Study On The Low-frequency Oscillation Features Of Precipitation In Serious Flood And Drought Pre-flood Season And Its Relationship With Atmospheric Low-frequency Oscillation Over South China

85-station Daily precipitation data from1961-2010provided by the National Meteorological Information Center are used to select2004,2010as the serious drought and flood year of pre-flood precipitation over South China respectively and to investigate the the low-frequency variability on pre-flood season precipitation.On this basis,by using the NCEP/NCAR200、2010daily reanalysis data,the characteristics of other elements of atmospheric circulation has been discussed and the sources of atmospheric low-frequency oscillation signal has been revealed.The main results are as follows:(1) The pre-flood season precipitation in2010is mainly controlled by10-20d and30-60d oscillation.In2004,20-30d is another significant low-frequency cycle.In flood year, The rainfall tends to be abundant (interrupt), when the two low frequency components superpose in the peak (valley) phase, and less when superposing in the inverse phase.In drought year, the rainfall tends to be abundant (interrupt), when the three low frequency components superpose in the peak (valley) phase, and less when two of them are superposing in the inverse phase.(2) In2010,10-20d and30-60d low-frequency precipitation has the positive correlation with850hPa wind field、water vapor flux、potential vorticity (PV)、vertical velocity and500hPa height field which share the same cycle,it also has negative correlation with OLR in the same cycle. The results show that the two low frequency component of the PV are correspond,other elements show that10-20d low frequency component is more active than that of30-60d. The situation in drought years is similar of rainy years.(3)10-20d low-frequency precipitation in2010is strongly influenced by Mascarene anticyclone (cyclone)-Somali cross-equatorial flow. South China Sea-West Pacific (SCS-WP) anticyclone (cyclone) and the cyclone (anticyclonic) in east of Lake Baikal.10-20d water vapor sources lie in the South India Ocean near30°S, the areas between Sumatra and Kalimantan Island near the equator(southwest source), the equatorial middle Pacific region (southeast source) and the southwestern Europe over the middle and high latitudes (cold air, northwest source). The source of the10-20d cold air in the low troposphere is in the North China to the Bohai Sea area. Inclined315K isentropic surfaces are the areas where vertical vorticity grows quickly most easily, it is the path along which the upper cold air intrudes.(4)In2004,10-20d low-frequency precipitation is influenced by SCS-WP anticyclone (cyclone), the cyclone (anticyclone) in east of Lake Baikal and east of Balkhash Lake.10-20d water vapor sources lie in west of the Red sea (northwest source), the areas between Lake Baikal and sea of Okhotsk (northeast source), and the bay of Bengal (southwest source), respectively. For the drought year, the high PV pools which exists in the low stratosphere is the main source. There is no cold air transportation in the low troposphere.