Modeling permafrost hydrothermal features considering the movement of freeze-thaw front
The freeze-thaw front within active layer is the interface between the frozen and the unfrozen soil lay-ers during the freeze-thaw process,and the hydrothermal parameters of the soil layers on both sides of freeze-thaw front are significantly different.Therefore,the accurate simulation of the freeze-thaw front movement in the land surface model is essential to improve models both in simulating the hydrothermal characteristics of per-mafrost and simulating the energy-water balance of the land surface.In this study,the simulation depth of the Noah-MP land surface model was extended to 20 m,and the 4 soil layers of the Noah-MP land surface model was increased to 19 soil layers,and the organic matter scheme and vegetation root scheme were introduced.Af-ter these modifications,in order to strengthen the ability of the Noah-MP land surface model on simulating freeze-thaw front,the Stefan method was coupled.Then,the simulation effect of the augmented Noah-MP land surface model on the hydrothermal process of the Xidatan permafrost site was evaluated.Two experiments,CTL experiment(coupled Stefan method)and STE experiment(not coupled Stefan method),were conducted to sim-ulate the soil temperature and soil liquid water content of 0～20 m in 2012,and the simulation results were veri-fied by the observed daily soil temperature and soil liquid water of 0～3.2 m and the observed yearly ground tem-perature of 3 m,6 m and 10 m.The results showed that the freeze-thaw front(0℃isotherm)obtained by inter-polation of soil temperature simulation values had obvious step-like characteristics,and its maximum freeze-thaw depth was larger than the measured.Coupling Stefan method enhanced the ability of Noah-MP model to simulate the freeze-thaw front,so that the model was able to better simulate the change trend and maximum depth of the freeze-thaw front.At the same time,coupling Stefan method also improved the simulation of soil temperature.The mean RMSE and the mean MBE of the soil temperature in the soil layers of 0～3.2 m decreased to 0.89℃(decreased by 44%)and-0.13℃(decreased by 86%)respectively,and yearly ground temperature of 3～20 m was closer to the measured.And it also improved the simulation of the soil liquid water content.The mean RMSE and the mean MBE of the soil liquid water content in the soil layers of 0～3.2 m decreased to 0.06 m3·m-3(decreased by 33%)and-0.01 m3·m-3(decreased by 67%)respectively,and the soil water melting time of 20 cm,40 cm,80 cm and 120 cm in the active layer was closer to the observed.It can be seen that coupling the Stefan method that can better model the movement process of freeze-thaw front in the land surface model can greatly improve the simulation ability of the model,which is one of the effective ways to improve the land sur-face process model.The results of this study can provide a reference for improving the simulation of the land sur-face model in the permafrost area.This study will provide a reference for improving the ability of land surface model to simulate hydrothermal processes of permafrost.
Stefan methodfreeze-thaw frontNoah-MPland surface modelXidatan
NETL
NSTL
万方数据
