基于水热变化的青藏高原土壤冻融过程研究进展

青藏高原近地层土壤冻融过程是高原地表最显著的陆面特征之一,也是判断冻土发育、存在以及反映气候变化的重要指标。近地层土壤昼夜、季节性的冻结、融化会导致青藏高原陆—气间能水平衡的变化甚至异常,从而显著影响高原地表水文过程、生态环境、碳氮循环以及高原及其周边区域的天气和气候系统。论文从观测、模拟以及对气候的影响3个角度来探讨1990年以来青藏高原土壤冻融过程的最新研究进展。结果表明：① 在一个完整的年冻融循环过程中,近地表各层土壤大体都经历了夏季融化期、春秋季融化—冻结期、冬季冻结期4个阶段。受局地因素的影响,不同站点的冻结或消融起止时间、速率、类型均有差异。② 多年冻土区和季节冻土区的日冻融循环过程差异较大,主要体现在日冻融循环持续时间上。③ 不同陆面模式都可以很好地抓住冻融过程中物理量的时空变化,但都需要针对高原陆面过程的特点进行参数化改进。④ 规避不稳定的迭代计算并根据热力学平衡方程确定冻融临界温度可以改进不合理的冻融参数化方案。基于已有研究回顾,发现增加高质量的观测站,利用卫星遥感等多种手段来反演高原土壤冻融过程以及加强陆面模式与区域气候模式和全球气候模式的耦合,并立足于高原冻融过程的特点发展相适应的参数化方案以及模拟结构的调整,能够有助于高原冻融过程的模拟。

Soil freezing-thawing processes on the Tibetan Plateau: A review based on hydrothermal dynamics

The freezing and thawing processes of near-surface soil are one of the most significant physical characteristics of the land surface on the Tibetan Plateau (TP), as well as an essential index for estimating the existence and development of the permafrost and reflecting the climate change. The seasonal and diurnal freezing-thawing processes of the near-surface soil cause the changes and even anomalies of water and energy balance between the land and the atmosphere on the TP, and thus significantly affect surface hydrological processes, ecological environment, carbon and nitrogen cycles, and the weather and climate system on the plateau and of the surrounding areas. This article discusses the observational and simulated changes and the impact on the climate by reviewing latest research progress in soil freezing-thawing processes (SFTPs) over the past 20 years. Our review shows that: 1) During a complete annual freezing-thawing cycle, each layer of soil generally experiences four stages: summer thawing period, autumn thawing-freezing period, winter freezing period, and spring thawing-freezing period. Due to the influence of local factors, the SFTPs show differences in the start and end dates, rate, and type of change. 2) Diurnal freezing-thawing cycles show large differences between the permafrost regions and the seasonally frozen regions, which are mainly reflected in the duration of diurnal freezing-thawing cycles. 3) Although different land surface models (LSMs) can well capture the spatiotemporal variations of physical quantity of SFTPs, all of them need to be revised according to the characteristic of LSMs of the TP. 4) Unreasonable freezing-thawing model parameterization schemes can be improved through avoiding the unstable iterative computation and determining the critical freezing-thawing temperature according to the thermodynamic equilibrium equation. According to the review of existing research, adding high-quality observation stations, using satellite remote sensing data to retrieve SFTPs and deepen the coupling of LSMs with regional climate models and global climate models, developing parameterization schemes that are suitable for SFTPs of the TP, and adjusting the model structures can be helpful for the simulation of SFTPs on the TP.