华南夏季12-30 d持续性强降水的低频特征分析

利用1982-2011年夏季（5-8月）中国气象观测站点逐日降水资料、NCEP/NCAR逐日再分析资料、NOAA逐日向外长波辐射和海表温度资料集，通过选取低频降水事件的方法，分析了华南夏季12-30 d持续性强降水事件的基本特征，然后利用位相合成法对持续性强降水期间伴随的低频大气环流型以及低频信号的来源和传播情况进行研究，同时也分析了低频海-气耦合过程对持续性强降水的影响。结果表明：（1）华南夏季降水具有显著的12-30 d低频振荡特征，持续性强降水事件在6月发生次数最多，低频降水期间的雨带自东南向西北传播。（2）在持续性强降水发生期间，华南及邻近海域低层受强大的低频气旋式环流控制，低频上升运动显著，而中国南海-菲律宾海一带则是强的低频反气旋式环流，其西侧向北的低频水汽输送不断将中国南海的水汽送至华南及邻近海域进行辐合上升。低层的低频信号来源于热带西太平洋和中国南海-菲律宾海一带低频振荡的西北向传播，同时伴随着西太平洋副热带高压明显的西伸东退过程。（3）在高层，华南北侧（22°-45°N，95°-130°E）区域强大的低频气旋式环流和孟加拉湾-中国南海一带的低频反气旋式环流相互配合，使华南高层处于强大的辐散环境中，从而加强了华南低层的辐合与低频上升运动，造成持续性强降水的增强。高层的低频信号来源于低频罗斯贝波列的东南向传播。（4）低频大气环流异常通过云辐射和热通量过程改变低频海表温度异常，而由大气强迫的低频海表温度异常通过影响低层大气的稳定性来对大气施加明显的反馈作用，该海-气耦合过程有利于大气低层低频信号向华南地区传播，从而影响了华南持续性强降水的发生、发展与结束。

Low frequency oscillation characteristics of 12-30 d persistent heavy rainfall over South China

Based on daily rainfall data collected at observational stations in China, NCEP/NCAR reanalysis data, NOAA daily outgoing long wave radiation (OLR) and sea surface temperature (SST) data from May to August during 1982-2011, the basic characteristics of 12-30 d persistent heavy rainfall are analyzed by selecting low frequency rain events. The corresponding low frequency atmospheric circulation pattern, and the source and propagation of low frequency signals are investigated using the phase-composite method. The impacts of the low frequency air-sea interaction on the persistent heavy rainfall are also studied. Results show that summer rainfall over South China has a significant characteristic of low frequency (12-30 d) oscillation, and persistent heavy rainfall event occurs most frequently in June. The rain band spreads from the southeast to the northwest of South China during the rainy period. Accompanied with the persistent heavy rainfall, a strong low frequency cyclone controls South China and adjacent sea area, leading to strong ascending motions; meanwhile, a low frequency anticyclone is located over the region from the South China Sea (SCS) to the Philippines Sea (PS) in the lower atmosphere. The strong southwesterly winds continuously bring water vapor from the SCS to South China, leading to water vapor convergence and ascending motions there. The low frequency signal in the lower atmosphere originates from the northwestward propagation of the low frequency oscillation over the SCS-PS and the tropical western Pacific, which is associated with the westward extension and eastward retreat of the western Pacific subtropical high. In the upper atmosphere, a strong low frequency cyclone centered over north of South China around (22°-45°N, 95°-130°E) is couple with a low frequency anticyclone located over the Bay of Bengal to SCS, which provides a strong divergent environment over South China that is favorable for the development of convergence in the lower atmosphere and ascending motion. As a result, persistent heavy rainfall occurs over South China. The low frequency signal in the upper atmosphere comes from the southeastward propagation of the low frequency Rossby wave train. The low frequency sea surface temperature anomaly (SSTA) can be modulated via the process of cloud radiation and heat fluxes driven by the low frequency circulation anomaly, while the low frequency SSTA can significantly influence the atmosphere in turn by changing the stability in the lower atmosphere. Such an air-sea interaction process on low frequency time scale favors the propagation of the low frequency signal in the lower atmosphere, thereby affects the development and termination of persistent heavy rainfall.