Problems Closing the Energy Balance over a Homogeneous Snow Cover during Midwinter

Abstract

pplication of the energy balance approach to estimate snowmelt inherently presumes that the external energy fluxes can be measured or modeled with sufficient accuracy to reliably estimate the internal energy changes and melt rate. However, owing to difficulties in directly measuring the internal energy content of the snow during melt periods, the ability to close the energy balance is rarely quantified. To address this, all of the external energy balance terms (sensible and latent heat fluxes, shortwave and longwave radiation fluxes, and the ground heat flux) were directly measured and compared to changes of the energy content within an extensive, homogeneous, snowpack of a level field near Saskatoon, Saskatchewan, Canada. The snow was observed to lose significant amounts of energy because of a persistent longwave radiation imbalance caused by low incoming fluxes during cold, clear-sky periods, while solar heating of the snow surface caused an increase in the outgoing fluxes. The sum of the measured turbulent heat fluxes, ground heat flux, and solar radiation fluxes were insufficient to offset these losses, however the snowpack temperatures were not observed to cool. It was concluded that an unmeasured exchange of sensible heat was occurring from the atmosphere to the snowpack. The exchange mechanism for this is not known but would appear to be consistent with the concept of a windless exchange as employed to close the energy balance in various snow models. The results suggest that caution should be exercised when using the energy balance method to determine changes in internal energy in cold snowpacks.