Modeling of Microwave Emission From Seasonally Frozen Ground Using Dense Media Radiative Transfer Theory (DMRT)

The freeze/thaw (F/T) transition of soil significantly affects water, energy, and carbon cycles at the land-atmosphere interface. The volumetric structure and vertical heterogeneity within the soil become apparent after soil freezing. This complicates the microwave radiative transfer process of frozen soil at different frequencies. In this study, a radiation transfer model, called SFS_DMRT, considering the volume scattering effects of seasonally frozen soil, is proposed based on dense media radiative transfer (DMRT) theory and the Mie spherical scattering model. The multiple scattering among discrete frozen soil clods is considered. This newly developed SFS_DMRT model is validated against ground radiometer measurements and compared with the advanced integral equation model (AIEM), a surface-scattering model, at three different experimental sites. Results show that in Sodankylä, where the soil is in a stable frozen state, the brightness temperature (Tb) simulated by SFS_DMRT has a higher agreement with observed Tb than that of AIEM. The emission of frozen soil is, moreover, better described by AIEM when the soil is undergoing diurnal F/T cycles in A’rou, in which the soil may freeze overnight and then thaw the next day. The Tb dependence on frequency was further examined, and results show that when simulating the passive microwave signature from the soil in a stable frozen state, which means the soil does not undergo intraday or diurnal F/T cycles, volume scattering effects can be ignored at the L-band; it should, however, be taken into consideration at Ku- and Ka-bands. The degree of volume scattering effects at C- and X-bands depends on the effective grain size of soil clods. The soil frost depth and microwave band penetration depth influence the attenuation of emissions from deeper unfrozen soil. The SFS_DMRT model developed in this study is vital for understanding the passive microwave signatures from frozen soil and can be used to obtain stratified profile information in layered soil.