青藏高原作为“亚洲水塔”，为数十亿人口提供水资源，所以其降水对气候变化的响应被广泛关注。在全球变暖的背景下，青藏高原主体经历了显著的变暖变湿；与此同时，高原的暖湿化导致其陆面过程（包括陆面状态和属性）也发生了明显的改变，进而可能反过来影响降水。本文基于野外观测、数据分析和模式模拟，首先对青藏高原夏季降水年代际变化特征及其相关的大气环流过程和海温异常进行了分析，然后进一步探究了高原降水对其陆面状态（以地表能量收支为例）和陆面属性（以土壤有机质为例）年代际变化的响应。主要结论如下：20世纪90年代中期以来，青藏高原夏季降水整体增加，但是高原内流区增加最明显，进而导致了该区域湖泊急剧扩张。高原内流区夏季降水的年代际变化与副热带西风急流的摆动有着密切的联系，而后者受到大西洋多年代际振荡（AMO）的影响，并引起了欧亚大陆上空的气旋和反气旋异常：位于中亚上空的气旋异常有利于来自阿拉伯海的水汽从西南部进入高原上空，增强了高原内流区的水汽辐合；同时，位于贝加尔湖上空的反气旋异常抑制了高原内流区东侧的水汽流出。因此，在上述有利的水汽和动力条件的共同作用下，高原内流区夏季降水增加。在变暖的气候下，由外部过程引起的青藏高原降水增加导致高原陆面过程也发生了相应变化，进而通过陆气反馈影响高原夏季降水年代际变化。本文发现青藏高原陆面状态地表能量收支在20世纪90年代中期左右发生了明显的年代际转折，具体表现为地表潜热通量的增加和地表感热通量的减小。借助于WRF数值模拟试验发现，高原地表能量收支年代际变化可以通过陆气反馈引起对流层低层异常反气旋和高层异常气旋，从而削弱南亚季风，使得从孟加拉湾进入高原的水汽减少，最终导致高原降水减少，一定程度上缓解了高原夏季降水的年代际增加。除了上述陆面状态变化外，青藏高原暖湿化导致其陆面属性土壤有机质（SOM）含量也发生了年代际变化，本文探究了其对高原夏季降水的影响。首先，不同于前人经验性地给定SOM参数，本文基于土壤观测发展了一个新的适用于青藏高原的SOM参数化方案，并发现以往研究低估了高原SOM的持水能力。评估结果表明，新方案可以明显提高Noah-MP陆面模式的模拟能力，减少WRF模式对降水模拟的湿偏差。将新方案引入WRF模式后模拟结果表明，高原SOM含量年代际变化会引起夏季降水整体略增加0.02 mm·d-1，但是在高原中东部会抑制降水。
As the “Asian Water Tower”, the Tibetan Plateau (TP) provides water resources for billions of people, so the response of the plateau precipitation to climate change has been of great interest. In the context of global warming, the TP has experienced significant warming and wetting; meanwhile, the warming and wetting of the plateau has led to obvious changes in its land surface processes (including land surface state and properties), which may in turn affect precipitation. Based on field observations, data analyses and model simulations, this study first investigates the interdecadal variation of summer precipitation over the TP and its associated atmospheric circulation processes and sea surface temperature (SST) anomalies. Then, the responses of precipitation to interdecadal variations of plateau land surface states (e.g., surface energy budget) and land surface properties (e.g., soil organic matter) are further explored in this study. The main conclusions are as follows.Summer precipitation over the TP has increased overall since the mid-1990s, but the increase has been most pronounced on the Inner TP (ITP), which has resulted in a dramatic expansion of lakes in this region. The interdecadal variation of ITP precipitation is closely related to the meander of the subtropical westerly jet, which is influenced by the Atlantic Multidecadal Oscillation (AMO) and causes cyclonic and anticyclonic anomalies over Eurasia: The cyclonic anomalies over Central Asia facilitate the water vapor from the Arabian Sea to enter the plateau from its southwest boundary and enhance the water vapor convergence over the ITP; meanwhile, the anticyclonic anomalies over Lake Baikal suppress water vapor outflow across the eastern boundary of the ITP. As a result, the combination of these favorable water vapor and dynamical conditions has led to an increase in summer precipitation on the ITP.Under a warming climate, the increased precipitation on the TP caused by external processes leads to changes in plateau land surface processes, which in turn influence the interdecadal variation of summer precipitation on the plateau through land-atmosphere feedbacks. It is found that the surface energy budget on the TP underwent a significant interdecadal transition around the mid-1990s, including increased surface latent heat flux and decreased surface sensible heat flux. With the help of WRF numerical simulations, it is demonstrated that the interdecadal variation of the plateau surface energy budget can cause anticyclonic anomalies in the lower troposphere while cyclonic anomalies in the upper troposphere through land-atmosphere feedbacks, thus weakening the South Asian monsoon and reducing the water vapor entering the plateau from the Bay of Bengal. These ultimately lead to reduced precipitation over the TP, and thus to some extent mitigate the interdecadal increase in the plateau precipitation.In addition to the aforementioned changes in land surface state, warming and humidification of the TP has led to interdecadal changes in its land surface properties, e.g., soil organic matter (SOM) contents, and this paper explores their effects on the plateau summer precipitation. First, unlike previous empirically given SOM parameters, a new SOM parameterization scheme applicable to the TP is developed based on soil observations, and it is found that previous studies generally underestimated the water-holding capacity of the plateau SOM. The evaluation results show that the new scheme can greatly improve the simulation capability of the Noah-MP land surface model and reduce the wet bias of the WRF model for precipitation simulations. After introducing the new scheme into the WRF model, simulations show that the interdecadal variation of the plateau SOM contents can cause a slight increase (0.02 mm·d-1) in summer precipitation over the TP, but it can inhibit precipitation in the central-eastern TP.
