Journal cover Journal topic
Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
Geosci. Model Dev., 8, 1493-1508, 2015
© Author(s) 2015. This work is distributed
under the Creative Commons Attribution 3.0 License.
Development and technical paper
21 May 2015
An improved representation of physical permafrost dynamics in the JULES land-surface model
S. Chadburn1, E. Burke2, R. Essery3, J. Boike4, M. Langer4,5, M. Heikenfeld4,6, P. Cox1, and P. Friedlingstein1 1Earth System Sciences, Laver Building, University of Exeter, North Park Road, Exeter EX4 4QE, UK
2Met Office Hadley Centre, Fitzroy Road, Exeter EX1 3PB, UK
3Grant Institute, The King's Buildings, James Hutton Road, Edinburgh EH9 3FE, UK
4Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI), 14473 Potsdam, Germany
5Laboratoire de Glaciologie et Géophysique de l'Environnement (LGGE) BP 96 38402 St Martin d'Hères CEDEX, France
6Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, UK
Abstract. It is important to correctly simulate permafrost in global climate models, since the stored carbon represents the source of a potentially important climate feedback. This carbon feedback depends on the physical state of the permafrost. We have therefore included improved physical permafrost processes in JULES (Joint UK Land Environment Simulator), which is the land-surface scheme used in the Hadley Centre climate models.

The thermal and hydraulic properties of the soil were modified to account for the presence of organic matter, and the insulating effects of a surface layer of moss were added, allowing for fractional moss cover. These processes are particularly relevant in permafrost zones. We also simulate a higher-resolution soil column and deeper soil, and include an additional thermal column at the base of the soil to represent bedrock. In addition, the snow scheme was improved to allow it to run with arbitrarily thin layers.

Point-site simulations at Samoylov Island, Siberia, show that the model is now able to simulate soil temperatures and thaw depth much closer to the observations. The root mean square error for the near-surface soil temperatures reduces by approximately 30%, and the active layer thickness is reduced from being over 1 m too deep to within 0.1 m of the observed active layer thickness. All of the model improvements contribute to improving the simulations, with organic matter having the single greatest impact. A new method is used to estimate active layer depth more accurately using the fraction of unfrozen water.

Soil hydrology and snow are investigated further by holding the soil moisture fixed and adjusting the parameters to make the soil moisture and snow density match better with observations. The root mean square error in near-surface soil temperatures is reduced by a further 20% as a result.

Citation: Chadburn, S., Burke, E., Essery, R., Boike, J., Langer, M., Heikenfeld, M., Cox, P., and Friedlingstein, P.: An improved representation of physical permafrost dynamics in the JULES land-surface model, Geosci. Model Dev., 8, 1493-1508, doi:10.5194/gmd-8-1493-2015, 2015.
Publications Copernicus
Short summary
Permafrost, ground that is frozen for 2 or more years, is found extensively in the Arctic. It stores large quantities of carbon, which may be released under climate warming, so it is important to include it in climate models. Here we improve the representation of permafrost in a climate model land-surface scheme, both in the numerical representation of soil and snow, and by adding the effects of organic soils and moss. Site simulations show significantly improved soil temperature and thaw depth.
Permafrost, ground that is frozen for 2 or more years, is found extensively in the Arctic. It...