Climatic Features and Cause Analysis of Extreme Rain Events in Shaanxi during the Midsummer of 2010

Based on analyzed NCEP/NCAR data, the mechanisms for extreme rain events during the midsummer of 2010 has been investigated. The results show an abnormal general circulation caused mainly by the sea surface temperature. There is a two ridges and one trough circulation pattern above mid-high latitude at 500 hPa, where the Eurasia and the Ural blocking highs and the low between the Aral Sea and Lake Balkhash were kept stable, and the small trough split from the low migrated eastwards along the westerly in mid-latitude Asia, converging with warm humid air masses from the northwest edge of the west Pacific subtropical high above Shaanxi. These factors directly resulted in extremely rainy weather during the high summer of 2010. The greater amounts of water vapor from the periphery of the Arabian Sea, the highs in the South China Sea and the western Pacific subtropics transported to Shaanxi at 850 hPa, and the formation of a strong cyclonic convergence zone in Shaanxi and its upstream region were all very conducive to the occurrence and continuation of extreme precipitation. At 700 hPa, two clear vapor conveyor belts and, upstream of Shaanxi, a generating and developing southwest vortex (the main system causing the rainstorm in Shaanxi) occurred. Decadal variations in precipitation during high summer also indicated that after 2010 Shaanxi was in a decadal period during which the possibility of extremely rainy weather was quite great. Further, during La Niña event years, the high summer in Shaanxi also favours extremely rainy weather.