SIXTEEN YEARS OF TOWER-BASED ELBARA-II L-BAND RADIOMETRY AT THE SODANKYLÄ SUPERSITE: FROM SOIL FREEZE/THAW TO FOREST CANOPY TRANSMISSIVITY ACROSS CONTRASTING BOREAL LANDSCAPES

K. Rautiainen¹, A. Kontu¹, H. Rytkönen¹, M. Holmberg¹, J. Lemmetyinen¹, T. Casal²

¹Finnish Meteorological Institute, ²ESA, ESTEC

The Finnish Meteorological Institute (FMI) has operated European Space Agency (ESA) owned tower-based ELBARA-II radiometers (ETH L-band radiometer for soil moisture research) [1] at the Sodankylä Supersite since 2009 to support calibration and validation of the Soil Moisture and Ocean Salinity (SMOS) mission [2] and to study boreal soil–snow–vegetation processes [3], [4]. Since 2018, the Sodankylä site has hosted two ELBARA-II instruments. The supersite provides permanent reference infrastructure across three main test sites where ELBARA-II radiometers have been deployed: a forest opening, a wetland, and a forest canopy. At the forest canopy site, two instrument positions—at ground level and atop a forest tower—enable observations from below and above the canopy. We report on a consolidated, multi-site record spanning these environments in northern Finland.

At L-band (≈1.4 GHz), land-surface emissivity—and thus brightness temperature—is strongly governed by the amount of free liquid water in the soil. The high permittivity of liquid water gives L-band strong sensitivity for monitoring soil moisture [2]. Similarly, the large permittivity contrast between liquid water and ice enables the detection of seasonal soil freeze–thaw (FT) transitions [3]. Building on this physics, we have assembled long, tower-based ELBARA-II time series at Sodankylä to support Arctic land applications across three environments (forest opening, wetland, closed coniferous canopy) and to extend analysis from soil moisture and FT to L-band vegetation optical depth (L-VOD). Recent L-VOD studies show that forest-canopy transmission varies with canopy temperature, with pronounced changes near 0 °C [4], which modulates the soil contribution seen at L-band. In this work our primary objective is FT: we synthesize the multi-site ELBARA-II record to characterize—and improve retrieval of—autumn and spring FT transitions while accounting for confounding canopy and snow effects.

We report on sixteen years of ELBARA-II observations across the Sodankylä test sites. Typically, we collected 2–3 years of continuous measurements at one site at a time, and since 2018 we have operated two sites concurrently. The dataset has supported, for example, improved monitoring of seasonal soil freeze–thaw (FT) transitions [3, 5], investigations of how a dry snow layer affects the L-band signal [6], retrieval of frozen-ground permittivity [6], and quantification of temperature-driven changes in forest-canopy transmissivity [4]. Building on these achievements, we continue to use the ELBARA-II record to reduce canopy/snow uncertainties in L-band soil moisture and FT retrievals and to examine links between snowpack properties, soil freezing, and winter methane emissions at a wetland site. In parallel, we are curating the 2009–present record into a harmonized, quality-controlled dataset with standardized metadata, site descriptors, and co-registered in situ variables. The resulting time series are being prepared for broad scientific reuse—supporting algorithm development and evaluation, canopy-transmissivity and L-VOD studies, and future boreal carbon-cycle assessments.

References

1. M. Schwank et al., ‘ELBARA II, an L-Band Radiometer System for Soil Moisture Research’, Sensors, vol. 10, no. 1, pp. 584–612, Jan. 2010, doi: 10.3390/s100100584.

2. Y. H. Kerr et al., ‘The SMOS Mission: New Tool for Monitoring Key Elements ofthe Global Water Cycle’, Proc. IEEE, vol. 98, no. 5, pp. 666–687, May 2010, doi: 10.1109/JPROC.2010.2043032.

3. K. Rautiainen et al., ‘Detection of soil freezing from L-band passive microwave observations’, Remote Sens. Environ., vol. 147, pp. 206–218, May 2014, doi: 10.1016/j.rse.2014.03.007.

4. M. Schwank et al., ‘Temperature effects on L-band vegetation optical depth of a boreal forest’, Remote Sens. Environ., vol. 263, p. 112542, Sept. 2021, doi: 10.1016/j.rse.2021.112542.

5. K. Rautiainen et al., ‘L-Band Radiometer Observations of Soil Processes in Boreal and Subarctic Environments’, IEEE Trans. Geosci. Remote Sens., vol. 50, no. 5, pp. 1483–1497, May 2012, doi: 10.1109/TGRS.2011.2167755.

6. J. Lemmetyinen et al., ‘Snow density and ground permittivity retrieved from L-band radiometry: Application to experimental data’, Remote Sens. Environ., vol. 180, pp. 377–391, July 2016, doi: 10.1016/j.rse.2016.02.002.