Surface energy fluxes at three distinct surfaces in Canada's Southern Arctic: an examination of the performance of the Canadian Land Surface Scheme

The response of Arctic ecosystems to climate change may include a number of possible feedbacks, from local to global scales, as changes in surface characteristics affect radiation, energy and carbon budgets. In this study we examine the performance of Version 3.5 of the Canadian Land Surface Scheme (CLASS) in simulating energy fluxes for three different terrestrial arctic ecosystems: mesic mixed tundra, mesic shrub tundra, and sedge fen, located within the same watershed near Daring Lake, NWT (64° 52' N, 111° 34' W) in Canada's Southern Arctic ecozone. Simulations focus on the May – September period for two different years when eddy covariance flux measurements were available for all three field sites. This period includes a snow-covered period, snow melt, the growing season, and the onset of senescence with daily average air temperature varying from as low as -16 °C in early May to as high as 20 °C during the summer.

Field measurements show a number of substantial differences in surface energy budgets for the three terrestrial ecosystems related to variations in surface moisture. During the snow-free growing season, average net radiation was up to 1.3 MJ m^-2 d^-1 less for the drier sites, which had higher albedo and greater upwelling longwave radiation when compared to the fen. The proportion of net radiation partitioned into convective fluxes was also dependent upon moisture, ranging from nearly 80% at the drier mixed tundra to 65% at the fen. Although atmospheric heating (the sum of latent and sensible heat fluxes) showed only small variations among the three terrestrial ecosystems, energy partitioning among latent and sensible heat fluxes varied considerably with daytime Bowen ratios varying from 0.57 (fen) to as high as 1.45 (mixed tundra). Shifts in energy partitioning were also observed between years, particularly at the drier sites, as the second study year was wetter and cooler than the first.

Terrestrial arctic ecosystems pose a number of challenges to land surface schemes designed to remain efficient enough to couple with General Circulation Models. For example in CLASS, there are no arctic-specific plant functional groups such that the prostrate ericaceous shrubs, herbs and sedges must be designated as needleleaf trees, broadleaf trees, or grasses. There is also no explicit representation of mosses. Sphagnum spp. occur as a nearly continuous ground cover at the sedge fen site and have a particular influence on latent heat fluxes by maintaining a moist surface through capillary rise while having no stomatal control over evapotranspiration rates. The number of soil layers has traditionally been limited to three, which imposes limitations on the ability to simulate accurately, permafrost and the annual freeze-thaw cycle. This study will assess the effect of increasing the number of soil layers, which became user definable in CLASS version 3.2.

The simulations are forced by observed meteorological data at 30 min time steps to produce surface energy fluxes to address the following questions: Which of the three sites is best represented by CLASS and which provides the biggest challenge for CLASS? Are these results consistent between years? Exploration of these questions will help us to evaluate the current applicability of CLASS for assessing land-atmosphere interactions in the arctic and will identify which new algorithms or parameterizations would be most likely to offer significant improvements.