Focus has recently increased on high-latitude climatic processes as awareness rose about the extreme sensitivity of the Arctic to climate change and its potential for major positive climate feedbacks. Modelling offers a powerful tool to assess the climatic impact of changes in the northern high-latitude regions, as well as to quantify the range of uncertainty stemming from the limits of our knowledge and representation of these environments. My PhD project, dedicated to the improvement of a land-surface model for high-latitude regions and the evaluation of its performances, tackles therefore an issue of concern both for science and society. Soil freezing is a major physical process of boreal regions, with climatic implications. Here, a parameterization of the hydrological effects of soil freezing is developed within the multi-layer hydrological scheme of the land-surface model ORCHIDEE, and its performance is evaluated against observations at different scales, including remotely-sensed data. Taking the hydrological impact of soil freezing into account improves our representation of soil moisture and river discharges over the pan-Arctic land-surface area. However, residual inaccuracies suggest that potential for improvement lies in the representation of temporary surface water reservoirs like floodplains, surface ponding, and, possibly, the introduction of a subgrid variability in soil freezing. Hydrological modelling at high latitudes would also benefit from a specific treatment of mountainous areas and a revision of soil textural input parameters to account for abundant coarse-grained soils in the High-Arctic. Concomitantly, the thermal parameterization of soil freezing in ORCHIDEE is revised and evaluated against field data: latent heat effects yield a reduction but no suppression of a model cold bias in winter soil temperatures, part of which is imputed to the coarse representation of snow in the model. A sensitivity study performed on the insulative properties of taiga vs. tundra snow over the pan-Arctic terrestrial domain confirms the thermal implications of snow and outlines its consequences for carbon cycling at high-latitudes, calling for an appropriate representation of snow-vegetation interactions. Snow is furthermore implicated in identified flaws of the modelled surface energy balance, the components of which are precisely compared with a one-year high quality dataset collected at an Arctic permafrost site in Svalbard. Inaccuracies are diagnosed to stem from the representation of albedo, surface roughness and liquid water percolation and phase change within the snowpack. These diverse
Representating high latitudes in a land-surface model : soil freezing and model evaluation