Evaluation of aMetOp ASCAT-Derived Surface Soil Moisture Product in Tundra Environments

Satellite-derived surface soil moisture data are available for the Arctic, but detailed validation
is still lacking. Previous studies have shown low correlations between in situ and modeled data. It is
hypothesized that soil temperature variations after soil thaw impact MetOp ASCAT satellite-derived surface
soil moisture (SSM) measurements in wet tundra environments, as C band backscatter is sensitive to
changes in dielectric properties. We compare in situ measurements of water content within the active layer
at four sites across the Arctic in Alaska (Barrow, Sagwon, Toolik) and Siberia (Tiksi), taken in the spring after
thawing and in autumn prior to freezing. In addition to the long-term measurement fields, where sensors
are installed deeper in the ground, we designed a monitoring setup for measuring moisture very close
to the surface in the Lena River Delta, Siberia. The volumetric water content (VWC) and soil temperature
sensors were placed in the moss organic layer in order to account for the limited penetration depth of
the radar signal. ASCAT SSM variations are generally very small, in line with the low variability of in situ
VWC. Short-term changes after complete thawing of the upper organic layer, however, seem to be mostly
influenced by soil temperature. Correlations between SSM and in situ VWC are generally very low, or even
negative. Mean standard deviation matching results in a comparably high root-mean-square error (on
average 11%) for predictions of VWC. Further investigations and measurement networks are needed to
clarify factors causing temporal variation of C band backscatter in tundra regions.