A soil water and heat transfer model including changes in soil frost and thaw fronts

Freeze-thaw processes in soils, including changes in frost and thaw fronts (FTFs), are important physical processes. The movement of FTFs affects soil hydrothermal characteristics, as well as energy and water exchanges between the land surface and the atmosphere and hydrothermal processes in the land surface. This paper reduces the issue of soil freezing and thawing to a multiple moving-boundary problem and develops a soil water and heat transfer model which considers the effects of FTF on soil hydrothermal processes. A local adaptive variable-grid method is used to discretize the model. Sensitivity tests based on the hierarchical structure of the Community Land Model (CLM) show that multiple FTFs can be continuously tracked, which overcomes the difficulties of isotherms that cannot simultaneously simulate multiple FTFs in the same soil layer. The local adaptive variable-grid method is stable and offers computational efficiency several times greater than the high-resolution case. The simulated FTF depths, soil temperatures, and soil moisture values fit well with the observed data, which further demonstrates the potential application of this simulation to the land-surface process model.