummer precipitation in Huanghuai area mainly locate in tropical India ocean and theWestern Pacific region, the Oyashio region, the central North Atlantic region. Underthe background of La Nina, when the precipitation is more in Huanghuai Area, theEASM and polar vortex are stronger,the Subtropical High is weaker. The key areaswhich influence summer precipitation in Huanghuai area mainly locate in SouthernIndia Ocean, near the North Equatorial current and equatorial the Atlantic.Accordance in the tropical India ocean SST will make the precipitation in HuanghuaiArea more under the background of El Nino. But the negative dipole in SouthernIndia Ocean will make the precipitation in Huanghuai Area more under thebackground of La Nina. (3) Under the background of El Nino, warmer(colder) SST in tropical IndiaOcean, result in stronger (weaker) South Asia High and Subtropical High.Thus thewater vapor transportation along the northwest side of Subtropical High is more (less)and the precipitation in Huanghuai Area is more(less). Under the background of LaNina, if negative dipole mode in subtropical India Ocean in early winter appeared,SST in India Ocean wuold be low by the summer. Anticyclone would be over thesouthern hemisphere India Ocean and the bay of Bengal, Arabia Sea. The strongerEast Asian summer Monsoon led to more precipitation in Huanghuai area. If positivedipole mode in subtropical India Ocean in early winter appeared, SST in India Oceanwuold be high by the summer. Cyclone would be over the southern hemisphere IndiaOcean. The stronger East Asian summer Monsoon led to more precipitation inHuanghuai area. The stronger cross-equator flow near105°E led to cycloniccirculation on the East of China. Subtropical High was easterly. The precipitation wasless in Huanghuai Area.(4) Based on the hindcast data of the second generation Coupled-Ocean GeneralClimate Model of National Climate Center, the historical reanalysis data, adownscling method is developed. One-year out cross-validation tests indicate that thedownscaling method is applicable in the prediction of summer precipitation anomalyacross most of China with high and stable accuracy,and is much better than the directCGCM/NCC prediction. The forecast accuracy to the precipitation in NorthernXinjiang, Southern Xinjiang, Qinghai, Tibet, North China, Southeast Coast, theYangtze River and Guangxi region exceed67%. But in Huanghuai Area the forecastaccuracy is only62.5%.(5)Based on the hindcast data of the second generation Coupled-Ocean GeneralClimate Model of National Climate Center, the historical reanalysis data, theCorresponding Period circulation factors, which have physical meaning to theprecipitation in Huanghuai Area in Summer and can be simulated by the Model,willbe selected. The trendancy of the factors and the predictand will be eliminated. Adownscaling method is developed. The forecast accuracy of this method to theprecipitation in Huanghuai Area in Summer can reach75～85%. It is feasible toimprove the ability to prediction of regional precipitation to research and developdownscaling method in the future.