Accurate representing freeze-thaw (FT) process is of great importance in cold region hydrology and climate studies. With the STEMMUS-FT model (Simultaneous Transfer of Energy, Mass and Momentum in Unsaturated Soil), we investigated the coupled water and heat transfer in the variably saturated frozen soil and the mechanisms of water phase change along with both evaporation and FT process, at a typical meadow ecosystem on the Tibetan Plateau. The STEMMUS-FT showed its capability of depicting the simultaneous movement of soil moisture and heat flow in frozen soil. The comparison of different parameterizations of soil thermal conductivity indicated that the de Vries parameterization performed better than others in reproducing the hydrothermal dynamics of frozen soils. The analysis of water/vapor fluxes indicated that both the liquid water and vapor fluxes move upward to the freezing front and highlighted the crucial role of vapor flow during soil FT cycles as it connects the water/vapor transfer beneath the freezing front and above the evaporation front. The liquid/vapor advective fluxes make a negligible contribution to the total mass transfer. Nevertheless, the interactive effect of soil ice and air can be found on the spatial and temporal variations of advective fluxes in frozen soils.
Yu, L., Zeng, Y., Wen, J. & Su, Z. 2018, Liquid-Vapor-Air Flow in the Frozen Soil, Journal of Geophysical Research : Atmospheres. Doi: 10.1029/2018jd028502, 23 p
Figure1 The calculation procedure and underlying physics of STEMMUS-FT expressed within one timestep. n is the time at the beginning of the time step, n+1 is the time in the end. The variables with the superscript (n+1/2) are the intermediate valuesFigure 2 Observed latent heat flux and simulated (a) latent heat flux and (b) surface soil (0.1cm) thermal and isothermal liquid water and vapor fluxes (LE, qVT, qVh, qLT, qLh) (c) surface soil (0.1cm) advective liquid water and vapor fluxes (qLa, qVa) of a typical five-day freezing period (from 8th to 12th Days after Dec. 1. 2015). LE is the latent heat flux, qVT, qVh are the water vapor fluxes driven by temperature and matric potential gradients, qLT, qLh are the liquid water fluxes driven by temperature and matric potential gradients, qLa, qVa are the liquid and vapor water fluxes driven by air pressure gradients. Positive/negative values indicate upward/downward fluxes.Figure3 Simulated vertical profiles of the thermal and isothermal liquid water and vapor fluxes, soil ice content at 1200 and 0000 h of a typical freezing period during 11th and 12th Days after Dec. 1. 2015. Positive/negative values indicate upward/downward fluxes. Solid lines and dot lines represent for the fluxes and soil moisture, temperature and ice content profile on the 11th and 12th Days after Dec. 1. 2015, respectively.Figure 4 . Spatial and temporal variations of (a) temperature gradient, (b) matric potential gradient and (c) air pressure gradient at surface soil layers (top 2cm, upper figure) and deeper soil layers (2-30cm, bottom figure), respectively, of a typical freezing period during 8th and 12th Days after Dec. 1. 2015.Figure 5 The spatial and temporal distributions of (a, and b) thermal liquid water, and vapor fluxes, (c, and d) isothermal liquid water, and vapor fluxes, (e, and f) advective liquid water, and vapor fluxes, at surface soil layers (top 2cm, upper figure) and deeper soil layers (2-30cm, bottom figure), respectively, of a typical freezing period during 8th and 12th Days after Dec. 1. 2015. Note that the unit for the fluxes is g cm^-2 s^-1.
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