Radiation Simulation Of Surface Freezing-thawing Process

The freeze-thaw cycle of near surface soil is a common natural process on the earth’s land surface.This process is important in water cycle,energy balance,carbon cycle and water heat exchange in the earth air system.China is a middle and high latitude country with rich types and wide distribution of frozen soil resources.Freeze-thaw cycles occur frequently in China.Researchers pay more attention to freeze-thaw cycles,frozen soil physical structure characteristics and other related research activities.Under the background of global warming,soil freeze-thaw cycles have also attracted the attention of a series of disciplines in other fields,such as hydrology,geology,environmental science and so on.The demand for freeze-thaw cycles monitoring and simulation research is increasing.This paper focuses on two aspects: the improvement of dielectric model of freeze-thaw soil and the simulation of multi-layer radiative transfer.The main research contents are as follows:(1)Improvement of freeze-thaw soil dielectric model.The freeze-thaw cycles of soil are the process of phase change of liquid water in soil,and the dielectric constant is highly sensitive to the change of soil water.Therefore,the dielectric model of frozen and thawed soil is very important to the study of soil freezing and thawing cycle.Based on the Zhang model,this paper improved the algorithm of soil unfrozen water content in the dielectric model,optimized the empirical parameters in the model with the measured dielectric data of soil samples,and finally compared and verified the dielectric simulation results of the improved model.The simulation ability of the improved freeze-thaw soil dielectric model has been improved.The error statistical results show that the soil complex relative permittivity predicted by the improved model is highly correlated with the measured values,and the correlation coefficient ranges from0.7944 to 0.9865.The predicted normalized root mean square errors are 13.72%(real part)and 25.41%(imaginary part).(2)Simulation of multi-layer soil radiative transfer in surface freeze-thaw cyclesMicrowave remote sensing has great application potential in the field of soil freezing and thawing monitoring.Due to the strong penetration ability of microwave radiation to soil and energy transfer,the process of soil freezing and thawing is a top-down process.Therefore,a multi-layer radiative transfer model of surface soil is of great significance in the simulation of surface freezing and thawing observation.In this paper,the improved freeze-thaw soil dielectric model,Wilheit radiative transfer model,AIEM surface roughness model and surface vegetation cover model were constituted the multi-layer radiative transfer model of surface soil.The surface brightness temperature observation data set of Qinghai Tibet Plateau scientific data center is used to simulate and verify the multi-layer radiative transfer model of surface soil.The results show that the model had a good simulation effect on the change of brightness temperature in the freeze-thaw cycles of surface soil,and has a strong ability to capture and monitor the change of freezing and thawing state of surface soil.The error statistical index shown that the correlation coefficient between the simulated brightness temperature value and the brightness temperature value observed by the surface radiometer is 0.9164(H polarization)and0.9543(V polarization),and the normalized root mean square error is 4.75%(H polarization)and 2.88%(V polarization),which proved the good brightness temperature simulation ability of the multi-layer radiative transfer model.