Study of circulation characteristics in autumn flood period over Weihe and Hanjiang River basins and the causes of flood by the heavy rains in 2011

Based on the meteorological and hydrological data, the spatial and temporal distribution characteristics and large-scale atmospheric circulation pattern of autumn rainfall in West China in September 2011 and the relationship between flood and autumn rainfall in the related basins are analyzed by using climatic statistics and synoptic analysis methods. The results show that the distributions of autumn rainfall in West China in 2011 are mainly located in the Weihe River basin, the upper basin of the Hanjiang River and Qujiang River basin with the main features of rainfall distribution concentration and severe precipitation duration concentration. Analysis on large-scale atmospheric general circulation indicates that geopotential height anomalies over the mid and high latitudes of Eurasia present a typical wave train distribution with“+, -, +”from northwest to southeast, and the rain belt location move is directly influenced by the subtropical high swinging between east and west and maintaining stably, and then the superposition of the westerly trough and the plateau trough result in a wide range severe precipitation. The changes of water level in the Weihe River basin and the upper basin of the Hanjiang River are corresponding with the severe precipitation processes in these basins. Comparing with flooding years in autumn flood season over the years, although the accumulated precipitation in the same season in the Weihe River basin and the upper basin of the Hanjiang River in 2011 is less than that in 2003, process
concentration and higher intensity of heavy rain result in the most serious flood in these basins since 1983.