2014年夏末长江中下游阴雨寡照天气分析

Rainy and Sunless Weather in the Middle and Lower Reaches of the Yangtze River in Late Summer of 2014

针对2014年夏末长江中下游地区长时间阴雨寡照天气的成因,利用全国站点观测资料和NCEP再分析资料分析得出:主要由于南亚高压、西太平洋副热带高压和东亚季风等大尺度环流异常所致。200 h Pa南亚高压中心与常年相比位置偏东南,形成有利于该区域降水的动力条件。500 h Pa北半球极涡强度偏强,导致影响中国的冷空气势力也明显偏强。西太平洋副热带高压比常年偏强、偏南、偏西,850 h Pa副高环流明显偏强。8月下半月欧亚中纬度环流形势可分为2个时段:时段1(16—20日)500 h Pa呈现两脊两槽分布,环流经向度明显强于常年,东端的低槽给中国中东部地区带来较强的冷空气。850 h Pa华南的偏西气流偏强,与副高气流共同增强了东海的西南风低空急流。时段2(21—31日)500 h Pa环流变成一脊一槽型,经向度减弱,造成小股冷空气不断到达长江中下游地区。两时段中纬度地区均存在θse锋区、锋生、锋前的垂直上升运动、中低层水汽辐合和由地面往上的湿舌。时段1各物理量表现更清楚,与天气表现相一致。

Based on the observations and NCEP reanalysis data, the authors analyzed the reasons for the rainy and sunless weather in the middle and lower reaches of the Yangtze River in late summer of 2014. The results showed that： the circulation anomalies were caused by abnormal large scale weather patterns, including the south Asia high, the western Pacific subtropical high and the east Asia monsoon. The location of south Asian high at 200 hPa was by southeast relatively to the normal, which was dynamically favorable for precipitations in the region; the polar vortex intensity at 500 hPa in the northern hemisphere was stronger than normal, leading to strong cold air masses intruding in China; both the western Pacific subtropical high and anti-cyclone circulation at 850 hPa were stronger and much by south and west to normal; two different circulation patterns in Eurasian mid-latitude displayed in the second half of August： period 1 （16-20）, the pattern with two ridges and two troughs showed at 500 hPa, with meridional circulations much stronger than normal and more favorable for cold air masses to enter central and east China; the strong westerly flow over south China at 850 hPa together with the subtropical high enhanced the southwest low level jet over the east China sea; period 2 （21-31）, only one ridge and one trough circulation existed at 500 hPa with meridional circulation weakening, which was favorable for weak cold air to reach the middle and lower reaches of the Yangtze River. There existed 0,~ frontal zone, frontogenesis, up lift, mid-low-level water vapor convergence and wet tongue from the ground during the two periods, with all the physical elements being much significant and consistent with the weather during period 1.