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

Abstract

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 (θ 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 (σ 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 σ 0 at both hh (σ 0 hh ) and vv (σ 0 vv ) polarizations. Analysis of measurements collected between December 2017 and March 2018 shows that 1) diurnal cycles are observed in both T B and σ 0 due to the change in surface θ liq caused by diurnal soil freeze-thaw transitions; 2) a negatively linear relationship is found between e and σ 0 regardless of frequency, polarization combinations, and observation angles; 3) slopes (β) of linearly fit equations between e H and σ 0 hh decrease with increasing observa-Index Terms-Active and passive, frozen soil, L-band microwave radiometry, sentinel, soil moisture active passive (SMAP), soil moisture and ocean salinity (SMOS), wide-band microwave scatterometer.

Modelling of Multi-Frequency Microwave Backscatter and Emission of Land Surface by a Community Land Active Passive Microwave Radiative Transfer Modelling Platform (CLAP)

“…It is known that AIEM assumes isotropic roughness properties for a homogenous dielectric half-space and does not account for the dielectric effects due to heterogeneities in the soil medium (e.g., composition, moisture content, and bulk density) and the resultant mismatch of impedance 110 between air and soil interface. To account for this, with the aid of comprehensive field observations involving ground-based L-band radiometry observations at an alpine meadow of Maqu site on the Eastern Tibetan Plateau (Su et al, 2020;Su et al, 2011), a physically-based surface dielectric roughness model named the air-to-soil transition (ATS) model (Zhao et al, 2021) Progress has been made to understand the scattering and emission processes in vegetated lands at a multi-120 annual scale (including freeze-thaw processes) by utilizing the coupled IEM with TVG models and the Maqu L-band radiometry data (Zheng et al, 2017;Zhao et al, 2021) and Maqu ground-based broad-band scatterometry data during the winter period (Zheng et al, 2021). A successful retrieval of soil moisture using Aquarius active and passive microwave data and the foregoing coupled model was achieved (Wang et al, 2019), which is proven consistent with satellite-derived precipitation and evaporation products on 125 the Tibetan plateau that is rarely achievable with current operational soil moisture products.…”

“…Considering the different nature of vegetation, soil and surface roughness status during the summer and winter periods, this study carries out model simulations for each period separately. We focused on simulations with one hour interval during the summer period from 20/07/2018 to 05/08/2018 and during 300 the winter period from 01/01/2018 to 15/01/2018, where the prior information is available (Hofste et al, 2021;Zheng et al, 2021). Accordingly, we also analyzed the performance of microwave multi-frequency signal forward modelling at the diurnal level.…”

“…Similar site-specific best-fit approaches were also applied for determining surface roughness in the 90 passive microwave case (Wigneron et al, 2017;Chaubell et al, 2020), and for parameterizing vegetation and soil variables in the water cloud model (Attema and Ulaby, 1978) in the active microwave case (Steele-Dunne et al, 2017;Bai et al, 2021).…”

Characteristics of Freeze–Thaw Cycles in an Endorheic Basin on the Qinghai-Tibet Plateau Based on SBAS-InSAR Technology

“…However, due to its coarse spatial resolution (~25 km), its application in fine-scale research is limited. Active microwave sensors, including synthetic aperture radar (SAR), mainly obtain the F-T state through the backscattering coefficient [46][47][48][49][50], generally have a fine spatial resolution (1 km or ~10 m for SAR) but are highly susceptible to scatterings from vegetation, snow, and surface roughness changes other than ground F-T state [51][52][53]. The combination of active and passive microwave technology has become a reliable method for monitoring the F-T state [38,[53][54][55][56][57][58][59].…”

“…The studies mentioned above focused on identifying the F-T state of the near-surface. Over permafrost terrain, in addition to the F-T state of the near-surface, the F-T state and F-T processes in the AL [44,53] are also very important but still lack investigation.…”