Measurements and modelling of snowmelt and turbulent heat fluxes over shrub tundra

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.

Modelled sensitivity of the snow regime to topography, shrub fraction and shrub height

“…Ground-based measurements, remote sensing and modelling studies in the Yukon Bewley et al, 2010), the Northwest Territories (Marsh et al, 2010), Alaska (Sturm et al, 2005a), Fennoscandia (Cohen et al, 2013) and the pan-Arctic (Loranty et al, 2011) have all shown that shrub branches exposed above snow decrease the surface albedo and increase the absorption of solar radiation. Shrubs can even absorb radiation while buried because short-wave radiation penetrates snow (Warren, 1982;Baker et al, 1991;Hardy et al, 1998).…”

“…Once exposed, branches can be 20 • C warmer than the surrounding snow , increasing turbulent heat and long-wave radiation fluxes from the exposed shrub canopy. Tall shrubs reduce the short-wave radiation reaching the snow surface by shading but increased long-wave radiation and sensible heat fluxes from the canopy to the snow can give higher melt rates for snow beneath shrubs than for unshaded snow (Bewley et al, 2010). For example, Sturm et al (2005b), Pomeroy et al (2006) and Marsh et al (2010) observed higher melt rates where shrubs were exposed above the snowpack than where shrubs were buried.…”

“…Bewley et al, 2007Bewley et al, , 2010Marsh et al, 2010;Ménard et al, 2012). However, one of the remaining difficulties in modelling shrub-tundra is how to represent sparse canopies and horizontal interactions between shrub and non-shrub surfaces.…”

“…However, this does not solve the problem for landscapes with exposed vegetation and patchy snow cover. Models that calculate a single energy balance for composite snowcovered and snow-free ground are known to fail to simulate late-lying snow patches (Marsh and Pomeroy, 1996;Bewley et al, 2010). This causes two issues in model performance: excessive melt rates and modelled sensible heat fluxes that are in the opposite direction to observations for patchy snow cover.…”

Micrometeorological processes driving snow ablation in an Alpine catchment

“…In recent years, extensive research into the main processes driving the spatial variability of snow water equivalent (SWE) has been carried out (Male and Granger, 1981;Pomeroy et al, 1998;Blöschl, 1999;Marks et al, 1999;Lehning et al, 2006;Bewley et al, 2010). Several studies focused on the relative importance of the energy contributed by solar radiation versus turbulent fluxes (Morris, 1989;Cline, 1997;.…”