【作者】 陈官军；

【导师】 周秀骥； 魏凤英；

【作者基本信息】 中国气象科学研究院 ， 气象学， 2014， 博士

【摘要】 中国南方地区大范围持续性的强降水过程是造成该地区洪涝灾害的主要原因。这种持续性灾害天气，与稳定、异常的大气环流形势有关，而以两周到两个月为周期的大气季节内振荡在其中扮演了关键性的角色。同时，季节内振荡这种大尺度、准周期性的变化为延伸预报的开展提供了有利条件。本文从中国南方区域持续性强降水的定义和时空分布特征出发，首先分析了区域持续性强降水与降水30-60天季节内振荡的关系以及影响降水季节内振荡的低、中、高纬度环流系统；然后进一步探讨了造成中国南方夏季降水季节内振荡的原因和机制，以及这种机制与区域持续性强降水的联系；最后，利用NCEP/CFSv2的回报产品，验证了上述机制在海气耦合气候预报模式中的体现，并采用统计和动力相结合的方法开展了中国南方地区夏季降水季节内振荡延伸预报试验。主要结论概括如下：（1）在明确区域持续性强降水事件性质的基础上，定义了中国南方地区区域持续性强降水。该定义综合考虑了降水强度、持续性和区域性特征。基于该定义统计并分析了1981-2013年发生在中国南方的区域持续性强降水事件，发现：①区域持续性强降水过程的发生受季风系统和地形的影响，主要出现在华南、江南和江淮三个典型区域；②南方区域持续性强降水的平均持续时间为4.8天，其中江南地区平均持续时间最长；③夏季是区域持续性强降水事件的高发期，并且三个区域的夏季区域持续性强降水事件发生频率接近。（2）中国南方夏季降水30-60天季节内振荡分量的空间分布特征与区域持续性强降水的三种类型基本一致，即可划分为华南、江南和江淮三个典型降水季节内振荡区域。区域持续性强降水的发生与典型区域降水季节内振荡的位相和强度关系密切，有87%以上的区域持续性强降水出现在降水30-60天季节内振荡的大于一个标准差的正位相。（3）从热带海洋性大陆地区经南海至中国东部35°N附近的区域是影响中国南方夏季降水季节内变化的关键区，南海夏季风的季节内振荡活动是在造成降水季节内变化关键因素。南海夏季风关键区850hPa纬向风EOF分解的前两主模态代表了南海夏季风季节内振荡的主要特征：热带大气季节内振荡（MJO）在南海和西太平洋地区的北传与西太平洋副热带高压系统的东西振荡相配合。这一过程造成了南海夏季风活跃期和中断期的交替出现，进而导致了中国南方地区夏季降水的季节内变化，而MJO北传的范围和西太副高振荡轴的纬度位置是造成南方夏季季节内降水分区特征的主要因素。（4）在降水季节内振荡的湿位相，南海夏季风低频环流系统为中国南方区域持续性强降水的发生提供了“充足的水汽”，“大范围的上升运动”和“持续时间”等必要的环流条件，造成超过70%的区域持续性强降水过程发生在该时段内。（5）南海夏季风季节内振荡的主要机制可能包括海气相互作用和大气内部动力作用两个方面，且都是建立在南海地区南海夏季风爆发后气候态东风垂直切变的基础上。其中，海气相互作用主要出现在西太平洋副热带高压振荡区域，而大气内部动力机制是造成MJO北传的主要原因。（6）东亚中高纬度大气季节内振荡的主模态包括“乌拉尔山阻高型”或称“欧亚阻高型”，“双阻型”和“鄂霍次克海阻高型”，他们在区域持续性强降水中的作用主要体现在稳定环流形势和提供冷空气上，但是某一区域的降水季节内变化并不完全与特定类型的中高纬度季节内环流形势相对应。在南海夏季风季节内振荡造成“湿位相”的背景下，中高纬度大气季节内振荡的强度是造成区域持续性强降水的关键因素。（7）NCEP/CFSv2模式回报资料反映的南海夏季风季节内振荡特征与观测分析结果一致。采用将实况观测和模式预报值投影到南海夏季风季节内振荡EOF空间结构上的方法，定义了南海夏季风季节内振荡的实时监测和预报指数（Real-timeSCSSM Oscillation index）RSO1和RSO2。利用南海夏季风季节内振荡实时监测和预报指数与模式直接预报降水量相结合的统计动力延伸预报方法能够有效提高降水季节内分量的预报效果，并且修正了大部分对模式预报降水直接进行带通滤波而导致的负相关现象，也避免了因带通滤波造成的数据的损失。同时，利用模式历史资料进行建模能够起到了消除模式系统误差的作用。该方法对于在延伸期预报中加强对区域持续性强降水过程的判断能力具有实际应用价值。更多还原

【Abstract】 Regionally persistent heavy rainfall (RPHR) is one of the main types of disastrousweather that is most likely to cause large-scale severe flooding and endangers the lives andproperty of the public. RPHR events in China are unpredictable, frequent, and persistent.They have been responsible for many catastrophic floods in southern China. As thestability of large-scale circulation is essential for the persistence of rainstorms,atmospheric intraseasonal oscillation (ISO) may also be a potential signal for RPHR.Meanwhile ISO is valuable for the extended range forecast (ERF) of rainfall due to itsquasi-periodicity, time scale, and spatial scale. A further understanding of the relationshipbetween RPHR and ISO would be helpful for improving ERF of persistent heavy rain. Inthe present study, basing the temporal and spatial structures of RPHR, the relationshipbetween ISO and RPHR in southern China was investigated firstly. Then the causes andmechanisms of ISO in the summer rainy season of southern China and its influence onRPHR were studied. Based on the mechanisms and the relationship between ISO andRPHR, using the reforecast from NCEP/CFSv2, a statistics-based ERF method for therainfall ISO was test. The major conclusions are as follows:(1) Defined the RPHR over the southern China by considering its range, intensity andduration. Based on the definition, the statistical characteristics of the RPHR are as follows:①The high incidence areas of RPHR are South China, the south of the Yangtze River,and the Yangtze–Huai River Basin;②The average duration of three areas is4.8days.The duration of RPHR in the south of the Yangtze River is longest;③Most of theRPHRs happen in summer, and the incidence rates are close in three areas during thesummer.(2) Based on the REOF modes of30–60-day bandpass filtered rainfall, southernChina can be divided into three typical regions which are similar to the high incidence areas of RPHR. The ISO rainfall index (IRI) was defined as the average regionalprecipitation in each region. The RPHR events that occurred in the typical regions werefound to be strongly related to the intraseasonal variation in the rainfall; indeed, more than87%of the RPHR events occurred when the IRI was larger than1SD.(3) The mapping of regression coefficients relating U850’ to IRIs indicated that theSouth China Sea summer monsoon (SCSSM) systems play an important role in regulatingintraseasonal rainfall over southern China. EOF analysis was performed on the850-hPazonal wind which was averaged over110°–120°E, to present the main mode of theSCSSM. Then, a pair of ISO modes of the SCSSM was identified by performing Morletwavelet analysis on the leading PCs of EOF1and EOF2. Composite analyses based onPC1and PC2revealed the evolution of the ISO in SCSSM: The accompanyingeastward-propagating MJO, and the east-west oscillation of the Western Pacificsubtropical high (WPSH), producing alternating dry and wet phases over southern China.These alternating periods last approximately40days and modulate the intraseasonalvariation in the rainfall. EOF1of the850hPa zonal wind over the SCS and southern Chinamainly represent the ISO mode controlling the intraseasonal variation of rainfall in thesouth of the Yangtze River, while EOF2leads to the intraseasonal out-of-phase rainfallover South China and the Yangtze–Huai River Basin.(4) The RPHR was found to be closely related to the ISO in SCSSM. In the threetypical regions, more than70%of RPHR events occur during wet phases from April toSeptember. During wet phases, moisture-rich air is transported to southern China at lowerlevels by the southwestward flow to the north of the WPSH. The unstable loweratmosphere coincides with an upper-tropospheric divergence and a low-level positivevorticity center that promotes heavy rainfall. The structures of ISO systems are reversedduring dry phases and suppress the rainfall over the corresponding regions. The region ofRPHR is determined by the zonal position of the WPSH.(5) The meridional-phase structure of the composite ISO modes indicated that thenorthward-propagating ISO and local air–sea interactions over the SCS are two keyfactors that promot the fluctuations in the geopotential height anomaly. Additionalnumerical simulations would be needed to identify the dominant mechanism.(6) Coupling between the ISO in SCSSM and the mid-and high-latitude systems,especially blocking situations, also play important roles in promoting the occurrence ofRPHR over southern China during wet phases. The main modes of the mid-andhigh-latitude ISO systems inclunde “Ural blocking”,“Ural-Okhotak double blocking” and “Okhotak blocking”. These modes provide cold air and stable circulation for theRPHR but do not exactly correspond to the RPHR of a certain region.(7) The SCSSM ISO is also found in the NCEP/CFSv2reforecasts and thecharacteristics of which are similar to the observation. The Real-time SCSSM ISOindeies (RSO1and RSO2) were defined by projecting the NCEP/CFSv2reforecasts onthe EOF spatial patterns of the SCSSM ISO. The statistics-based ERF method for therainfall ISO, by using these indeies, provides useful forecasts of precipitationintraseasonal anomalies up to10to90days of lead times in the southern China.更多还原