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

Yonghong Yi,Mahta Moghaddam,John S Kimball,Charles E Miller,Richard H Chen

doi:10.5194/tc-13-197-2019

Abstract

Abstract. 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 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 (∼<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 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.

Highlights

Warming in the northern high latitudes is occurring at roughly twice the global rate, leading to widespread soil thawing and permafrost degradation (Liljedahl et al, 2016)
The accuracy of the gap-filled MODerate resolution Imaging Spectroradiometer (MODIS) snow cover extent (SCE) product was cross-checked using the two MODIS sensors (Terra and Aqua); the downscaled snow depth data were evaluated using in situ SNOTEL observations across Alaska
The modelsimulated soil-freezing process and active-layer thickness (ALT) dynamics were conducted over a smaller Arctic Alaska domain and evaluated using a diverse set of regional observations

Summary

Introduction

Warming in the northern high latitudes is occurring at roughly twice the global rate, leading to widespread soil thawing and permafrost degradation (Liljedahl et al, 2016). Increasing soil warming and thawing potentially expose vast soil organic carbon (SOC) stocks in permafrost soils to mobilization and decomposition, which may promote large positive climate feedbacks (Schuur et al, 2015). The timing, magnitude, location and form of this potential permafrost carbon feedback remain highly uncertain due to many poorly. Y. Yi et al.: Sensitivity of active-layer freezing process to snow cover in Arctic Alaska understood mechanisms that control permafrost thaw and subsequent organic carbon decomposition (Lawrence et al, 2015). Despite recent improvements in modeling permafrost soil thermal and carbon dynamics, global model projections of near-surface permafrost loss by 2100 range from 30 % to 99 % and associated carbon release ranges from 37 to 174 Pg C under the current climate warming trajectory (Representative Concentration Pathway RCP 8.5) (Koven et al, 2013; Schuur et al, 2015). Most observational and modeling studies in the Arctic–boreal zone (ABZ) have emphasized the shorter growing season, while cold-season soil respiration may account for more than 50 % of the annual carbon budget (Zona et al, 2016)

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