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: English
• Published: 2019
• Subjects: Arctic; Merra; Alaska North Slope; north slope; permafrost; Tundra; Alaska
• Online Access: https://doi.org/10.5194/tc-13-197-2019; https://tc.copernicus.org/articles/13/197/2019/

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 , <math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><mtext>bias</mtext><mo>=</mo><mo>-</mo><mn mathvariant="normal">0.01</mn></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="64pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="1a3da0dc9f7711d4ab0ae129e41b767d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tc-13-197-2019-ie00001.svg" width="64pt" height="10pt" src="tc-13-197-2019-ie00001.png"/></svg:svg> 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 the surface ( <math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>∼</mo><mo><</mo><mn mathvariant="normal">0.1</mn></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="35pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="e9cc1cbe469720fec5b17639f79d84f3"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tc-13-197-2019-ie00002.svg" width="35pt" height="10pt" src="tc-13-197-2019-ie00002.png"/></svg:svg> m ) soil between late August and early October was negatively correlated with SCF in September ( <math xmlns="http://www.w3.org/1998/Math/MathML" id="M25" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>R</mi><mo>=</mo><mo>-</mo><mn mathvariant="normal">0.77</mn></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="b640556f9be3aa920bcfcb3165d43fbe"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tc-13-197-2019-ie00003.svg" width="52pt" height="10pt" src="tc-13-197-2019-ie00003.png"/></svg:svg> , p <0.01 ); areas with lower SCF generally showed larger ε reductions, indicating earlier surface soil freezing. On regional scales, the simulated zero curtain in the upper ( <0.4 m ) soils showed large variability and was closely associated with variations in early cold-season snow cover. Areas with earlier snow onset generally showed a longer zero-curtain period; however, the soil freeze onset and zero-curtain period in deeper ( >0.5 m ) soils were more closely linked to maximum thaw depth. Our findings indicate that a deepening active layer associated with climate warming will lead to persistent unfrozen conditions in deeper soils, promoting greater cold-season soil carbon loss.