Soil Frost Detection Algorithm for SMOS
Rautiainen, Kimmo1; Pulliainen, Jouni1; Juha, Lemmetyinen1; Kontu, Anna1; Menard, Cecile B.1; Ikonen, Jaakko1; Schwank, Mike2; Mätzler, Christian2; Wiesmann, Andreas2
1Finnish Meteorological Institute, FINLAND; 2Gamma Remote Sensing, SWITZERLAND

Soil freezing and thawing, including the winter-time evolution of soil frost, are important characteristics influencing hydrological and climate processes at the regions of seasonal frost and permafrost, which include major land areas of North America and northern Eurasia. Changes in the seasonal behaviour of soil frost have a major effect on the surface energy balance, as well as on the intensity of CO2 and CH4 fluxes. The monitoring of seasonal frost and the permafrost active layer is currently based mostly on sparse in-situ observations. Some research using satellite observations for global and continuous coverage has been conducted in the past using active and passive microwave data. The European Space Agency's (ESA) SMOS satellite (Soil Moisture and Ocean Salinity) is the first passive instrument using a low microwave frequency band (1.403 - 1.424 GHz) for Earth remote sensing. The output signal of the SMOS payload instrument MIRAS (Microwave Imaging Radiometer with Aperture Synthesis) is highly sensitive to changes in soil permittivity (i.e. soil liquid water content), with relatively low influence of surface vegetation. Due to the instrument's low operating frequency, information on the soil processes can be achieved from deeper layers than from previously available satellite instruments. SMOS sensitivity to changes in soil permittivity and the deeper soil layer monitoring capabilities ensure new possibilities for soil freeze/thaw cycle monitoring.

Within the frame of SMOS programme, ESA has initialized several innovation projects to extend the applicability of SMOS data. One such project is SMOS+ Innovation Permafrost, coordinated by the Finnish Meteorological Institute (FMI) with GAMMA Remote Sensing as a Swiss partner. The main objectives of the project are (1) to develop methods and algorithms for detection and monitoring of soil freezing/thawing processes using L-band passive microwave data and (2) to demonstrate the developed methods with soil frost maps derived from SMOS observations representing the whole Northern Hemisphere.

Continuous tower-based measurements using the SMOS reference radiometer, ELBARA-II [1], have played a crucial role in the development of a novel soil state detection algorithm. [2]. An ELBARA-II instrument has been operational since October 2009 at the Finnish Meteorological Institute Arctic Research Center, close to the town of Sodankylä in Northern Finland. During the first three years, until August 2012, the observed area was a forest opening, representing typical boreal soil composed of a mixture of sand (70%), silt (29%) and clay (1%), with a thin (2-10 cm) organic top layer. In August 2012, ELBARA-II was relocated to a test site on a peatbog, the second most dominant type of land cover in the region.

The freezing of soil affects the passive L-band signal in two clearly noticeable ways: Firstly, the brightness temperature increases with decreasing permittivity of the soil, stabilizing to a constant value for the remainder of the freezing period.. Secondly, the detected polarization difference decreases. These two main characteristics form the basis of the frost detection algorithm developed here. The first version of our soil frost detection algorithm was developed using the ELBARA-II tower-based observations, complemented by numerous in situ measurements of the relatively well defined environment of the test site. The algorithm was then applied to SMOS CATDS level 3 brightness temperature data (Centre Aval de Traitement des Données SMOS). The derived maps of soil freezing were evaluated against a network of frost tube measurements over the geographical area of Finland The Finnish Environment Institute operates a network of over 600 manually measured frost tubes in 100 different locations spread evenly throughout the country. Using the results of the evaluation, algorithm parameters were fine-tuned to suit the differing measurement geometries between SMOS and the tower-based observations and to account for the heterogeneous sceneries observed by SMOS. Finally soil frost maps for the whole Northern Hemisphere were produced. The results indicate that the event and development of soil freezing can be monitored with SMOS. However, heterogeneous vegetation, land cover and soil type have a significant effect on the soil freezing processes and need to be taken into account. Future work is needed to improve the algorithms from the present experimental demonstration to an operational product.


[1] M. Schwank, A. Wiesmann, C. Werner, C. Maetzler, D. Weber, A. Murk, I. Voelksch and U. Wegmueller, "ELBARA II, an L-band radiometer system for soil moisture research", Sensors, 10, pp. 584 - 612, doi:10.3390/s100100584, 2010.

[2] K. Rautiainen, J. Lemmetyinen, J. Pulliainen, J. Vehviläinen, M. Drusch, A. Kontu, J. Kainulainen, and J. Seppänen, "L-Band Radiometer Observations of Soil Processes in Boreal and Subarctic Environments," IEEE Trans. Geosci. Remote Sensing, vol. 50, pp. 1483-1497, 2012