Abstract
Abstract. Climate change is profoundly transforming the carbon-rich Arctic tundra landscape, potentially moving it from a carbon sink to a carbon source by increasing the thickness of soil that thaws on a seasonal basis. However, the modeling capability and precise parameterizations of the physical characteristics needed to estimate projected active layer thickness (ALT) are limited in Earth system models (ESMs). In particular, discrepancies in spatial scale between field measurements and Earth system models challenge validation and parameterization of hydrothermal models. A recently developed surface–subsurface model for permafrost thermal hydrology, the Advanced Terrestrial Simulator (ATS), is used in combination with field measurements to achieve the goals of constructing a process-rich model based on plausible parameters and to identify fine-scale controls of ALT in ice-wedge polygon tundra in Barrow, Alaska. An iterative model refinement procedure that cycles between borehole temperature and snow cover measurements and simulations functions to evaluate and parameterize different model processes necessary to simulate freeze–thaw processes and ALT formation. After model refinement and calibration, reasonable matches between simulated and measured soil temperatures are obtained, with the largest errors occurring during early summer above ice wedges (e.g., troughs). The results suggest that properly constructed and calibrated one-dimensional thermal hydrology models have the potential to provide reasonable representation of the subsurface thermal response and can be used to infer model input parameters and process representations. The models for soil thermal conductivity and snow distribution were found to be the most sensitive process representations. However, information on lateral flow and snowpack evolution might be needed to constrain model representations of surface hydrology and snow depth.
Highlights
In Arctic tundra, the thickness of the soil layer that reaches above 0 ◦C, defined as the active layer thickness (ALT), largely determines the volume of carbon stores available for decomposition
In this paper we summarize our ModEx experience involving the detailed use of subsurface temperature and snow cover field data to develop and test process-rich simulations of ALT dynamics, such that observational data and necessary physical dynamics are incorporated into the model
The lowland, cold continuous permafrost tundra at Barrow Environmental Observatory (BEO) was established as the end-member of the NGEE-Arctic sites, which follow a bioclimatic gradient that extends to the warm discontinuous permafrost, shrub tundra environment of the Seward Peninsula
Summary
In Arctic tundra, the thickness of the soil layer that reaches above 0 ◦C, defined as the active layer thickness (ALT), largely determines the volume of carbon stores available for decomposition. Site-specific properties of Arctic soils, such as porosity, bulk thermal conductivity, and water retention characteristics, have been measured in lab settings from samples taken in the field (Hinzman et al, 1991; Letts et al, 2000) Those field and lab measured properties were used in ESMs in order to predict future ALT and permafrost conditions (Beringer et al, 2001; Lawrence and Slater, 2008; Subin et al, 2013). We use an iterative procedure that integrates finely resolved models with field observations and measurements to develop a process-rich model with physical mechanisms and parameters consistent with measurements from the Department of Energy Office of Science – Generation Ecosystem Experiment (NGEE-Arctic) site, Barrow Environmental Observatory (BEO), Barrow, Alaska (Fig. 1). We further complete the ModEx cycle by discussing how future data needs can reduce system uncertainty and refine our understanding of process behavior
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