1998年长江流域二度梅建立过程中西太平洋副热带高压异常调整的机制分析

MECHANISM STUDY FOR ANOMALOUS VARIATION OF THE WESTERN PACIFIC SUBTROPICAL HIGH DURING THE REBUILDING PROCESS OF THE MEIYU OVER THE YANGTZE RIVER VALLEY IN 1998

基于NCEP2再分析资料,利用全型线性位势诊断方程详细分析了1998年7月长江流域二度梅建立过程中西太平洋副热带高压(副高)异常调整的物理机制。研究表明,1998年二度梅的重新建立是由于副高南撤增强所致,期间以副高南、北两侧位势高度的"此长彼消"为显著特征。这一调整过程可以分为副高的减弱南撤和增强维持两个阶段,而且是由高、低纬天气系统共同影响的结果。地转涡度项和摩擦力散度项在这其中起到重要贡献,尤以前者贡献最大。在不考虑边界条件作用下,地转涡度项对位势高度的贡献量级为101gpm,而摩擦力散度项则为100 gpm,后者的影响仅局限于对流层低层。对于北侧位势高度而言,受中高纬度Rossby波调整所产生的低压槽影响,地转涡度项使其持续减弱,而南侧位势高度由于赤道反气旋加强北上,地转涡度项使其由减弱转为增强。摩擦力散度项则通过正反馈作用,分别使得上述的减弱趋势和增强趋势更明显。

Based on the NCEP Reanalysis 2 data, the variation of geopotential heights during the rebuilding process of the Meiyu over the Yangtze River valley in July 1998 is analyzed by using the linear geopotential equation. It is revealed that the rebuilding of the Meiyu is due to the southward retreat and enhancement of the western Pacific subtropical high （WPSH）, which is mainly characterized by respective decrease and increase of geopotential heights in the northem and southern part of the WPSH. This process consists of two stages, i.e., the first stage of decreasing and southwardretreating, and the second stage of increasing and maintaining. Moreover, the weather systems in high and low latitudes play a collective role in the adjustment process. The vorticity-related geostrophic mode and the friction divergence are the two most important items. Specifically, without considering the boundary effect, the former＇s contribution order is 101 gpm and the latter＇s is 10~ gpm, yet with the contribution constrained to the lower level of the troposphere. Due to deep depression troughs induced by Rossby waves in the middle and high latitudes, the geopotential height to the north of WPSH tends to decrease persistently under the impact of a vorticity-related geostrophic mode. Similarly, it is also this item that makes geopotential heights to the south turn to increase after reaching a minimum because of the northward movement of an equatorial anticyclone. Through positive feedback, the friction divergence item tends to accelerate the aforementioned decrease and increase trends.