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

Abstract

Application 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.

Field-scale water balance closure in seasonally frozen conditions

“…Twine et al, 2000;Barr et al, 2012) on a daily basis, which increased the measured seasonal evaporation fluxes by 39 and 35 % in 2013 and 2014, respectively. Biases for the other seasons were not corrected since the evaporation fluxes over the frozen ground surface were very small, and the turbulent heat fluxes are much more uncertain over snow (Helgason and Pomeroy, 2012). Precipitation (mm) was measured by a Geonor T200-B weighing gauge.…”

Surface-layer turbulence, energy balance and links to atmospheric circulations over a mountain glacier in the French Alps

“…In complex terrain, several studies have shown that atmospheric flow interactions with the orography can generate such features and they may reach the surface and enhance turbulent mixing. In such a case, mixing may not scale with local flow characteristics (Andreas, 1987;Smeets et al, 1999;Poulos et al, 2007;Helgason and Pomeroy, 2012). Near the surface, these eddies would be horizontally deformed due to its blocking effect and to the large shear in the surface layer (Högström et al, 2002): they would disturb the surface layer by inducing large horizontal velocity fluctuations, while vertical velocity would be less influenced.…”

“…Helgason and Pomeroy, 2012;Conway and Cullen, 2013). Although numerical simulations have shown the profile method was reliable in estimating the surface fluxes in the presence of flux divergence below a wind-speed maximum (Denby and Greuell, 2000), the effects of katabatic oscillations and outer-layer interactions remain poorly documented over mountain glaciers (Smeets et al, 1999(Smeets et al, , 2000.…”

A study of turbulent fluxes and their measurement errors for different wind regimes over the tropical Zongo Glacier (16° S) during the dry season

“…Such an independent estimation may help in understanding the origin of biases evidenced herein. Since ice or snow temperature measurements are challenging due to solar radiation contamination heating of the sensors below the surface (Helgason and Pomeroy, 2012), the potentially large conduction flux below the surface remained unknown during the 2007 campaign on Zongo Glacier. This prevented us from conducting an energy balance closure check.…”