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. [ABSTRACT FROM AUTHOR]