Abstract
Abstract. Numerical simulations are essential tools for understanding the complex hydrologic response of Arctic regions to a warming climate. However, strong coupling among thermal and hydrological processes on the surface and in the subsurface and the significant role that subtle variations in surface topography have in regulating flow direction and surface storage lead to significant uncertainties. Careful model evaluation against field observations is thus important to build confidence. We evaluate the integrated surface/subsurface permafrost thermal hydrology models in the Advanced Terrestrial Simulator (ATS) against field observations from polygonal tundra at the Barrow Environmental Observatory. ATS couples a multiphase, 3D representation of subsurface thermal hydrology with representations of overland nonisothermal flows, snow processes, and surface energy balance. We simulated thermal hydrology of a 3D ice-wedge polygon with geometry that is abstracted but broadly consistent with the surface microtopography at our study site. The simulations were forced by meteorological data and observed water table elevations in ice-wedge polygon troughs. With limited calibration of parameters appearing in the soil evaporation model, the 3-year simulations agreed reasonably well with snow depth, summer water table elevations in the polygon center, and high-frequency soil temperature measurements at several depths in the trough, rim, and center of the polygon. Upscaled evaporation is in good agreement with flux tower observations. The simulations were found to be sensitive to parameters in the bare soil evaporation model, snowpack, and the lateral saturated hydraulic conductivity. Timing of fall freeze-up was found to be sensitive to initial snow density, illustrating the importance of including snow aging effects. The study provides new support for an emerging class of integrated surface/subsurface permafrost simulators.
Highlights
Permafrost soils underlie approximately one-quarter ( ∼ 15 million km2) of the land surface in the Northern Hemisphere (Brown et al, 1997; Jorgenson et al, 2001) and store a vast amount of frozen organic carbon (Hugelius et al, 2014; Schuur et al, 2015)
We evaluate integrated surface/subsurface permafrost thermal hydrology models implemented in the Advanced Terrestrial Simulator (ATS) v0.88 using soil temperature (Romanovsky et al, 2017; Garayshin et al, 2019), water level (Liljedahl and Wilson., 2016; Liljedahl et al, 2016), snowpack depth (Romanovsky et al, 2017), and evapotranspiration (Dengel et al, 2019; Raz-Yaseef et al, 2017) data collected over several years at the Next-Generation Ecosystem Experiment–Arctic (NGEE Arctic) study site in polygonal tundra near Utqiagvik, Alaska
Distribution among the center, rim, and trough locations agrees well with the observations. These results indicate that the ATS models for snowpack dynamics and snow distribution are reasonably representative
Summary
Permafrost soils underlie approximately one-quarter ( ∼ 15 million km2) of the land surface in the Northern Hemisphere (Brown et al, 1997; Jorgenson et al, 2001) and store a vast amount of frozen organic carbon (Hugelius et al, 2014; Schuur et al, 2015). Warming in Arctic regions is expected to lead to permafrost thawing, as has been observed from field data during the past several decades (Lachenbruch and Marshall, 1986; Romanovsky et al, 2002; Osterkamp, 2003; Hinzman et al, 2005; Osterkamp, 2007; Wu and Zhang, 2008; Batir et al, 2017; Farquharson et al, 2019). The thermal stability of these regions is a primary control over the fate of the stored organic matter. Since most of this organic carbon is stored in the upper 4 m of the soil (Tarnocai et al, 2009), degradation of permafrost can result in the decomposition of large carbon stocks, potentially releasing this carbon to the atmosphere (Koven et al, 2011). As climate models generally indicate accelerating warming in the 21st century, there is an urgent need to understand these impacts
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