青藏高原地区融冻的气候效应及机理研究

The anomalies of soil moisture induced by the variations of active layer of frozen soil and seasonally permafrost region with extensive and long-term characteristics are the key factor and basic process that significantly influence the surface diabatic heating over the Tibetan Plateau (TP), it is also an main component that is responsible for the interannual variations of surface diabatic heating over the TP. The thawing of frozen soil and snow-melting during spring is an essential component of land-atmosphere coupling over TP. This project will focus on the anomalies of surface diabatic heating induced by spring thawing of frozen soil and snow-melting processes, summarize the main progresses acquired in the past projects, analyze the land-atmosphere interactions over the TP and its relation with the low- and mid-latitudes atmospheric circulations. Besides, the variations of soil moisture and the associated surface diabatic heating during different periods (frozen and non-frozen period) will also be analyzed to distinguish the relative contributions of thawing of frozen soil, snow-melting and precipitation to soil moisture, the physical framework that includes “preceding (autumn and winter) precipitation—persistence of soil moisture anomalies—local feedback between soil moisture and precipitation during spring and summer” energy and water cycle processes will also be established; Based on relationship between surface diabatic heating anomalies over the TP and low- and mid-latitudes atmospheric circulations during the period of frozen soil thawing and snow-melting, the impacts of the anomalies of frozen soil thawing and snow-melting on low- and mid-latitudes atmospheric circulations, and the their influences on the climate of the surrounding region of TP and the Northern Hemisphere, are also analyzed and integrated.

青藏高原（简写为高原）地区陆面长时间、大面积的季节性冻土和多年冻土活动层引起的土壤水分变化，是制约高原非绝热加热变化的基本过程和要素，也是高原非绝热加热年际变化的根本原因之一。高原春季融冻、融雪是高原地-气耦合中必不可或缺的过程和部分。本项目拟围绕融冻、融雪过程导致的非绝热加热异常关系，总结过去的研究成果，探讨以融雪融冻引起的非绝热变化为核心的高原地区陆气相互作用，集中于高原地区冻融循环的不同阶段，分析土壤水分的演化及其与之相联系的地表非绝热加热变化，定量分辨融冻和融雪对土壤湿度变化的相对贡献和降水对土壤湿度的影响；建构高原地区跨季节土壤水分循环和由此引起的非绝热加热异常反映的能量循环的物理关系；深化高原融冻、融雪异常与中、低纬度大气环流的关系，集成高原融冻、融雪过程对周边和北半球气候效应的系统性认识。

本项目以揭示青藏高原（简写为高原）季节性冻融过程的气候效应为目标，利用观测资料、再分析资料和数值模拟，分析了广泛分布季节性冻土和多年冻土的高原地区的土壤冻融过程的水热特征，得出了高原地区冻土具有缓慢变化的特征，土壤水分相变和热量传输直接与之相联系，并显著影响地表感、潜热通量，通过陆气相互作用对高原及周边地区的大气环流产生影响，从而形成了高原季节性冻融过程的跨季节气候效应。本项目量化了冻融过程对土壤水分的存储作用，在揭示前秋、前冬和春季的青藏高原土壤湿度的时空变化特征的基础上，给出了不同季节土壤湿度异常和中国东部夏季降水的关系；揭示出了青藏高原春季土壤湿度和夏季降水耦合存在呈东、西相反的模态，在高原中-东部，春季土壤湿度和夏季降水呈正耦合，而西部则呈负耦合；揭示了高原春季融冻异常对积雪异常持续性的促进作用，当春季高原土壤冻融异常偏晚（早），会引起高原东部土壤湿度干（湿）异常，且持续性相比积雪反照率效应更长，导致后期高原东部地表非绝热加热偏弱（强），进而引起高原东—西热力对比差异，激发异常的次级环流，并通过土壤湿度—水汽局地和非局地反馈过程，导致了高原初夏东—西降水反相异常变化，南亚夏季风偏弱（强），从而深化了融冻、融雪异常对大气反馈机理的认识；通过同化冻融过程信息验证了高原土壤冻融过程可有效改进跨季节气候预测。