Soil freezing, wintertime evolution of soil frost and thawing 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 behavior of frost have a major effect on the surface energy balance, as well as on the intensity of CO2 and CH4 fluxes. Space-borne L-band radiometers, such as SMOS, provide a unique opportunity to monitor soil freezing and thawing processes on a global scale, covering regions of seasonal soil frost and sporadic / continuous permafrost.

The objective of the SMOS + Permafrost activity is to improve existing methods and develop new algorithms for the detection and monitoring of soil freezing / thawing processes using L-band passive microwave data. Radiative transfer modeling and algorithm development will be based on observations from the ELBARA-II SMOS reference radiometer data together with extensive in situ information available for the Sodankylä - Pallas region in northern Finland. First results obtained from the Sodankylä test site show that the L-band passive signature tends to saturate for the winter period after the soil frost depth has reached a depth of 25 - 35 cm; the maximum seasonal soil frost in this area typically extends to about 1.5 - 3 meters depending on the year.

For the interpretation of SMOS observations detailed information on the land cover and soil type is required, since the microwave emissivity from barren mountain regions, bogs and forests can vary substantially. In addition, the contribution from open and ice covered water bodies have to be taken into account. For the whole of Northern Finland land cover and soil type information is available in 25 x 25 m resolution. First SMOS-based frost depth estimates have been derived for Northern Europe for 2010 and 2011 (Fig. 1). The examples for 26 November 2010 and 2011 show the late onset of winter in the Northern European areas in 2011. The results are currently being validated using frost tubes installed in the Northern part of Finland. Within the activity it is planned to produce a demonstration data set for the Northern Hemisphere.