L-Band Microwave Emission of Soil Freeze–Thaw Process in the Third Pole Environment

Soil freeze–thaw transition monitoring is essential for quantifying climate change and hydrologic dynamics over cold regions, for instance, the Third Pole. We investigate the L-band (1.4 GHz) microwave emission characteristics of soil freeze–thaw cycle via analysis of tower-based brightness temperature (
$T_{{{{\mathrm {B}}}}}^{p}$
) measurements in combination with simulations performed by a model of soil microwave emission considering vertical variations of permittivity and temperature. Vegetation effects are modeled using Tor Vergata discrete emission model. The ELBARA-III radiometer is installed in a seasonally frozen Tibetan grassland site to measure diurnal cycles of L-band
$T_{{{{\mathrm {B}}}}}^{p}$
every 30 min, and supporting micrometeorological as well as volumetric soil moisture (
$\theta $
) and temperature profile measurements are also conducted. Soil freezing/thawing phases are clearly distinguished by using
$T_{{{{\mathrm {B}}}}}^{p}$
measurements at two polarizations, and further analyses show that: 1) the four-phase dielectric mixing model is appropriate for estimating permittivity of frozen soil; 2) the soil effective temperature is well comparable with the temperature at 25 cm depth when soil liquid water is freezing, while it is closer to the one measured at 5 cm when soil ice is thawing; and 3) the impact on
$T_{{{{\mathrm {B}}}}}^{p}$
caused by diurnal changes of ground permittivity is dominating the impact of changing ground temperature. Moreover, the simulations performed with the integrated Tor Vergata emission model and Noah land surface model indicate that the
$T_{{{{\mathrm {B}}}}}^{p}$
signatures of diurnal soil freeze–thaw cycle is more sensitive to the liquid water content of the soil surface layer than the
in situ
measurements taken at 5 cm depth.