BAROTROPIC AND BAROCLINIC PATTERNS OF ATMOSPHERIC CIRCULATIONS RELATED TO MONSOON TYPES

In the context of 1982—1994 NCEP/NCAR wind at 12-level isobaric surfaces on a global basiscalculation is made of the barotropic(mass-weighed vertical mean)and baroclinic components(difference between the actual wind at each level and barotropic component)of atmospheric flowfields,followed by dealing with the distribution features of barotropic and baroelinie patternsglobally in winter and summer,alongside with the classification of global monsoons according tothe surface barotropic/baroclinic patterns.Evidence suggests that the seasonal variation of bothcomponents will lead to the reversal of a prevailing wind between winter and summer,thus causinga related monsoon:the baroclinie flow pattern is indicative of a thermal circulation driven byatmospheric inhomogeneous heating chiefly from land-sea thermal contrast whilst the barotropiccounterpart represents the result mainly from dynamic effects,which is helpful to theunderstanding of monsoon nature.And further study shows that the classical monsoon regions intropical Asia,Africa and South America fall into a baroclinic category,those in the bi-hemispheriesubtropical Pacific into a barotropic type and the East Asian subtropical monsoon generated underthe joint action of both the patterns falls into a mixed category.     

华西秋雨区域性极端降水的环流特征

利用1961—2010年中国743个测站的逐日降水资料和NCEP/NCAR再分析资料,采用百分位方法挑选出了发生在华西秋雨期(8月9日—10月12日)的161次区域性极端降水事件,将其分为两类分别讨论了同期和前期的异常环流特征,并与一般性降水进行了对比。结果表明,对于第一(二)类极端降水,华西地区气旋性距平环流(气旋性风切变)形成低层风场辐合,同时高层为辐散,由此产生强烈的上升运动,来自热带海洋的水汽输送偏强(偏弱),在华西秋雨区形成水汽辐合,副热带高压位置偏西、偏北使得华西地区西南暖湿气流偏强,巴尔喀什湖以北地区为正(负)高度异常,形成一槽一脊(二槽一脊)环流型,西北气流由此加强,冷暖空气交汇于华西地区从而形成极端降水。两类极端降水均与一般性降水具有相似的异常环流特征,只是造成极端降水的异常环流强度均比一般性降水更强。两类极端降水的上述异常环流型在极端降水发生前一天就已经存在,可以作为华西秋雨极端降水短期预测的强信号。