Modeling thermokarst lake expansion on the Qinghai-Tibetan Plateau and its thermal effects by the moving mesh method

doi:10.1016/j.coldregions.2015.10.012

Cold Regions Science and Technology

Volume 121, January 2016, Pages 84-92

https://doi.org/10.1016/j.coldregions.2015.10.012 Get rights and content

Highlights

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A dynamic model to simulate thermokarst lake growth was presented.
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Thermokarst lake expansion depended on lake-bottom temperature and permafrost temperature.
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Active thermokarst caused a greater thermal disturbance to permafrost.
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Lake expansion was not a good indicator for climate warming.

Abstract

The increasing number of engineering activities on the Qinghai-Tibetan Plateau, along with a warming climate, tend to induce significant thermokarst processes. However, thermokarst lake development on the plateau and the long-term influence of thermokarst dynamics on the surrounding permafrost have not been fully understood. Based on the moving mesh method, we developed a dynamic model of lake growth with phase change to investigate the morphologic processes of a thermokarst lake and its long-term influence on the local permafrost thermal regime. Our numerical results indicated that lake expansion due to thermokarst development was fairly rapid, and it depended on many factors including lake-bottom temperature and permafrost temperature. The depth and radius of the simulated lake increased gradually from the end of May to the end of the following January, and remained stable from February to May due to the lower lake-bottom temperature. Our results also showed that lake expansion was not very sensitive to climate warming in the first 50 years after the formation of a thermokarst lake, but the lake expansion was clearly affected by a warming climate in the long run (200 years). The development of a thermokarst lake is shown to significantly enhance the thermal effects on the surrounding permafrost. Compared with a fixed-boundary thermokarst lake, an active (moving-boundary) thermokarst lake caused greater thermal disturbance to the permafrost around and beneath it.

Introduction

Thermokarst lakes are topographic depressions which develop as a result of thawing of ice-rich permafrost or melting of massive ground ice. Thermokarst lakes area major component of ice-rich permafrost landscapes and are widespread on the Qinghai-Tibetan Plateau, and are seen as sensitive indicators of environmental change (Niu et al., 2011). Thermokarst lakes have a significant influence on surrounding permafrost environments (Osterkamp et al., 2000), mainly on changes in thermal regimes and permafrost degradation (Zhou and Huang, 2004). These cause a series of environmental problems, such as water table depression, vegetation degradation, land desertification, and infrastructure instability (Burn, 2005, Lin, 2011, Lunardini, 1996, Osterkamp et al., 2009). In recent years, human activities and changed climate along the Qinghai-Tibet engineering corridor have initiated significant thermokarst processes (Niu et al., 2011, Wang and Mi, 1993) and have resulted in extensive and serious disturbances in sensitive permafrost environments on the Qinghai-Tibetan Plateau. These disturbances have seriously affected the stability of nearby infrastructure and the performance of built structures in that vicinity (Lin et al., 2012).

Thermokarst processes usually develop rapidly after the formation of thaw ponds. Both the rates of thermokarst lake development and changes in the dimensions of thermokarst lakes play an important role in the local permafrost thermal regime and permafrost degradation. For example, the thermal impacts of thermokarst lakes on nearby engineering works can show a visible difference due to the changes in the lake expansion rate and a decreasing distance between an engineered structure and a thermokarst lake (Lin et al., 2012). Thermokarst expansion may accelerate the release of greenhouse gases sequestered in permafrost and can disperse a significant amount of methane into the atmosphere (Zimov et al., 1997). To predict the future environmental effects of thermokarst dynamics, it is necessary to understand the processes involved in thermokarst lake development and their thermal effects.

Several models have already been developed to study thermokarst lakes and their effects on ground temperatures. Lin et al. (2012) employed a heat transfer model with phase change to predict the thermal regime changes beneath a frozen ground roadbed, and possible subgrade defects. Their results showed that a thermokarst lake caused thermal erosion of permafrost under the roadbed and the amount of thermal erosion depended mainly on the annual average lake-bottom temperature and the distance from the roadbed to the lake edge. Ling and Zhang (2003) simulated the long-term influence of shallow thermokarst lakes on the thermal regime of permafrost and talik development on the Alaskan Arctic Coastal Plain, by an unsteady finite-element heat transfer model with phase change. Zhou and Huang (2004) developed a transient heat transfer model to simulate a multimedia system including snow cover, thermokarst lake, and frozen soil, which revealed the impacts of thermokarst lakes on the thermal regime and the formation of talik and lake ice. However, most thermokarst lake models ignore thermokarst lake dynamics and treat the lake morphology as unchanging.

Similar to other thermokarst processes, the evolution of a thermokarst lake is a sediment redistribution process initiated by thawing of ice-rich permafrost (Sun et al., 2012). As a heat source, thermokarst lakes provide continuous heat flow to surrounding permafrost and result in the increase in thawing depth and ground temperature (Lin et al., 2010). Consequently, thermokarst lakes increase in area and depth over time due to long-term slumping and collapse. Increased engineering disturbance in recent years has induced the formation of many thermokarst ponds or lakes on the Qinghai-Tibetan Plateau, which in turn has caused serious thermal erosion and thaw settlement, resulting in decreased bearing capacity of permafrost under engineered embankments (Niu et al., 2011).

Past investigations also indicated that climate warming has resulted in increased numbers of thermokarst lakes in the continuous permafrost region along the Qinghai-Tibet Railway (Wang and Mi, 1993). Thermokarst lakes have been increasingly studied in recent years, but the future thermokarst development on the Qinghai-Tibetan Plateau is not yet fully understood. The morphology of thermokarst lakes in permafrost regions is in a dynamic change. The depth, radius, and area of such lakes vary significantly over time. Lakes on the Qinghai-Tibetan Plateau have been found to be mostly enlarging (Niu et al., 2008).

To understand the evolution of thermokarst lakes on the Qinghai-Tibetan Plateau, we employed moving mesh technology to assess the dynamic morphology during thermokarst lake evolution. We developed a coupled numerical model for heat transfer with phase change and thaw subsidence to simulate thermokarst lake expansion and the effects of thermokarst dynamics on local permafrost temperature. To simulate the impact of lake expansion on the thermal regime of permafrost under and surrounding the lake, we compared the thermal effects of a dynamic morphology (moving-boundary) thermokarst lake with those of a fixed morphology (fixed-boundary) thermokarst lake. We conducted a series of simulation cases and investigated the influence of lake-bottom temperature, permafrost temperature, and rates of climate warming on thermokarst dynamics.

Section snippets

Moving mesh method

Dynamic fluid grids are commonly used for the solution of flow problems with moving boundaries, including blood flow circulation, parachute dynamics, airfoil oscillations, flutter prediction, and a large class of free-surface flow problems (Batina, 1990). When the fluid mesh undergoes large displacements and/or deformations, the existing grids are allowed to deform to follow the computational domain geometries. We used the spring analogy approach (Bohn and Moritz, 2005) to serve for the mesh

Themorphology variation of thermokarst lakes

Fig. 4 shows the morphology evolution process of a simulated thermokarst lake. Fig. 4(a) displays changes in the radius of the thermokarst lake over time. Our numerical results indicated that lake expansion due to thermokarst development was fairly rapid and the radius of the simulated lake increased linearly at a rate of 0.38 m/year. It was assumed that the ice-rich permafrost was spread uniformly in the computation domain, so the simulated lake expanded continuously. Past experiences also

Conclusions

We developed an axis-symmetrical heat transfer dynamic model with phase change to simulate a thermokarst lake expansion and its thermal effects. We conducted a series of simulation cases at different lake-bottom temperatures, permafrost temperatures, and rates of climate warming. Based on the thermal simulation results, some useful conclusions can be drawn as follows:

Our numerical results indicated that lake expansion due to the development of thermokarst was fairly rapid, and the radius of the

Acknowledgments

This research project was supported by the Natural Science Foundation of China (Grant Nos. 41171059 and 41471061), the National Key Basic Research Program of China (Grant No. 2012CB026101), the 100 Talented Young Scientists Project granted to Dr. Zhi Wen (Grant No. Y251561001), the Science and Technology Project of the State Grid Corporation of China (Grant No. SGJSJS (2010)935-936), the Research Project of the State Key Laboratory of Frozen Soils Engineering (SKLFSE-ZT-16 and SKLFSE-ZY-12) and

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Modeling thermokarst lake expansion on the Qinghai-Tibetan Plateau and its thermal effects by the moving mesh method

Highlights

Abstract

Introduction

Section snippets

Moving mesh method

Themorphology variation of thermokarst lakes

Conclusions

Acknowledgments

Cold Reg. Sci. Technol.

Geomorphology

Unsteady Euler airfoil solutions using unstructured dynamic meshes

AIAA J.

Algebraic method for efficient adaption of structured grids to fluctuating geometries

Proc. Inst. Mech. Eng. A J. Power Energy

Lake-bottom thermal regimes, western Arctic coast, Canada

Permafr. Periglac. Process.

Causes and consequences of rapid thermokarst development in permafrost on glacial terrain

Permafr. Periglac. Process.

Study of Thermokarst Lake and its Influence on the Permafrost Environment and Engineering

Thermal regime of a thermokarst lake and its influence on permafrost, Beiluhe Basin, Qinghai-Tibet Plateau

Permafr. Periglac. Process.