Understanding the Mass, Momentum and Energy Transfer in the Frozen Soil with Three Levels of Model Complexities

Posted by Yijian Zeng in the category Case Studies

Frozen ground covers a vast area of Earth surface and has its important ecohydrological implications for cold regions under changing climate. However, it is challenging to characterize the simultaneous transfer of mass and energy in frozen soils. Within the modeling framework of STEMMUS (Simultaneous Transfer of Mass, Momentum and Energy in Unsaturated Soil), the complexity of soil heat and mass transfer model varies from the basic coupled (termed as BCM), to the advance coupled heat and mass transfer (ACM), and further to the explicit consideration of airflow (ACM-AIR). The impact of different model complexities on understanding the mass, momentum and energy transfer in frozen soil was investigated. The model performance in simulating water and heat transfer and surface latent heat flux was evaluated over a typical Tibetan Plateau meadow site. Results indicate that the ACM considerably improved the simulation of soil moisture, temperature and latent heat flux. The analyses of heat budget reveal that the improvement of soil temperature simulations by ACM is attributed to its physical consideration of vapor flow and thermal effect on water flow, with the former mainly functions above the evaporative front and the latter dominates below the evaporative front. The contribution of airflow-induced water and heat transport (driven by the air pressure gradient) to the total mass and energy fluxes is negligible. Nevertheless, given the explicit consideration of airflow, vapor flow and its effects on heat transfer were enhanced during the freezing-thawing transition period.

Yu, L., Zeng, Y., & Su, Z. (2020). Understanding the mass, momentum, and energy transfer in the frozen soil with three levels of model complexities. Hydrology and Earth System Sciences, 24(10), 4813-4830.

Figure 1 Conceptual illustration of the model setup, the surface/bottom boundary conditions, driving forces, and vertical discretization.
Figure 2 Comparison of measured (Obs) and model simulated freezing front propagation (FFP) using Basic Coupled Model (BCM), Advanced Coupled Model (ACM) and Advanced Coupled Model with Air flow (ACM-AIR). Note the measured FFP was seen as the development of zero degree isothermal lines from the measured soil temperature field.
Figure 3 Comparison of observed and model simulated mean diurnal variations of surface evapotranspiration by Basic Coupled Model (BCM), Advanced Coupled Model (ACM), and Advanced Coupled Model with Air flow (ACM-AIR).
Figure 4 Time series of model simulated heat budget components at the soil depth of 5cm using (a &d) Basic Coupled Model (BCM), (b &e) Advanced Coupled Model (ACM), and (c &f) Advanced Coupled Model with Air flow (ACM-AIR) simulations during the typical 6-day freezing (left column) and freezing-thawing transition (right column) periods. HC, change rate of heat content, CHF, conductive heat flux divergence, HFL, convective heat flux divergence due to liquid water flow, HFV, convective heat flux divergence due to water vapor flow, HFa, convective heat flux divergence due to air flow, LHF, latent heat flux divergence. Note that for graphical purposes, HFL, HFV, HFa, and LHF were enhanced by a factor of 10 during the freezing period.



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