利用19812010年中国753站逐日降水观测资料、 NCEP/NCAR第二套逐日再分析资料及实况天气图等, 选取长江中下游32次大范围持续性强降水过程, 分析了该类强降水过程与月内(10~30天)低频降水的联系, 并重点讨论了形成该类强降水过程的可能机制.结果表明: (1)长江中下游夏季降水具有显著的月内低频振荡周期.大范围持续性强降水过程基本位于降水低频振荡的峰值阶段.(2)梅汛期(6-7月)月内低频降水峰值位相前期, 西太平洋副热带高压(下称西太副高)西伸北进, 高低空急流发展加强.在强降水过程发生期, 高中低层配置出现垂直方向上的最佳耦合; 而台汛期(8-9月)低频降水峰值位相前期, 西太副高东退南撤, 低空急流逐渐南落至长江中下游东南部, 与高空急流相配合, 为强降水过程的发生提供了有利条件.(3)梅汛期东北亚低频位势高度低值区南下, 与中纬太平洋西传的低频波列在长江中下游汇合.同时西太副高发展加强, 造成了长江中下游降水峰值位相南高北低的低频位势高度分布, 有利于强降水过程的发生; 台汛期伴随从热带西太平洋到日本海低频波列的西北向移动, 菲律宾东北部的低频气旋及其北侧低频反气旋的降水峰值位相分别移至长江中下游和东北亚地区, 导致暖湿、 干冷气流在长江流域交汇, 进而造成强降水过程.(4)菲律宾以东洋面低频强对流可作为梅汛期和台汛期强降水过程发生的前期热带信号, 提前低频降水峰值位相10天左右.
Using daily observed precipitation data from 753 stations of China from 1981 to 2010, NCEP/NCAR daily reanalysis data and real-time weather map, selecting 32 large-scale persistent heavy rainfall processes in the middle-lower Reaches of the Yangtze River, the relationship between the processes and intra-monthly (10~30 d) low-frequency rainfall and possible forming mechanism of the process are analyzed. The results suggest that: (1) Rainfall in the middle-lower Reaches of the Yangtze River has significant intra-monthly low-frequency oscillation (LFO). Most large-scale persistent heavy rainfall processes are in its peak stage. (2) Before the peak phase of LFO rainfall for Meiyu flood period, the west Pacific subtropical high extends northwestward, upper and lower jet streams strengthen. At peak phase of LFO rainfall, upper, middle and lower circulation coordinate with each other very well. While before the peak phase of LFO rainfall for typhoon flood period, with the southeastward retreating of subtropical high, lower jet streams extend southward, cooperating with the upper jet streams, which provide favorable conditions for the heavy rainfall. (3) At Meiyu flood period, LFO wave train in north Pacific propagates westly and LFO vortex in the northeast Asia propagates southward. They converge at the middle-lower Reaches of the Yangtze River. At the same time, with the northward propagating of high value system of LFO geopotential height in South China Sea, the corresponding distribution of "south high north low" in LFO geopotential height field contributes to the heavy rainfall process. For typhoon flood period, with the northwestward propagation of the LFO wave train from the tropical west Pacific to the Japan Sea, LFO cyclone at northeast of Philippines with LFO anticyclone on its north moves to the middle-lower reaches of the Yangtze River and Northeast Asia at peak phase of LFO rainfall, which contributes to the severe convergence and results in the heavy rainfall process. (4) Positive anomalous convection over east of Philippines can be used as previous tropical signals of the heavy rainfall process at Meiyu and typhoon flood period,which is in advance of rainfall peak phase about 10 days.
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