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 (similar to 50 m) long-wave 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 (similar to 0.5 degrees) 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 m). In situ tundra soil dielectric constant (epsilon) 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 degrees 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 0.45m 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 epsilon profile along this transect in 2014 and 2015, which is sensitive to near-surface soil liquid water content and freezethaw status. The epsilon difference in radar retrievals for the surface (similar to<0.1 m) soil between late August and early October was negatively correlated with SCF in September (R = 0.77, p < 0.01); areas with lower SCF generally showed larger epsilon 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.
1.CALTECH, Jet Prop Lab, 4800 Oak Grove Dr, Pasadena, CA 91109 USA 2.Univ Montana, Numer Terradynam Simulat Grp, Missoula, MT 59812 USA 3.Univ Southern Calif, Dept Elect Engn, Los Angeles, CA USA
Recommended Citation:
Yi, Yonghong,Kimball, John S.,Chen, Richard H.,et al. Sensitivity of active-layer freezing process to snow cover in Arctic Alaska[J]. CRYOSPHERE,2019-01-01,13(1):197-218