Sensitivity of active-layer freezing process to snow cover in Arctic Alaska

• Published in: The Cryosphere
• Main Authors: Yi, Yonghong, Kimball, John S, Chen, Richard H., Moghaddam, Mahta, Miller, Charles E.
• Format: Text
• Language: unknown
• Published: ScholarWorks at University of Montana 2019
• Subjects: Arctic; Merra; Alaska North Slope; north slope; permafrost; Tundra; Alaska
• Online Access: https://scholarworks.umt.edu/ntsg_pubs/409; https://doi.org/10.5194/tc-13-197-2019; https://scholarworks.umt.edu/context/ntsg_pubs/article/1409/viewcontent/Yi_Cryosphere_2019.pdf; https://scholarworks.umt.edu/context/ntsg_pubs/article/1409/filename/0/type/additional/viewcontent/tc_13_197_2019_supplement.pdf

The contribution of cold-season soil respiration to the Arctic–boreal carbon cycle and its potential feedback to the global climate remain poorly quantified, partly due to a poor understanding of changes in the soil thermal regime and liquid water content during the soil-freezing process. Here, we characterized the processes controlling active-layer freezing in Arctic Alaska using an integrated approach combining in situ soil measurements, local-scale (∼50 m) longwave radar retrievals from NASA airborne P-band polarimetric SAR (PolSAR) and a remote-sensing-driven permafrost model. To better capture landscape variability in snow cover and its influence on the soil thermal regime, we downscaled global coarse-resolution (∼0.5∘) MERRA-2 reanalysis snow depth data using finer-scale (500 m) MODIS snow cover extent (SCE) observations. The downscaled 1 km snow depth data were used as key inputs to the permafrost model, capturing finer-scale variability associated with local topography and with favorable accuracy relative to the SNOTEL site measurements in Arctic Alaska (mean RMSE=0.16 m, bias=-0.01" role="presentation">bias=−0.01 m). In situ tundra soil dielectric constant (ε) profile measurements were used for model parameterization of the soil organic layer and unfrozen-water content curve. The resulting model-simulated mean zero-curtain period was generally consistent with in situ observations spanning a 2∘ latitudinal transect along the Alaska North Slope (R: 0.6±0.2; RMSE: 19±6 days), with an estimated mean zero-curtain period ranging from 61±11 to 73±15 days at 0.25 to 0.45 m depths. Along the same transect, both the observed and model-simulated zero-curtain periods were positively correlated (R>0.55, p<0.01) with a MODIS-derived snow cover fraction (SCF) from September to October. We also examined the airborne P-band radar-retrieved ε profile along this transect in 2014 and 2015, which is sensitive to near-surface soil liquid water content and freeze–thaw status. The ε difference in radar retrievals for ...