Abstract
Abstract. Measurements of snowmelt and turbulent heat fluxes were made during the snowmelt periods of two years at two neighbouring tundra sites in the Yukon, one in a sheltered location with tall shrubs exposed above deep snow and the other in an exposed location with dwarf shrubs covered by shallow snow. The snow was about twice as deep in the valley as on the plateau at the end of each winter and melted out about 10 days later. The site with buried vegetation showed a transition from air-to-surface heat transfers to surface-to-air heat transfers as bare ground became exposed during snowmelt, but there were daytime transfers of heat from the surface to the air at the site with exposed vegetation even while snow remained on the ground. A model calculating separate energy balances for snow and exposed vegetation, driven with meteorological data from the sites, is found to be able to reproduce these behaviours. Averaged over 30-day periods the model gives about 8 Wm−2 more sensible heat flux to the atmosphere for the valley site than for the plateau site. Sensitivity of simulated fluxes to model parameters describing vegetation cover and density is investigated.
Highlights
Shrub tundra occupies the latitudes and altitudes above the coniferous forest treeline
The results show consistent differences in pre-melt snow accumulation between the sites, with more snow accumulating in the valley than on the plateau
Eddy covariance systems (Campbell Scientific CSAT3 sonic anemometers and KH2O or LiCor-7500 absorption hygrometers) were installed 3 m above the ground surface in the valley and 2.3 m on the plateau to provide measurements of sensible and latent heat fluxes between the surface and the atmosphere, calculated by finding 30-min covariances between vertical wind speed fluctuations and temperature or vapour density fluctuations sampled at 10 Hz
Summary
Tundra may be snow-covered for 6 to 8 months per year in sub-arctic regions (Pomeroy et al, 2006) and up to 9 months in the Arctic (McFadden et al, 2001). Previous field and modelling studies have identified the effect of shrubs on snow accumulation, snowmelt, hydrology and surface energy balance. McCartney et al (2006) showed that tall shrub canopies exert an inordinately large control on the timing and magnitude of streamflow discharge in tundra basins because of their enhancement of snow accumulation and rapid meltwater production which overwhelms the infiltration capacity of soils. Melt rates determine the degree of vegetation exposure and fractional snow covered area, which in turn strongly influence albedo. Energy-balance snowmelt models have been used to simulate shrub effects on surface energy and moisture fluxes during snowmelt, representations of important natural processes in such models are tentative and require evaluation by field measurements. The measurements are used to evaluate the performance of an energy balance model in simulating melt rates and energy fluxes to the atmosphere for the two sites, and the model is used to investigate how fluxes depend on shrub canopy structure
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