Retrieving frozen ground surface temperature under the snowpack in the Arctic permafrost area from SMOS observations

Abstract. We developed and evaluated a new method to retrieve ground surface temperatures Tg below the snowpack from Soil Moisture and Ocean Salinity (SMOS) satellite L-band brightness temperatures (BTs). The study was performed over 21 reference sites providing in situ ground temperatures Tg-insitu in Northern Alaska from 2011 to 2020, representative of Arctic tundra underlined by continuous permafrost, and with various open water fractions. Values of Tg were obtained by inverting two types of microwave emission models (MEMs) tailored for winter Arctic tundra environments. The first MEM assumed homogeneous SMOS pixels and optimized the surface roughness Hr,gs. We observed the important influence of the frozen water bodies on Tg retrievals. Accordingly, we used a second more advanced MEM that accounts for the water surfaces within the SMOS pixels and describes their emission using an optimized water–ice interface roughness parameter, Hr,wi. For sites with water fraction < 0.04, our methods (median R = 0.60) outperformed the European Centre for Medium-Range Weather Forecasts reanalysis (ERA5) product (median R = 0.51) with respect to the reference sites. The bias between retrieved and in situ temperature was slightly negative (median bias = −0.2 °C). For sites with water fraction > 0.20, our water fraction correction reduced the bias, but the correlation of the Tg retrievals remained lower than that of ERA5. This study opens a new avenue for monitoring Tg below the snowpack in the Arctic using L-band BT, by inversion of a relatively simple MEM and limited auxiliary data. Extending this study to the whole Arctic area and taking advantage of the 15 years of SMOS data to study spatio-temporal variability of winter Tg in Arctic environments is extremely promising.