Year-long, broad-band, microwave backscatter observations of an alpine meadow over the Tibetan Plateau with a ground-based scatterometer

Abstract

Abstract. A ground-based scatterometer was installed on an alpine meadow over the Tibetan Plateau to study the soil moisture and temperature dynamics of the top soil layer and air–soil interface during the period August 2017–August 2018. The deployed system measured the amplitude and phase of the ground surface radar return at hourly and half-hourly intervals over 1–10 GHz in the four linear polarization combinations (vv, hh, hv, vh). In this paper we describe the developed scatterometer system, gathered datasets, retrieval method for the backscattering coefficient (σ0), and results of σ0. The system was installed on a 5 m high tower and designed using only commercially available components: a vector network analyser (VNA), four coaxial cables, and two dual-polarization broad-band gain horn antennas at a fixed position and orientation. We provide a detailed description on how to retrieve the backscattering coefficients for all four linear polarization combinations σpq0, where p is the received and q the transmitted polarization (v or h), for this specific scatterometer design. To account for the particular effects caused by wide antenna radiation patterns (G) at lower frequencies, σ0 was calculated using the narrow-beam approximation combined with a mapping of the function G2/R4 over the ground surface. (R is the distance between antennas and the infinitesimal patches of ground surface.) This approach allowed for a proper derivation of footprint positions and areas, as well as incidence angle ranges. The frequency averaging technique was used to reduce the effects of fading on the σpq0 uncertainty. Absolute calibration of the scatterometer was achieved with measurements of a rectangular metal plate and rotated dihedral metal reflectors as reference targets. In the retrieved time series of σpq0 for L-band (1.5–1.75 GHz), S-band (2.5–3.0 GHz), C-band (4.5–5.0 GHz), and X-band (9.0–10.0 GHz), we observed characteristic changes or features that can be attributed to seasonal or diurnal changes in the soil: for example a fully frozen top soil, diurnal freeze–thaw changes in the top soil, emerging vegetation in spring, and drying of soil. Our preliminary analysis of the collected σpq0 time-series dataset demonstrates that it contains valuable information on water and energy exchange directly below the air–soil interface – information which is difficult to quantify, at that particular position, with in situ measurement techniques alone. Availability of backscattering data for multiple frequency bands (raw radar return and retrieved σpq0) allows for studying scattering effects at different depths within the soil and vegetation canopy during the spring and summer periods. Hence further investigation of this scatterometer dataset provides an opportunity to gain new insights in hydrometeorological processes, such as freezing and thawing, and how these can be monitored with multi-frequency scatterometer observations. The dataset is available via https://doi.org/10.17026/dans-zfb-qegy (Hofste et al., 2021). Software code for processing the data and retrieving σpq0 via the method presented in this paper can be found under https://doi.org/10.17026/dans-xyf-fmkk (Hofste, 2021).

Monitoring Water and Energy Cycles at Climate Scale in the Third Pole Environment (CLIMATE-TPE)

“…WP 1: Observation and modelling of microwave scattering and emission under complex terrains including permafrost and freeze and thawing A data record of L-band radiometry [16] A data record of 1-10 GHz scatterometry [28] Analysis results of ELBARA observations [29,30] New retrieval methods by merging existing satellite data of different frequencies PhD theses WP2: Advancement of physical understanding and quantification of changes of water and energy budgets in TPE Results of observation and analysis of water and energy balance of the lakes in TPE [31] A consistent soil moisture data set for the Tibetan Plateau [32][33][34] A high-resolution land surface energy and water fluxes from satellite data using EO data using the SEBS model [35,36] Results of joint diagnoses of the products from ( 2)-( 3) were conducted to improve model physics, parameterisation and parameters for climate analysis A STEMMUS-FT model for detailing freeze-thaw dynamics [21,22] PhD theses WP3: Advancement of quantifying changes in surface characteristics and monsoon interactions…”

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

“…In August 2017, the ground-based scatterometer was installed also on the tower and continued to operate until December 2018, providing broad-band ranging from L-band (1.5-1.75 GHz), S-band (2.5-3.0 GHz), C-band (4.5-5.0 GHz), to X-band (9.0-10.0 GHz) backscatter observations (with hourly interval) at the four linear polarizations (i.e., HH, VV, VH and HV) (Hofste et al, 2021). Considering the effect of the scatterometer antenna pattern, especially a large pattern angle in the low frequency, Hofste et al (2021) 185 https://doi.org/10.5194/hess-2022-333 Preprint. Discussion started: 21 October 2022 c Author(s) 2022.…”

“…derived the effective incidence angle characterized from the antenna to the calculated footprint center, where the surface projected antenna beam intensity is equal to half its maximum value. The incidence angle range (i.e., minimum and maximum angles in Table 1) that determine the footprint area was also obtained for each frequency (Hofste et al, 2021). Although the backscatter at both cross-polarization (i.e., VH and HV) were measured, they exhibit the difference in terms of frequency and land surface condition 190 change (Hofste et al, 2021).…”

“…The incidence angle range (i.e., minimum and maximum angles in Table 1) that determine the footprint area was also obtained for each frequency (Hofste et al, 2021). Although the backscatter at both cross-polarization (i.e., VH and HV) were measured, they exhibit the difference in terms of frequency and land surface condition 190 change (Hofste et al, 2021). This paper only focuses on the observation at VH polarization for simplicity.…”

“…Where 𝑉𝑉𝑉𝑉𝑉𝑉_𝑚𝑚 is the mean vegetation water content (kg/kg), A is the amplitude of the VWC fluctuation 215 (the range from maximum or from minimum to the average temperature). In this case, based on the in situ measurement conducted by Hofste et al (2021), 𝑉𝑉𝑉𝑉𝑉𝑉_𝑚𝑚 is valued at 0.6, and 𝐴𝐴 is valued at 0.05. 𝜔𝜔 is the radial frequency, which is 2π times the actual frequency. In the case of diurnal variation, the period is 86,400 sec (24 hour), so ω= 2π/86,400 = 7.27 × 10 −5 /sec.…”