UAV-BASED L-BAND RADIOMETER MEASUREMENTS THE NORTHERN BOREAL WETLAND ENVIRONMENT DURING THE FREEZING SEASON

A. Rimali¹, K. Rautiainen¹, J. Lemmetyinen¹

¹Finnish Meteorological Institute, Earth Observation Research, Finland, Corresponding Author: aleksi.rimali@fmi.fi

Keywords: UAV, L-band, freezing season, boreal wetland

Challenge

Seasonal soil freezing strongly affects water and energy exchange between atmosphere and land. In northern boreal wetlands, freeze–thaw dynamics are crucial for understanding greenhouse gas fluxes, especially during shoulder seasons [1]–[6]. In situ measurements are sparse and fail to capture spatial variability in these heterogeneous landscapes, while remote sensing products often lack the resolution to detect local differences. UAV-based L-band radiometer observations offer an intermediate solution, bridging the gap between in situ and satellite data. They cover larger areas than ground measurements while resolving fine-scale variability. The low L-band frequency enables penetration through dry snow and surface vegetation to monitor freeze–thaw states, though wet snow complicates interpretation during spring melt.

Methodology used

We employed a UAV equipped with a Portable L-Band Radiometer (PoLRa) [7] (1.4 GHz) to monitor freeze–thaw dynamics and spatial variability in a boreal wetland. Between 2022 and 2025, UAV surveys were conducted at least monthly during autumn to track freezing progression in two areas of the Halssiaapa wetland, Sodankylä, Finland. Measurements started before snowfall and continued through winter into late spring.

The study sites are heterogeneous wetlands with flarks, strings, and sparse pine forest. Each UAV campaign covered ~200 × 250 m, with a spatial resolution of 21 × 27 m at 25 m altitude. The radiometer antenna recorded vertical (Tbv) and horizontal (Tbh) brightness temperatures. Soil liquid water content affects permittivity, producing distinct signatures between frozen and thawed states, enabling detection of soil condition.

To complement UAV data, continuous observations were made with the ELBARA-II [8] radiometer near one site, and manual soil frost tube measurements supported validation.

Results

Initial results show that brightness temperature varied across the wetland during freezing, reflecting differences in vegetation, soil moisture, and microtopography. Contrasts between flarks and strings highlighted site heterogeneity. Fluctuating air temperatures produced alternating freeze–thaw cycles, visible in radiometric and in-situ data. Early-winter snow timing and thickness influenced freezing dynamics, frost depth, and mid-winter thawing episodes. As winter progressed, temperatures fell, snow accumulated, and soils froze. Later spring snowmelt increased liquid water in the snowpack, limiting ground-state detection with L-band.

Main conclusions

Freeze–thaw in boreal wetlands is shaped by soil liquid water, vegetation, snowfall timing, and air temperature fluctuations. While UAV-based L-band radiometry can be affected by interference, wind, cold, or late-winter snow, it offers clear advantages over in situ and satellite observations: reduced field labour, flexible resolution, and access to remote areas. UAVs thus provide a critical link between point-scale in situ data and coarse-resolution satellites, offering validation for spaceborne radiometer missions.

We will continue UAV observations at Halssiaapa and expand to varied environments. Future campaigns will test different flight altitudes to optimize resolution. In parallel, UAVs will be coupled with fixed L-band radiometers to build a comprehensive framework for monitoring freeze–thaw processes in northern regions.

Figure 1. (a) PoLRa radiometer mounted on a DJI Matrice 600 UAV. Photograph by: Elmeri Viuho. (b) Brightness temperature (vertical polarization) measured over study area 1 in the Halssiaapa wetland on 20 November 2024. Note: the background map is not from the same time period.

1. I. Allison, R. G. Barry, and B. E. Goodison, Climate and Cryosphere (CliC) Project Science and Co-ordination Plan: Version 1, vol. 114. Geneva, Switzerland: Joint Planning Staff for WCRP, World Meteorological Organization, 2001, p. 76

2. E. D. Yershov, General Geocryology. Cambridge, U.K.: Cambridge Univ. Press, 2004, p. 608.

3. T. Sinha and K. A. Cherkauer, “Time series analysis of soil freeze and thaw processes in Indiana,” J. Hydrometeorol., vol. 9, no. 5, pp. 936–950, 2008.

4. M. G. Öquist and H. Laudon, “Winter soil frost conditions in boreal forests control growing season soil CO₂ concentration and its atmospheric exchange,” Glob. Change Biol., vol. 14, no. 12, pp. 2839–2847, 2008.

5. H. N. Kreplin, C. S. Santos Ferreira, G. Destouni, S. D. Keesstra, L. Salvati, and Z. Kalantari, “Arctic wetland system dynamics under climate warming,” Wiley Interdiscip. Rev.: Water, vol. 8, no. 4, p. e1526, 2021.

6. M. Meredith, M. Sommerkorn, S. Cassotta, C. Derksen, A. Ekaykin, A. Hollowed, G. Kofinas, A. Mackintosh, J. Melbourne-Thomas, M. M. C. Muelbert, G. Ottersen, H. Pritchard, and E. A. G. Schuur, “Polar Regions,” in IPCC Special Report on the Ocean and Cryosphere in a Changing Climate, H.-O. Pörtner, D. C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, and N. M. Weyer, Eds. Cambridge, UK and New York, NY, USA: Cambridge Univ. Press, 2019, pp. 203–320.

7. D. Houtz, R. Naderpour, and M. Schwank, ‘Portable L-Band Radiometer (PoLRa): Design and Characterization’, Remote Sens., vol. 12, no. 17, p. 2780, Aug. 2020, doi: 10.3390/rs12172780.

8. 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.