Characteristics of Surface Water and Heat Exchange during Soil Freezing And Thawing of Maqu Station in the Source Area of the Yellow River

Abstract

Land surface moisture content and energy budget are significantly affected by the soil freeze-thaw process.From August 2017 to July 2018， soil temperature， soil moisture， eddy data from observation and the simulation data of Community Land Model 5.0 （CLM5.0） of Maqu were used to analyze the time evolution characteristics of soil temperature and moisture， radiation and energy flux in soil freeze-thaw process， and to explore the characteristics of surface water and heat exchange during freezing and thawing.The observed data of radiation and energy flux are replaced by simulated data during the freezing process.The results show that： （1） Soil freeze-thaw process includes four stages： the freezing， completely frozen， thawing， and completely thawed stages.Soil temperature， soil moisture， radiation and energy flux have obvious diurnal changes at each stages.In the freezing and thawing stages， the soil moisture change with soil temperature shows an clear diurnal freeze-thaw cycles.（2） The water phase change in the soil during freezing and thawing changes the nature of the underlying surface， and affects the surface radiation budget.The phase transition of water affects the surface latent heat flux， in the completely thawed stage， the surface energy exchange is dominated by latent heat flux.The surface energy exchange is dominated by sensible heat flux in the completely freezed stage.Compared to freezing stages， which surface energy exchange is dominated by latent heat flux， the net radiation gradually increase in thawing stage， leading to sensible heat flux increase.The soil water melts during the day and freezes at night， resulting in frequent release of latent heat flux， affecting the surface energy exchange.The absolute value of soil heat flux in freezing （G0=-9.1 W·m-2） and thawing stage （G0=3.4 W·m-2） is greater than that in completely thawed stage （G0=1.2 W·m-2）.Daily freeze-thaw cycle strengthens surface heat flux.（3） The energy closure rate is the ratio of the sum of sensible heat and latent heat flux to the difference between net radiation flux and soil heat flux.The average energy closure rates of freezing process， completely frozen， thawing process and completely thawed stages were 1.44， 1.59， 0.99 and 0.81.The energy closure rate in thawing and completely thawed stages is close to 1.The daily freeze-thaw cycle affects the soil water phase trasition.Heat released by soil water freezing， resulting in overestimation （underestimation） of soil heat flux and overestimation of energy closure rate.Heat absorbed by soil water thawing， resulting in underestimation of soil heat flux and underestimation of energy closure rate， thus the freeze-thaw process affecting the balance of surface energy budget.

Keywords

• soil freeze-thaw process
• energy flux
• water and heat exchange
• numerical simulation