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 ( $theta_{liq}$ ) 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 SUPERSCRIPT ZERO) 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 TB and copolarized sigma SUPERSCRIPT ZERO at both hh (sigma $_{hh}$ SUPERSCRIPT ZERO) and vv (sigma $_{vv}$ SUPERSCRIPT ZERO) polarizations. Analysis of measurements collected between December 2017 and March 2018 shows that 1) diurnal cycles are observed in both T $_{B}$ and sigma SUPERSCRIPT ZERO due to the change in surface theta liq caused by diurnal soil freeze-thaw transitions; 2) a negatively linear relationship is found between e and sigma SUPERSCRIPT ZERO regardless of frequency, polarization combinations, and observation angles; 3) slopes (beta ) of linearly fit equations between eH and sigma $_{hh}$ SUPERSCRIPT ZERO decrease with increasing observation angles of ELBARA-III, while the ones between eV and sigma $_{vv}$ SUPERSCRIPT ZERO increase with increasing observation angles; and 4) correlations between e and sigma SUPERSCRIPT ZERO increase with decreasing microwave frequency of sigma SUPERSCRIPT ZERO measurements and ELBARA-III observation angles, and magnitudes of diurnal sigma SUPERSCRIPT ZERO cycles also increase with decreasing microwave frequency. Moreover, the calibrated Tor Vergata model shows capability to reproduce both diurnal e and sigma SUPERSCRIPT ZERO variations as well as to quantify their relationships at different frequencies and observation angles.