青藏高原热力作用对我国南方持续性强降水影响机制研究

To advance the understanding of mechanisms governing influence of the Tibetan Plateau (TP)’s heating effect on the development and maintenance of persistent heavy rainfall over Southern China (SC) is of scientific and practical importance. This project will investigate roles played by the TP’s heating in the formation and maintenance of the persistent heavy rainfall and explore the underlying physical mechanisms by conducting the following research:.First, using long-term multiple observational and reanalysis datasets, we will analyze the relationships between the TP’s sensible heating and the warm-moist air transportation near the TP’s southern slope, the detouring flow near the TP’s northern slope, and the activities of cold and warm air masses and the persistent heavy rainfall over SC..Second, for the persistent heavy rainfall events over Southern China during June 1998, July 1998, and June 2010, respectively, several closely related studies will be carried out..a) Design and conduct convection-permitting CONTROL-ensemble simulations, and evaluate the ensemble-mean in terms of the simulated precipitation, large-scale circulation, and weather systems by comparing with observational analyses to validate the simulated results..b) Design and conduct multiple sensitivity-experiments of ensemble simulation that differ in thermal characteristics of the plateau’s ground surface over three sub-regions of TP, namely, the major plateau, the plateau’s southern slope, and the plateau’s northern slope, respectively. Then compare the precipitation, large-scale circulation, and weather systems in each sensitivity-ensemble with the CONTROL-ensemble focusing on changes in the warm-moist air transportation near the TP’s southern slope, the detouring flow around the TP’s northern slope, the activities of cold and warm air masses over SC, and the heavy rainfall processes over SC, which will reveal the influences of the plateau’s surface heating over each sub-region. ..c) Dynamical diagnostics will be made using outputs from the ensemble-means, and from some individual members if needed. Combining the dynamical diagnoses with theoretical analysis, physical mechanisms governing the influences of the plateau’s surface heating on the persistent heavy rainfall will be investigated..d) Comparisons will be made between each pair of the sensitivity-ensemble, which will reveal the differences/similarities in the influences caused by the Plateau’s surface heating over the different sub-regions and the associated physical mechanisms. Relative importance among the plateau’s sub-regions and similarities/differences in their influencing mechanisms will be investigated..Finally, comparisons will be made among the results obtained from the abovementioned studies for the June 1998, July 1998, and June 2010 to investigate the similarities/differences in the influence of the TP’s heating on the different persistent heavy rainfall events over SC..

深刻认识青藏高原热力作用对我国南方持续性强降水的影响机制具有重要的科学价值和现实意义，因此，将利用长时间序列观测和再分析资料，分析高原热力作用与其南侧暖湿空气输送、北侧扰流产生的波动、我国南方冷暖空气活动及南方持续性强降水的关系；然后针对1998年6月、7月及2010年6月我国南方持续性强降水过程，设计并实施显式对流集合模拟“控制试验”，验证其结果的可用性；设计并实施多组集合模拟“敏感性试验”，分别改变高原不同地区（主体、南坡、北坡）的地表热力特征，通过诊断模拟结果和理论分析，研究高原不同地区热力作用影响高原南侧暖湿空气输送、北侧扰流产生的波动、我国南方冷暖空气活动，进而影响大尺度和中尺度系统及强降水过程的物理机制；对比不同组的敏感性试验结果，揭示高原不同区域热力作用影响我国南方持续性强降水过程的相对重要性及物理机制的异同；对比三次强降水过程的研究结果，分析其异同和原因。

本项目重点研究了1998-2015年4-6月24次华南区域性极端降水过程的天气尺度波动与青藏高原的关联、卫星辐射资料同化对青藏高原天气要素预报准确性的影响；并利用“第三次青藏高原大气科学试验”提供的那曲地区降水回波垂直结构的最新观测资料，获得了高原中部夏季对流性降水和层状降水的统计特征。有两个亮点成果。.一、华南区域性极端降水事件与天气尺度波动之间的关系。选取华南区域平均日降水量最高5%的24次极端降水过程，分析逐次过程的天气尺度波动异常环流，根据主要的异常天气系统对极端降水事件分类，分为15次“气旋型”和8次“深槽型”极端降水事件，及1次“反气旋型”事件，15次气旋型事件又分为“低层涡度异常型”（11）和“高层涡度异常型”（4），进一步揭示每一次事件的天气尺度波动异常环流场的时间演变，归纳出三种主要类型的演变规律。11次低层涡度异常型的事件中，有9次可追溯到青藏高原下游的气旋性异常，另外2次从南海移到华南；对流层高层涡度异常型的高层涡度异常也可以追溯到青藏高原东部，此类型在极端降水日也有低层气旋性涡度异常影响华南，并发生在高层正温度异常之下。深槽型极端降水的产生与华北强气旋异常所形成的深槽异常密切相关，该强气旋异常亦可追溯到青藏高原东南部，在往东北方向移动的过程中逐步增强，在极端降水日形成强深槽扰动影响华南。这些结果对于了解华南前汛期华南区域性极端降水的天气尺度扰动的起源、结构和演变具有重要意义。.二、那曲地区雷达降水回波分类与垂直结构分析。利用“第三次青藏高原大气科学试验”2014年外场观测试验期间布设在那曲地区的云降水观测设备，包括C波段垂直指向调频连续波雷达（C-FMCW）、雨滴谱仪、C波段体扫雷达、和时间加密探空观测资料，将C-FMCW观测的降水回波垂直廓线分为层状降水、强对流降水、弱对流降水，发现包含亮带的层状降水回波占总降水回波发生频次的52%，地面总降水量的54%；根据强对流回波的垂直结构判断混合相云微物理过程的活跃程度，并估计对流的强度，那曲强对流比北美中纬度大陆地区的强对流要弱，同时显著强于热带海洋性的强对流、略微强于热带大陆性的强对流；探空资料分析表明，虽然高原强地表感热加热有利于下午触发对流，但是大气可降水量很低、对流有效位能很小，导致对流云内上升运动迅速减弱，强对流快速转变为弱对流和层状降水。这些结果提供了关于高原对流活动特征的新的认识。