Abstract
In the majority of glacier surface energy balance studies, parameterisation rather than direct measurement is used to estimate one or more of the individual heat fluxes, with others, such as the rain and ground heat fluxes, often deemed negligible. Turbulent fluxes of sensible and latent heat are commonly parameterised using the bulk aerodynamic technique. This method was developed for horizontal, uniform surfaces rather than sloped, inhomogeneous glacier terrain, and significant uncertainty remains regarding the selection of appropriate roughness length values, and the validity of the atmospheric stability functions employed. A customised weather station, designed to measure all relevant heat fluxes, was installed on an alpine glacier over the 2014 melt season. Eddy covariance techniques were used to observe the turbulent heat fluxes, and to calculate site-specific roughness values. The obtained dataset was used to drive a point ablation model, and to evaluate the most commonly used bulk methods and roughness length schemes in the literature. Modelled ablation showed good agreement with observed rates at seasonal, daily, and sub-daily timescales, effectively closing the surface energy balance, and giving a high level of confidence in the flux observation method. Net radiation was the dominant contributor to melt energy over the season (65.2%), followed by the sensible heat flux (29.7%), while the rain heat flux was observed to be a significant contributor on daily timescales during periods of persistent heavy rain (up to 20% day-1). Momentum roughness lengths observed for the study surface (snow: 10-3.8 m; ice: 10-2.2 m) showed general agreement with previous findings, while the scalar values (temperature: 10-4.6 m; water vapour: 10-6 m) differed significantly from those for momentum, disagreeing with the assumption of equal roughness lengths. Of the three bulk method stability schemes tested, the functions based on the Monin-Obukhov length returned mean daily flux values closest to those observed, but displayed poor performance on sub-daily timescales, and periods of substantial flux overestimation.
Highlights
The observed large-scale retreat of mountain glaciers is a primary indicator of current climate change, and the loss of mass from these systems may have significant future impacts on sea level rise, freshwater supply, hydroelectric power production, and aquatic habitats (IPCC, 2013)
Precipitation was confined to three frontal systems and sporadic convective showers, with a trace amount of snowfall on July 24th (DOY 205)
Little change is observed in the temperature profile over time, with a slight warming trend commencing a few days after the removal of the snow layer (DOY 196)
Summary
The observed large-scale retreat of mountain glaciers is a primary indicator of current climate change, and the loss of mass from these systems may have significant future impacts on sea level rise, freshwater supply, hydroelectric power production, and aquatic habitats (IPCC, 2013). Utilizing a surface energy balance (SEB) model can reduce the reliance on local, empirically-derived melt relationships used in other modeling techniques (e.g., temperature index models) which can vary spatially and temporally, requiring calibration for each glacier, and limiting transferability. In situ measurements of the complete SEB are rare on glacier surfaces, necessitating the use of parameterization methods for one or more of the energy fluxes in the majority of studies. The parameterization methods commonly used to estimate these fluxes contain substantial uncertainties, with a lack of observed data to evaluate their performance
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