CryoGrid

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

The distribution and condition of permafrost, is directly linked to the snow and vegetation cover, topography, water bodies, the geothermal heat flux and the air temperature. Therefore, the prediction of permafrost sensitivity to a warming climate is highly complex with many uncertainties (Boike et al., 2013). Moreover, climate change has a direct impact on the water, heat and nutrient budget of boreal ecosystems (Pearson et al., 2013). The boreal forests of Siberia are expected to expand to the North under warming climatic conditions (Holtmeier and Broll, 2005; Kruse et al., 2016). Extensive ecosystem changes such as a change in composition, density and distribution of Arctic vegetation are already reported all over the Arctic and Sub-Arctic (Goetz et al., 2011; Pearson et al., 2013). These changes and transitions trigger multiple feedback mechanisms of different magnitudes (Chapin, 2005; Pearson et al., 2013). The numerous interactions and inter-dependencies of the atmosphere, the larch dominated vegetation, and the permafrost ground in Eastern Siberia make predictions and interpretations of the current and future state highly challenging. In a multi-proxy and multidisciplinary approach the climate-vegetation-soil interactions in permafrost-dominated areas will be studied over a large transect of Eastern Siberia.

The hypothesis focused on is that larch stands modify the local hydrological and thermal conditions in a way that protects permafrost from further degradation. To prove or disprove this hypothesis further understanding of the heat and water exchange between the atmosphere, larch forests and the permafrost is needed. As a first step, (i) the installation of two microclimate stations and further soil temperature sensors, as well as the sampling and describing of the forest types across a transect in North-Eastern Siberia will provide data to study and understand the heat and water transfer processes in larch dominated boreal forest areas. (ii) Secondly, the coupling of the permafrost model CryoGrid 3 and the larix vegetation simulator LAVESI will be implemented with model runs based on different climate scenarios to understand future and past developments of larch forests at different locations in Siberia and predict changes to the biome and the permafrost extent as well as tree line movements. (iii) Thirdly, the coupled model will be used to provide insight into future developments in terms of the changing fire cycle or flooding and permafrost thawing and its effects on the biome.

Therefore, the PhD project aims at understanding and answering the following questions in three work packages as listed below:

  1. What are the interactions between the larch forests and the underlying permafrost ground? And how do the larch trees influence or even change the soil structure, depth of the permafrost table or the fire cycle?
  2. Which parameters of the permafrost land surface model CryoGrid 3 have to be adapted to successfully simulate larch forests in North-Eastern Siberia? How can the LAVESI model be coupled to CryoGrid 3 based on these parameters? How will the forests change under differing future climate scenarios?
  3. What are possible future scenarios for the larch forests under the influence of i.e. changing fire cycles or flooding?

Literature:

Boike, J. et al. (2013). “Baseline characteristics of climate, permafrost and land cover from a new permafrost observatory in the Lena River Delta, Siberia (1998-2011)”. Biogeosciences, 10(3), pp. 2105–2128

Chapin, F.S. et al (2005). “Role of Land-Surface Changes in Arctic Summer Warming”. Science 299(310), pp. 657– 660. url: http://www.ncbi.nlm.nih.gov/pubmed/12637742

Goetz, Scott J. et al. (2011). “Recent Changes in Arctic Vegetation: Satellite Observations and Simulation Model Predictions”. In: Eurasian Arctic Land Cover and Land Use in a Changing Climate. Ed. by G. Gutman and A. Reissell. Springer Netherlands: Dordrecht, pp. 9–36. url: https://link.springer.com/chapter/10.1007/978-90-481-9118-5_2

Holtmeier, Friedrich Karl and Gabriele Broll (2005). “Sensitivity and response of northern hemisphere altitudinal and polar treelines to environmental change at landscape and local scales”. Global Ecology and Biogeography 14(5), pp. 395–410

Kruse, Stefan, Mareike Wieczorek, Florian Jeltsch, and Ulrike Herzschuh (2016). “Treeline dynamics in Siberia under changing climates as inferred from an individual-based model for Larix”. Ecological Modelling 338, pp. 101–121. url: http://dx.doi.org/10.1016/j.ecolmodel.2016.08.003

Pearson, Richard G., Steven J. Phillips, Michael M. Loranty, Pieter S.A. Beck, Theodoros Damoulas, Sarah J. Knight, and Scott J. Goetz (2013). “Shifts in Arctic vegetation and associated feed- backs under climate change”. Nature Climate Change 3(7), pp. 673–677. url: http://dx.doi.org/10.1038/nclimate1858