Based on daily precipitation data over North China in summer and the NCEP/NCAR reanalysis atmospheric circulation data, by using of power spectral analysis, Butterworth band-pass filter and other related methods, the low-frequency characteristics of daily precipitation over North China in summer and the features of atmospheric low-frequency circulation evolutions are investigated. The results indicated that low-frequency characteristics of summer rainfall in North China is mainly quasi-biweekly oscillation. In making extended-range forecasts, in addition to focusing on 30-60 day atmospheric oscillation signal, we also should pay attention to quasi-biweekly atmospheric oscillation signal. On 500 hPa height field, atmospheric low-frequency circulation are closed high and low pressure systems, simple and easy to identify. On 850 hPa, low frequency current field changes are a bit more complicated, but they regularly change along with the high level low-frequency systems moving. On 500 hPa height field, north of 35°N, the low-frequency system moves eastward, sometimes extended from Okhotsk Sea to the west. However those south of 35°N appears to be from the Northwestern Pacific to northwest, and then moved to west. In North China, the summer precipitation is mainly directly affected by the low-frequency system from high latitudes, and those from low latitudes is difficult to directly affect the North China, but through changes of the southerly or northerly airstream they can effects water vapor transport or cold air activities to North China, thereby increasing or decreasing precipitation intensity in North China. When the low-frequency low pressure on 500 hPa within 40°N-60°N moving from the west to the east near North China, it will induce cyclone or shear line build on 850 hPa flow field, so obvious precipitation appears in North China. When the low frequency low pressure on 500 hPa moves eastward out of North China, on 850 hPa the low level flow field will switch to divergence field or a consistent northerly wind, precipitation process is finished. Precipitation occurs, the low-frequency circulation of high level and low level has a good matching, these can be applied to extended-range forecasts of summer rainfall over North China.
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