Modulation of Intraseasonal Oscillation Moisture Budget on Persistent Extreme Precipitation Intensity over the Middle and Lower Reaches of the Yangtze River
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摘要: 利用中国1979—2019年逐日降水格点数据，考量降水的时空聚集性强度之客观监测方法，识别中国东部区域极端降水事件，发现长江中下游是夏季持续3 d及以上区域持续性极端降水(PREP)发生最频繁的区域。以此为研究对象，基于水汽收支理论，利用ERA-Interim再分析资料和带通滤波方法，诊断日降水及其水汽收支的季节内特征，研究关键尺度对流层高低空系统配置及演变。结果显示：PREP日降水强度及其演变被证实与区域大气柱水汽辐合量相一致。10~30 d准双同振荡(QBWO)和30~90 d(MJO)的水汽辐合在事件发生前1~3 d和7~9 d开始由负位相转为正位相。区域南、北边界QBWO(MJO)水汽输送在事件发生前2~4 d(9~10 d)由水汽输出转为输入。南海的QBWO对流北传至长江中下游，促使源于西北太平洋的对流层低层QBWO反气旋式环流系统向西南移动，其西北侧的西南风使区域南边界水汽输送在事件开始日达最大。事件发生前10 d左右，对流层低层MJO反气旋式环流系统出现在西北太平洋，并向西南移动，长江中下游气旋系统加强维持，对流层中层孟加拉湾MJO尺度低槽开始加深，三者的配合使区域南边界的MJO水汽输入逐渐增强并维持到事件发生后。MJO尺度的强水汽输入结合QBWO反气旋式环流促使长江中下游地区的持续性极端降水事件的发生。研究结果对持续性极端降水事件延伸期的预报提供参考。Abstract: Based on the gridded daily precipitation dataset in China from 1979 to 2019, the extreme precipitation events in eastern China are identified by using an objective method that comprehensively considers the temporal-spatial strength of gathering. Persistent regional extreme precipitation (PREP) events lasting for at least three days in summer are found most frequently in the middle and lower reaches of the Yangtze River (MLRYR). Based on moisture budget theory, the intraseasonal characteristics of daily precipitation and its moisture budget are diagnosed, and the configuration and evolution of the troposphere systems at primary intraseasonal scales are revealed. The daily precipitation intensity and evolution of PREP proved to be consistent with regional atmospheric column moisture convergence. The 10~30 day quasi-biweekly oscillation (QBWO) and 30~90 day Madden-Julian oscillation (MJO) moisture convergence converted from negative phase to positive phase 1~3 days and 7~9 days prior to the event. The QBWO-related (MJO-related) moisture transport on the region's southern and northern boundary changed from outward to inward 2~4 days (9~10 days) prior to the event. The QBWO-related convection in the South China Sea propagated northward to the MLRYR, prompting corresponding anticyclonic circulation that originated from the northwest Pacific and was in the lower troposphere to move southwestward. Due to the southwest wind in the northwest, the moisture transport in the southern boundary reached the maximum on the onset date of the event. Ten days before the event, the MJO-related anticyclonic circulation in the lower troposphere appeared in the northwestern Pacific Ocean and moved southwestward, the cyclone system in the MLRYR strengthened, while the MJO-related trough that was in the Bay of Bengal and in the middle troposphere began to deepen. Together, these factors gradually increased the MJO-related moisture inward transport in the southern boundary and maintained it until the end of the event. Overall, the combination of the strong MJO-related moisture inward transport and QBWO-related anticyclonic circulation contributed to the occurrence of PREP in the MLRYR.
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图 5 长江中下游地区PREP事件合成的区域平均多尺度降水及大气柱水汽辐合的时间演变
a. 降水量；b. 水汽辐合。单位：kg·m-2·d-1。横坐标表示滞后事件发生日的天数(d)。
图 7 长江中下游地区PREP事件合成的到达区域东西南北四个边界的QBWO(a)与MJO(b)频段水汽输送通量净输入(105kg·s-1，正值表示净输入，负值表示净输出)以及区域净水汽辐合(kg·m-2·d-1)的时间演变
横坐标表示滞后于事件发生日的天数(d)。
图 8 长江中下游地区PREP事件合成QBWO分量下OLR(阴影，单位：W·m-2)和大气柱水汽输送通量(矢量，单位：150 kg·m-1·s-1)空间分布
a~e分别为滞后事件发生日-9、-6、-3、0和3 d，红色箭头表示合成的QBWO风矢量的显著性通过90%信度水平。
图 11 长江中下游地区PREP合成QBWO分量下850 hPa风场(左列，矢量，单位：m· s-1)和水汽辐合(阴影；单位：kg·m-2· hPa-1·d-1)；500 hPa位势高度(中列，红蓝线分别代表正负，黑线为原始场；单位：m2·s-2)；200 hPa势函数(右列，阴影；单位：m·s-1)和位势高度(红蓝线分别代表正负；单位：m2·s-2)异常的时空演变
第1~3行分别对应滞后事件发生日-3、0、3 d。左列中红色箭头与打点、中列中阴影以及右列中打点(对应于势函数)皆表示合成的QBWO对应要素场显著性通过90%信度水平。
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