Analysis on Atmospheric Circulation Characteristics of Rainy Year and Drought Year in Midsummer in Shaanxi Province

Based on the NCEP/NCAR 1979-2004 re-analysis data,the circulation characteristics of precipitation anomaly in flood season are analyzed against the condition of precipitation in midsummer(from July to August) in Shaanxi province.The results show that in rainy year,the main synoptic systems which impact precipitation show the type of three strong and one weak,i.e.western Pacific subtropical is high,westerly belt Mongolian trough and the Ural blocking high are strong and Tibetan high is weak.On the contrary,in drought year,the main synoptic systems which impact precipitation show the type of three weak and one strong,i.e.western Pacific subtropical is high,westerly belt Mongolian trough and the Ural blocking high are weak and Tibetan high is strong.In rainy year,500 hPa wind is convergence field in areas of the center of Shaanxi province or the flow field has a shear line from east to west.There are two moisture channels at 850 hPa,one is in areas of Bengal Bay to Sichuan basin and Shaanxi province.The other is in areas of South China Sea to northwest of Hubei province and Shaanxi province.In drought year,the center of Hetao Area is controlled by the northwest wind at 500 hPa and the moisture channel is not obvious or the moisture transporting becomes weak at midway at 850 hPa.