Variability of the Surface Energy Balance in Permafrost Underlain Boreal Forest

Abstract

Abstract. Boreal forests in permafrost regions make up around one-third of the global forest cover and are an essential component of regional and global climate patterns. Further, climatic change can trigger extensive ecosystem shifts such as the partial disappearance of near surface permafrost or changes to the vegetation structure and composition. Therefore, our aim is to understand how the interactions between the vegetation, permafrost, and the atmosphere stabilize the forests and the underlying permafrost. Existing model set-ups are often static or are not able to capture important processes such as the vertical structure or the leaf physiological properties. There is a need for a physically based model with a robust radiative transfer scheme through the canopy. A one-dimensional land surface model (CryoGrid) is adapted for the application in vegetated areas by coupling a multilayer canopy model (CLM-ml v0) and is used to reproduce the energy transfer and thermal regime at a study site (N 63.18946, E 118.19596) in mixed boreal forest in Eastern Siberia. We have in-situ soil temperature and radiation measurements, to evaluate the model and demonstrate the capabilities of a coupled multilayer forest-permafrost model to investigate the vertical exchange of radiation, heat, and water. We find that the forests exert a strong control on the thermal state of permafrost through changing the radiation balance and snow cover phenology. The forest cover alters the surface energy balance by inhibiting over 90 % of the solar radiation and suppressing turbulent heat fluxes. Additionally, our simulations reveal a surplus in longwave radiation trapped below the canopy, similar to a greenhouse, which leads to a comparable magnitude in storage heat flux to that simulated at the grassland site. Further, the end of season snow cover is three times greater at the forest site and the onset of the snow melting processes are delayed.

Comment on gmd-2021-304

“…As fires become more intense and frequent under global warming, spruce or other species may become dominant rather than shade-intolerant larch species. In the currently naturally deciduous, larch covered areas, evergreen taxa may invade and change the heat fluxes and energy balance with the threat of entering a positive feedback loop such as a deepening of the ALT (Bonan, 2008, Stocker et al, 2013, Stuenzi et al, 2021b.…”

“…Parametersdefining the canopy, snow, and soil properties were set according to literature values, model documentation, and own measurements (seeStuenzi et al 2021b for constants and multilayer canopy parameter choices). Table2summarizes the parameter choices for the three different sites.…”

Comment on gmd-2021-304

“…Here, we aim at creating a model system that can accurately assess detailed thermal and hydrological fluxes between permafrost and forest cover as recently developed by Stuenzi et al (2021aStuenzi et al ( , 2021b. It will include a dynamic vegetation model, which has a full life-cycle to allow intraspecies and interspecies interactions at all stages (seed-seedling-mature tree) leading to nonlinear behaviour of population dynamics as well as resolving a 3-dimensional landscape that is available for Siberian treeline areas developed by Kruse et al (2016Kruse et al ( , 2019aKruse et al ( , 2019b.…”

“…In combination with CryoGrid the canopy model replaces the standard surface energy balance scheme while soil state variables are passed back to the forest module. FollowingStuenzi et al (2021b), a realistic canopy structure is simulated by allowing fractional compositions of deciduous and evergreen taxa within a simulated forest stand. This entire model setup has previously been extensively validated for different study sites throughout our study region, including Nyurba (63.08°N, 117.99°E), Spasskaya (62.14°N, 129.37°E), and Ilirney (67.40°N, 168.37°E)(Stuenzi et al 2021a and2021b).…”

“…FollowingStuenzi et al (2021b), a realistic canopy structure is simulated by allowing fractional compositions of deciduous and evergreen taxa within a simulated forest stand. This entire model setup has previously been extensively validated for different study sites throughout our study region, including Nyurba (63.08°N, 117.99°E), Spasskaya (62.14°N, 129.37°E), and Ilirney (67.40°N, 168.37°E)(Stuenzi et al 2021a and2021b). Validation exercises were carried out based on measured and modelled ground surface temperature (GST), activelayer thickness (ALT), soil moisture, bowen ratio, and short-and longwave radiation below and above the canopy.…”