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

Abstract. We developed and evaluated a new method to retrieve ground surface temperatures T_g below the snowpack from Soil Moisture and Ocean Salinity (SMOS) L-band brightness temperatures (BT). The study was performed over 21 reference sites providing with in situ ground temperatures T_g-insitu in Northern Alaska from 2011 to 2020, representative of Arctic tundra underlined by continuous permafrost, and with various open water fractions. T_g were obtained by inverting two types of microwave emission models (MEM) tailored for winter Arctic tundra environments. The first MEM assumed a homogeneous SMOS pixel and optimized the surface roughness H_r,gs. We observed the important influence of the frozen water bodies on T_g retrievals. Accordingly, we used an advanced MEM that accounts for the water surfaces within the SMOS pixels and describes their emission using an optimized water-ice interface roughness parameter, H_r,wi. For sites with water fraction < 0.04, our methods (median correlation 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 T_g retrievals remained lower than that of ERA5. This study opens a new avenue for monitoring T_g 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 T_g in Arctic environments is excessively promising.