华北平原夏季降水准双周振荡与低频环流演变特征

The Quasi-Biweekly Oscillation of Daily Precipitation and Low-Frequency Circulation Characteristics over North China Plain in Summer

利用华北地区夏季日降水资料和NCEP/NCAR再分析环流资料,采用功率谱分析、Butterworth带通滤波等方法,对华北地区夏季降水低频特征与大气低频环流演变进行了综合分析。结果表明:(1)华北地区夏季降水存在低频特征,主要以准双周振荡为主。(2)500 h Pa高度场大气低频环流表现为闭合的高、低压系统;850 h Pa低频流场演变要复杂一些,但它们伴随高层低频系统移动会发生有规律的变化。500 h Pa高度场上,在35°N以北,低频系统向东移动,有时也从鄂霍次克海附近向西的扩展;在35°N以南,低频系统表现为由西北太平洋向西北移动,然后再向西移动。华北夏季降水主要是直接受中高纬度低频系统影响,低纬度低频系统很难直接影响华北地区,但它们可以通过偏南偏北气流的变化影响向华北地区的水汽输送或冷空气活动,进而加强或减弱华北地区的降水强度。(3)当500h Pa 40°N-60°N范围内有低频低压从西向东移近华北地区,并诱发850 h Pa气旋或切变线生成,从而在华北地区出现明显的降水过程;当500 h Pa低频低压向东移出而后部低频高压移来时,华北地区低层850 h Pa流场就会转为辐散气流或为一致的偏北气流,降水过程结束。(4)华北地区降水发生时,高、低空低频环流有较好的配合。

Based on daily precipitation data over North China in summer and the NCEP / NCAR reanalysis atmospheric circulation data,by using of power spectral analysis,Butterw orth 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-biw eekly oscillation. In making extended-range forecasts,in addition to focusing on 30-60 day atmospheric oscillation signal,we also should pay attention to quasibiw eekly atmospheric oscillation signal. On 500 h Pa height field,atmospheric low-frequency circulation are closed high and low pressure systems,simple and easy to identify. On 850 h Pa,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 h Pa height field,north of 35°N,the low-frequency system moves eastw ard,sometimes extended from Okhotsk Sea to the west. How ever 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 h Pa 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 h Pa flow field,so obvious precipitation appears in North China.When the low frequency low pressure on 500 h Pa moves eastw ard out of North China,on 850 h Pa the low level flow field will sw itch 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.