Ben W. Brock

An enhanced temperature-index glacier melt model including the shortwave radiation balance : development and testing for Haut Glacier d'Arolla, Switzerland

An enhanced temperature-index glacier melt model, incorporating incoming shortwave radiation and albedo, is presented. The model is an attempt to combine the high temporal resolution and accuracy of physically based melt models with the lower data requirements and computational simplicity of empirical melt models, represented by the ‘degree-day’ method and its variants. The model is run with both measured and modelled radiation data, to test its applicability to glaciers with differing data availability. Five automatic weather stations were established on Haut Glacier d’Arolla, Switzerland, between May and September 2001. Reference surface melt rates were calculated using a physically based energy-balance melt model. The performance of the enhanced temperature-index model was tested at each of the four validation stations by comparing predicted hourly melt rates with reference melt rates. Predictions made with three other temperature-index models were evaluated in the same way for comparison. The enhanced temperature-index model offers significant improvements over the other temperature-index models, and accounts for 90–95% of the variation in the reference melt rate. The improvement is lower, but still significant, when the model is forced by modelled shortwave radiation data, thus offering a better alternative to existing models that require only temperature data input.

Meteorology and surface energy fluxes in the 2005–2007 ablation seasons at the Miage debris-covered glacier, Mont Blanc Massif, Italian Alps

monitored at 25 points with debris thickness of 0.04–0.55 m, spread over 5 km 2 of the glacier. The radiative fluxes were directly measured, and near‐closure of the surface energy balance is achieved, providing support for the bulk aerodynamic calculation of the turbulent fluxes. Surface‐layer meteorology and energy fluxes are dominated by the pattern of incoming solar radiation which heats the debris, driving strong convection. Mean measured subdebris ice melt rates are 6–33 mm d �1 , and mean debris thermal conductivity is 0.96 W m �1 K �1 , displaying a weak positive relationship with debris thickness. Mean seasonal values of the net shortwave, net longwave, and debris heat fluxes show little variation between years, despite contrasting meteorological conditions, while the turbulent latent (evaporative) heat flux was more than twice as large in the wet summer of 2007 compared with 2005. The increase in energy output from the debris surface in response to increasing surface temperature means that subdebris ice melt rates are fairly insensitive to atmospheric temperature variations in contrast to debris‐free glaciers. Improved knowledge of spatial patterns of debris thickness distribution and 2 m air temperature, and the controls on evaporation of rainwater from the surface, are needed for distributed physically based melt modeling of debris‐covered glaciers.

Modelling seasonal and spatial variations in the surface energy balance of Haut Glacier d’Arolla, Switzerland

Abstract The impact of spatial and temporal variations in the surface albedo and aerodynamic roughness length on the surface energy balance of Haut Glacier d’Arolla, Switzerland, was examined using a semi-distributed surface energy-balance model (Arnold and others, 1996). The model was updated to incorporate the glacier-wide effects of albedo and aerodynamic roughness-length variations using parameterizations following Brock (1997). After the model’s performance was validated, the glacier-wide patterns of the net shortwave, turbulent and melt energy fluxes were examined on four days, representative of surface conditions in late May, June July and August. In the model, meteorological conditions were held constant on each day in order that the impact of albedo and aerodynamic roughness-length variations could be assessed independently. A late-summer snowfall event was also simulated. Albedo and aerodynamic roughness-length variations, particularly those associated with the migration of the transient snowline and the decay of the winter snowpack, were found to exert a strong influence on the magnitude of the surface energy fluxes The importance of meteorological conditions in suppressing the surface energy fluxes and melt rate following a fresh snowfall was highlighted

Quantified time scale for glacial valley cross-profile evolution in alpine mountains

Space is used as a proxy for time in determining the role of glaciation in shaping valley cross profiles in the Two Thumb Range (Southern Alps), New Zealand. The range is undergoing rapid tectonic transport and uplift as it is advected toward the Alpine fault. Repeated cycles of glacial erosion during the Quaternary have fashioned U-shaped valleys in the north of the range, close to the Main Divide, while valleys in the less glaciated south of the range have rounded divides and convex fluvial cross profiles. Tectonic transport and uplift rates, coupled with an offshore oxygen isotope (δ18O) record and glacial- geological reconstructions, allow time constraints to be applied to valley development along the range. Valley width and depth measurements and power-law equations were used to quantify the shape of cross-profile transects in 37 valleys. Valleys evolved into a characteristic U-shaped cross profile formed over ≥400 k.y. of glacial occupancy. This, regardless of other models of glacial flattening of valley long profiles, suggests that alpine glaciation excavates greater volumes of rock, indicating that glaciers are a more effective erosional agent than fluvial activity in alpine mountain belts.

Rock Strength and Development of Glacial Valley Morphology in the Scottish Highlands and Northwest Iceland

Abstract Using data from the Scottish Highlands and northwest Iceland, the present study indicates that bedrock strength properties are an important control on the morphology of glacial valleys. Results indicate that on closely jointed metasedimentary bedrock of low rock mass strength, broad U‐shaped valleys are developed, whilst steeper sided, narrower cross‐profiles have been developed on igneous bedrock of high rock mass strength. Findings suggest it is the interplay of the mass strength of the subglacial bedrock and the dynamic properties of the eroding glacier that control valley morphological development. The implication is that realistic models of topographic development beneath ice sheets need to consider the rock mass strength properties of the eroded bedrock as well as the glaciological variables.