Observing the Heterogeneity of Snow-Atmosphere Interactions over Wheat Stubble and Patchy Snowcover during Melt

The rate of snowmelt is governed by the surface energy balance. Prediction of melt rates can be especially challenging in areas of patchy snow cover where boundary layers are not fully developed. Most snow energy balance models only consider vertical exchange and ignore the significant complexity associated with dynamic heterogeneous surfaces. Previous studies considering energy advection from snow free to snow-covered areas during snowmelt have focused exclusively on advection of sensible heat with varying degrees of complexity and success. The role of latent heat advection from ponded meltwater or wet soils to snow is ignored. An intensive field campaign in March 2015 contrasted the energy balance of a melting prairie snowcover with short (15 cm) and tall (40 cm) wheat stubble treatments in Saskatchewan, Canada. Near surface eddy correlation observations quantified the bulk vertical turbulent fluxes. This, with high-resolution snowcovered area maps, based upon unmanned aerial vehicle imagery, allowed the disaggregation of the turbulent fluxes to their respective sources with two-dimensional footprint modeling. The contrasting stubble heights influenced the bulk turbulent terms and are related to differences in surface roughness and temperature. A portable instrument array (32 fine wire thermocouples and 12 gas intakes tested by a common LI-6262 gas analyzer) was deployed over a variety of surface transitions to sample the heterogeneity of two-dimensional near surface (<2m) profiles of air temperature and water vapour. The profiles confirmed the classic conceptual model of unstable profiles over snow-free surfaces and stable profiles over snowcover surfaces and displayed internal boundary layer development downwind of snow transitions. The water vapor profiles provide indirect evidence for latent heat advection to a snowpack. The latent heat advection flux was related to the fetch length, vegetation characteristics and presence of ponded meltwater. Challenges in observing heterogeneity of near surface snow-atmosphere interactions in this environment will be discussed with respect to the scale and variability of the processes, and uncertainties in the instrumentation.