Abstract
Abstract. A new, two-layer canopy module with thermal inertia as part of the detailed snow model SNOWPACK (version 3.2.1) is presented and evaluated. As a by-product of these new developments, an exhaustive description of the canopy module of the SNOWPACK model is provided, thereby filling a gap in the existing literature. In its current form, the two-layer canopy module is suited for evergreen needleleaf forest, with or without snow cover. It is designed to reproduce the difference in thermal response between leafy and woody canopy elements, and their impact on the underlying snowpack or ground surface energy balance. Given the number of processes resolved, the SNOWPACK model with its enhanced canopy module constitutes a sophisticated physics-based modeling chain of the continuum going from atmosphere to soil through the canopy and snow. Comparisons of modeled sub-canopy thermal radiation to stand-scale observations at an Alpine site (Alptal, Switzerland) demonstrate improvements induced by the new canopy module. Both thermal heat mass and the two-layer canopy formulation contribute to reduce the daily amplitude of the modeled canopy temperature signal, in agreement with observations. Particularly striking is the attenuation of the nighttime drop in canopy temperature, which was a key model bias. We specifically show that a single-layered canopy model is unable to produce this limited temperature drop correctly. The impact of the new parameterizations on the modeled dynamics of the sub-canopy snowpack is analyzed. The new canopy module yields consistent results but the frequent occurrence of mixed-precipitation events at Alptal prevents a conclusive assessment of model performance against snow data. The new model is also successfully tested without specific tuning against measured tree temperature and biomass heat-storage fluxes at the boreal site of Norunda (Sweden). This provides an independent assessment of its physical consistency and stresses the robustness and transferability of the chosen parameterizations. The SNOWPACK code including the new canopy module, is available under Gnu General Public License (GPL) license and upon creation of an account at https://models.slf.ch/.
Highlights
In the Northern Hemisphere, around 19 % of the annually snow-covered areas are forested (Rutter et al, 2009)
It is designed to reproduce the difference in thermal response between leafy and woody canopy elements, and their impact on the underlying snowpack or ground surface energy balance
We develop a two-layer canopy representation in the aforementioned SNOWPACK model, which proposes a very detailed physical and microphysical representation of the snowpack
Summary
In the Northern Hemisphere, around 19 % of the annually snow-covered areas are forested (Rutter et al, 2009). The snowpack insulates the underlying soil from winter cold air temperature, with implications for the ecosystem in terms of vegetation cover and dynamics (Rasmus et al, 2011; Grippa et al, 2005), litter decomposition (e.g., Saccone et al, 2013), or carbon cycling (e.g., Kelley et al, 1968). The representation of this insulation is one of the critical uncertainties of the modeling of the global soil carbon cycle and its evolution in permafrost environments (Lawrence and Slater, 2010; Gouttevin et al, 2012). The northwards migration of shrubs observed in the last decades at high latitudes (e.g., ACIA, 2005) indicates that snow–forest interactions are to become more and more a concern for climate modeling in the context of global warming
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