CryoGrid is a one-dimensional land surface model dedicated to simulate ground temperatures in permafrost environments.
The model calculates the surface energy balance in order  to  represent energy transfer processes between the atmosphere and the ground. These processes include the radiation balance, the exchange of sensible heat, as well as evaporation and condensation. For a realistic representation of the thermal dynamics of the ground, the model includes processes such as the phase change of soil water and an insulating snow cover during winter.
Further developments of CryoGrid already enable the model to represent processes such as ground subsidence and the formation of thermokarst due to melting of excess ground ice. The implemented lake module (FLAKE) allows CryoGrid also to simulate heat transfer processes of tundra landscapes that are densely populated by lakes and ponds.

Simulation Results

The animation shows the seasonal variations in soil temperature at various depths - under a thermokarst lake and in the non-lake-covered tundra environment. The simulation starts in 1950 and runs up to the year 2100 assuming strong climate warming (RCP8.5 scenario). (The animation shows in extracts the first and last ten years, as well as the decade from 2017 to 2027, in which the ground under the lake is no longer freezing completely in winter for the first time.) Red shades illustrate thawed areas whereas blue shades show frozen areas in the ground and the lake respectively.
Underneath thermokarst lakes, the ground heats up much stronger than in the surroundings. As a result, the thawing front reaches significantly deeper soil layers. This is especially to be expected in warmer climates, when the lake does not longer freezes to the ground. and increases in depth due to the melt of excess ground ice (blue dashed line). Under these circumstances permanently thawed areas (so-called taliks) start to develop.
The different soil temperature profiles are the result of the surface energy balance, which is controlled by the net radiation flux and the sensible and latent heat fluxes (colored arrows). In addition, lateral heat flow in the subsurface leads to a partial convergence of the temperature profiles.

CryoGrid/LARIX - Heat and water exchange processes in larch dominated permafrost ecosystems

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. The forests are usually considered to efficiently insulate the underlying permafrost. The canopy exerts shading by reflecting and absorbing most of the downward solar radiation, changes the surface albedo, decreases the soil moisture by intercepting precipitation and increasing evapotranspiration and promotes the accumulation of an organic surface layer which further insulates the soil from the atmosphere.

Changing climatic conditions can promote an increasing active-layer depth or trigger the partial disappearance of the near-surface permafrost. Extensive ecosystem shifts such as a change in composition, density or the distribution of vegetation and resulting changes to the below- and within-canopy radiation fluxes have already been reported. These changes to the vegetation – permafrost dynamics can have a potentially high impact on the numerous feedback mechanisms between the two ecosystem components. Increased soil carbon release from thawing permafrost through the delivery of soil organic matter to the active carbon cycle is modified by vegetation changes, which can compensate for carbon losses due to an increased CO2 uptake or even further accelerate total carbon loss.

The aim of this interdisciplinary project is to understand how the interactions between the vegetation, permafrost and the atmosphere stabilize the forests and the underlying permafrost. A one-dimensional land surface model (CryoGrid) is adapted for the application in vegetated areas by coupling a multilayer canopy model (CLM-ml v0; Community Land Model) and is used to reproduce the energy transfer and thermal regime at a study site in mixed boreal forest in eastern Siberia.

In Stuenzi et al. (2021) we find that the forests exert a strong control on the thermal state of permafrost through changing the radiation balance and snow cover phenology.  In a further step, we are investigating how changes in forest cover impact the feedback processes in permafrost underlain boreal forests across a wide transect in eastern Siberia. We there-by deliver important insights into the range of possible changes to the permafrost condition that can be expected following landscape changes such as deforestation through fires or land use changes, afforestation in currently unforested grasslands or the climate-induced densification of forested areas. We find that varying density and tree composition have  a  significant  effect  on  the  thermal  and  hydrological  state  of  permafrost  and therefore, play an important role in the fate of this ecosystem, the forests function as a carbon sink and permafrost persistence, which may induce feedback mechanisms such as swamping, droughts, fires or forest loss.

For further reference see our open-access publication in Biogeosciences:

Stuenzi, S. M., Boike, J., Cable, W., Herzschuh, U., Kruse, S., Pestryakova, L. A., Schneider von Deimling, T., Westermann, S., Zakharov, E. S., and Langer, M.: Variability of the surface energy balance in permafrost-underlain boreal forest, Biogeosciences, 18, 343–365,, 2021