The effect of freezing and thawing on the microwave signatures of bare soil

Abstract

To study the effect of a frozen soil layer on the microwave signatures of bare soil, we carried out measurements on frozen and unfrozen fields in winter. Our ground-based radiometer — scatterometer system, RASAM, works at frequencies between 3 GHz and 11 GHz. The brightness temperature was measured at horizontal and vertical polarization, the backscattering coefficient at HH, VV, HV, and VH polarization, all at incidence angles from 0° to 70°. In addition, we measured soil parameters such as moisture, roughness, and texture. The results are subdivided into three sections. First the microwave parameters measured during two diurnal freeze — thaw cycles of the soil are shown. For thawing the soil has three layers wet/frozen/wet and therefore different microwave properties than for freezing. Second frozen soil (thickness of the frozen layer is larger than the penetration depth of the radiation) is compared with unfrozen soil. For the horizontal polarization we separate the reflectivity into the Fresnel reflectivity and a roughness factor. The active data are compared with the geometrical-optics model at the higher frequencies and with the small-perturbation model at the lower frequencies. In addition the data are used to determine the sensitivity of the microwave parameters to soil liquid water content. Third the growth of a frozen soil layer is monitored. These data are used to test layered media models. Due to the scale height of surface roughness, which is of the order of the penetration depth of the radiation, the models do not agree well with the measurements. We find much better fits with a semiempirical formula.