华北地区低云量的变化特征及其影响因子分析

Change Characteristic of Low Cloud over North China Region and Its Relationship with Large Scale Circulation

利用1961—2005年华北地区42个测站月平均低云量资料,采用趋势分析、Morlet小波分析等方法分析了华北地区四季低云量的年际变化和周期特征,并利用奇异值分解方法研究了秋季华北地区低云量与500hPa环流指数的关系,重点分析了西太平洋副热带高压(下称西太副高)面积如何影响秋季华北地区低云量变化。结果表明,近45年华北地区四季低云量减弱趋势显著,以秋季减少最多;在年代际尺度上华北地区四季低云量均存在准16年周期,夏季低云量在年际尺度上有显著的准2～4年周期特征;秋季华北地区低云量主要受西太副高面积指数、强度指数、Ⅰ区极涡面积指数、Ⅱ区极涡强度指数和北半球极涡强度指数影响;当秋季西太副高面积偏大(小)时,华北地区上空500hPa高度场偏高(低),上升运动减弱(增强),850hPa南风减小(增大),低层经向水汽输送随之减弱(增强),并且可降水量减少(增大)不利于(有利于)低云的生成;极涡主要是通过影响低层经向风,进而影响华北地区低云量。

Using 43-year observational data in North China region,the temporal variations of the low cloud are examined.The relationship between low cloud in North China and large scale circulation indexes are analyzed with SVD method,and the effect of the western pacific subtropical high（WPSH） for low cloud over North China is studied.The analyzed result shows that the low cloud over North China region has remarkably decreased in the past 45 years,especially in autumn.On the decadal time scale,quasi-sixteen-year periodic oscillation is observed in seasonal low cloud amount over North China region,while 2~4-year periodic oscillations are found in summer.Among all the large scale factors,both coverage and intensity of the western Pacific subtropical high,the coverage of polar vortex region I,the intensity of region II and Northern Hemisphere polar vortex are strongly correlated with the autumn low cloud over North China region.With the anomalously large area of Western Pacific subtropical high,the geopotential heights over North China region is much higher with stronger ascending motion,correspond in lower atmosphere the south wind weak with less water vapor transport,which lead to decreased low cloud over the area.Polar vortex influences the low cloud through the south wind over North China region.