青藏高原热融湖对冻土热状况长期作用的数值模拟研究

Thermokarst lakes are one of the most obvious manifestations of the hydrological system at work in high latitude and high altitude regions where ice-rich permafrost exists. These lakes have considerable disturbances to the surrounding permafrost, giving rise to open talik formation below some thermokarst lakes. The effect of thermokarst lakes on thermal regime of permafrost in cold regions is an advanced subject，and has captured the interests of many researchers and scientists recently. As the quantitative analysis method, numerical simulation provides a powerful tool for simulating permafrost thermal regime and talik development under and around thermokarst lakes. There are significant differences in permafrost thermal regime and talik development under thermokarst lakes between on the Alaskan Arctic Coastal Plain and on the Qinghai-Tibet Plateau. Therefore, quantifying the long-term impacts of thermokarst lakes on permafrost thermal regime and talik development on the Qinghai-Tibet Plateau with a well-tuned model and the precious monitoring below-lake ground temperature data could obtain valuable information about local environment conditions and climate changes. This would also have very important impacts for further investigating the thermal stability of structures constructed on Qinghai-Tibet Plateau and the variations in local ecological conditions. Based on a simplified two-dimensional unsteady finite element model under a cylindrical coordinate system for thermal regime of permafrost and talik under thermokarst lakes in the Alaskan Arctic developed by the proposer, and the long-term observed data from a representative thermokarst lake in the Beiluhe Basin on the Qinghai-Tibel Plateau measured by the State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, this study calibrates the thermal and physical parameters of the model for thermal regime of ground beneath and around thermokarst lakes in the Beiluhe Basin on the Qinghai-Tibet Plateau; simulates talik development processes under thermokarst lakes; analyzes responses of talik development and change to variations in environment; discusses characteristics of permafrost lateral thaw processes beneath thermokarst lakes after the formation of open taliks, and explores features of talik freeze-up and permafrost response under drained thermokarst lake.

热融湖对多年冻土区土壤热状况长期作用的效果是国内外气候与环境学者近年来普遍关心的前沿性课题。数值模拟作为定量分析研究的科学方法，为这一问题的研究提供了有力工具。青藏高原热融湖下融区的发展与阿拉斯加北极的情形有明显区别，充分利用现有观测资料和成熟的数值模型分析热融湖对冻土环境的影响，可望获取关于区域环境和气候变化的宝贵信息，对进一步研究青藏高原工程建筑物的热稳定性及生态环境的变化具有重要作用。本项研究拟以申请人开发的模拟阿拉斯加北极热融湖对冻土环境作用的数值模型为工具，以申请人收集的中国科学院寒区旱区环境与工程研究所在青藏高原北麓河盆地对典型热融湖周边及湖下土壤热状况持续监测的资料为模型输入，标定能正确描述青藏高原热融湖对冻土热状况作用的模型的热和物理参数；模拟湖下融区的发展过程；分析融区发展对环境变化的响应；查明贯通融区形成后湖下冻土侧向融化的特征；探索热融湖消退后融区热状况的变化规律。

数值模拟是量化分析研究的科学方法，为研究热融湖对多年冻土区周边土壤热状况长期作用的效果提供了有力工具。利用观测资料和数值模型分析青藏高原热融湖对冻土环境的影响，可望获取关于区域环境变化的宝贵信息，对进一步研究青藏高原工程建筑物的热稳定性及生态环境的变化具有重要作用。本项研究以青藏高原北麓河盆地典型热融湖周边及湖下土壤热状况持续监测的资料为基础，用数值模拟方法研究青藏高原热融湖下土壤热状况的演化和融区的扩展过程；通过试验分析多年冻土的物理力学性质；并探索求解数学模型和处理时间序列数据的新方法。研究表明：热融湖是多年冻土融化的热源，导致热融湖下形成锅底状融区。多年冻土热状况和融区扩展过程对多年冻土的厚度具有敏感性，冻土厚度增加不仅延后热融湖下贯通融区形成的时间，而且将推迟贯通融区形成后多年冻土的侧向融化过程；不同半径热融湖湖下土壤热状况和融区发展有较大差异，精确模拟热融湖下土壤热状况和融区发展的数学模型必须考虑热融滑塌引起的热融湖横向扩展作用；对含盐冻结粉质砂土进行三轴压缩试验研究表明，初始切线模量E0和0.5倍强度处割线模量E0.5随围压的变化规律均可分为3个阶段,第1和第2阶段均随围压增大而增大,且第2阶段变化率均较第1阶段大,第3阶段随围压增大而减小,但E0.5在整个围压范围内随围压的变化率均较初始模量小,在工程计算中若采用E0.5作为变形指标可在较大围压范围内取常数,而E0必须考虑围压的影响；根据热融湖的横向扩张的平均速率随时间线性增加的关系，对柱坐标系下伴有相变的热传导方程作了改进，建立了模拟不断横向扩张的热融湖对周边多年冻土热状况长期作用的模型。并给出了利用建立的模型模拟热融湖的持续扩张过程以及周边多年冻土热状况动态演化过程的算例；研究了具有周期边界条件的一维相变导热问题的有限差分求解方法，获得了冻融交界面和温度场的演化过程，分析了Stefan数和周期边界条件对温度分布和移动界面的影响。