Ninth International Conference on Permafrost ... - IARC Research

Improving the Parameterization of Snow Processes to Model the Implications ofShrub-Tundra Expansion on Soil TemperaturesCecile MenardCentre for Ecology and Hydrology, Wallingford, UKRichard EsserySchool of GeoSciences, University of Edinburgh, UKDouglas ClarkCentre for Ecology and Hydrology, Wallingford, UKIntroductionField observations, satellite remote sensing, and modelssuggest that the recent warming of the Arctic has caused anincrease in shrub cover (Sturm et al. 2005, Jia et al. 2006,Tape et al. 2006). This change in vegetation structure isexpected to significantly affect snow distributions andinteractions between the land surface and the atmosphere,with consequences for the hydrology, ecology, carbon, andenergy balances of the region. Shrubs capture wind-blownsnow, increasing snow depths, and decreasing winter waterlosses through sublimation. The low thermal conductivityof snow insulates the soil, deepening the active layer andaffecting the permafrost regime. Thus, snow/permafrostinteractions will be at the core of feedback loops leading tofurther shrub expansion. For example, warmer winter soiltemperatures lead to increased microbial activity and henceto greater nutrient availability, which will further stimulateshrub growth (Chapin et al. 2005, Tape et al. 2006). Carboncycling will also be affected, although the environmentaleffects of greater shrub abundance are uncertain. Sturmet al. (2005) suggest that the Arctic may become a carbonsink because of increasing production of above-groundshrub biomass. On the other hand, thawing of permafrostis expected to liberate large amounts of carbon currentlysequestered in frozen organic soils (Solomon et al. 2007).Land surface models (LSMs) are required to calculateenergy and water fluxes between the land and the atmospherein global climate models (GCMs), but the representation ofcryospheric processes is generally crude in current LSMs.In this paper, two different snow schemes are tested offlineto assess the implications for soil processes of the predictednorthward expansion of shrub-tundra.MethodsField siteMeteorological measurements and soil temperatures wereobtained at two sites in the Wolf Creek Research Basin(60°36′N, 134°57′W), Yukon Territory, Canada (Pomeroy etal. 2004):1. An alpine tundra site (1615 m a.s.l.), characterized by0.01–0.3 m tall vegetation (willow, dwarf birch, grass, andlichen) and bare rock, within the widespread discontinuouspermafrost zone (Lewkowicz & Ednie 2004). Soiltemperatures were measured at 3 cm depth.2. A shrub tundra site (1250 m a.s.l.), with 0.4–3 m tallvegetation (willow, sparse white spruce, dwarf birch andgrass), within a sporadic discontinuous permafrost zone.Soil temperatures were measured at 11 cm depth.Data for the one-year period starting on 1 August 1998 areused here. Air temperatures at both sites were similar fromNovember to February, but the alpine site was generally2°C colder than the shrub tundra site for the rest of the year.Because the alpine site is more exposed, the annual averagewind speed was greater by 2.3 ms -1 . Although the snowfall isalmost the same at the two sites, increased wind ablation andreduced trapping by shrubs give lower snow depths at thealpine site. The importance of snow insulation is reflectedin differences in soil temperatures; the greatest differencesoccurred in March, for which average soil temperatureswere -9°C at the alpine site but -4°C at the shrub tundrasite. Summer differences were smaller, and down to 0.4°Cin June and August (7°C at the alpine site and 6.6°C at theshrub tundra site).Model parameterizationSnow depths and soil temperatures at the two sites havebeen simulated with the JULES (Joint UK Land EnvironmentSimulator; Blyth et al. 2006) LSM using two different snowschemes. Simulations were first performed with the presentversion of JULES, which represents snow as a compositewith the top soil layer. The insulating properties of snoware incorporated by adjusting the thermal conductivity andthickness of the layer (Cox et al. 1999). The temperature ofthis top layer is taken to be at the layer midpoint, whethersnow is present or not, hence it may reflect the soil or thesnow temperature depending on snow depth. A new snowmodel using a multilayer representation of snow has nowbeen developed for JULES. Simple representations of snowcompaction, and retention and refreezing of liquid water, allof which were neglected in the original model, have beenincluded.Model resultsFigures 1 and 2 compare simulated snow depths and soiltemperatures with measurements. Simulations with theoriginal snow model underestimate winter soil temperatures,particularly for the alpine site where the measurement depthlies within the model’s composite snow-soil layer, but alsofor the shrub tundra site where the measurements are belowthis layer. The greater snow insulation simulated by the newmodel greatly improves the soil temperature simulations,209