Active and Passive Microwave Signatures of Diurnal Soil Freeze-Thaw Transitions on the Tibetan Plateau

Active and passive microwave characteristics of diurnal soil freeze-thaw transitions and their relationships are crucial for developing retrieval algorithms of the soil liquid water content (
$\theta _{\mathrm {liq}}$
) and freeze/thaw state, which, however, have been less explored. This study investigates these microwave characteristics and relationships via analysis of ground-based measurements of brightness temperature (
$T_{B}$
) and backscattering coefficients (
$\sigma ^{0}$
) in combination with simulations performed with the Tor Vergata discrete radiative transfer model. Both an L-band (1.4 GHz) radiometer ELBARA-III and a wide-band (1–10 GHz) scatterometer are installed in a seasonally frozen Tibetan meadow ecosystem to measure diurnal variations of
$T_{B}$
and copolarized
$\sigma ^{0}$
at both hh (
$\sigma _{\mathrm {hh}}^{0}$
) and vv (
$\sigma _{\mathrm {vv}}^{0}$
) polarizations. Analysis of measurements collected between December 2017 and March 2018 shows that 1) diurnal cycles are observed in both
$T_{B}$
and
$\sigma ^{0}$
due to the change in surface
$\theta _{\mathrm {liq}}$
caused by diurnal soil freeze-thaw transitions; 2) a negatively linear relationship is found between
$e$
and
$\sigma ^{0}$
regardless of frequency, polarization combinations, and observation angles; 3) slopes (
$\beta$
) of linearly fit equations between
$e^{H}$
and
$\sigma _{\mathrm {hh}}^{0}$
decrease with increasing observation angles of ELBARA-III, while the ones between
$e^{V}$
and
$\sigma _{\mathrm {vv}}^{\mathrm {0 {}}}$
increase with increasing observation angles; and 4) correlations between
$e$
and
$\sigma ^{0}$
increase with decreasing microwave frequency of
$\sigma ^{0}$
measurements and ELBARA-III observation angles, and magnitudes of diurnal
$\sigma ^{0}$
cycles also increase with decreasing microwave frequency. Moreover, the calibrated Tor Vergata model shows capability to reproduce both diurnal
$e$
and
$\sigma ^{\mathrm {0 {}}}$
variations as well as to quantify their relationships at different frequencies and observation angles.