The Verification and Significance of Three Approaches to Longitudinal Stresses in High–resolution Models of Glacier Flow

With the purpose of improving the ice physics underpinning time–dependent glacier flowline models, three independent approaches for solving longitudinal stresses in glaciers are discussed and verified by application to Haut Glacier d'Arolla. To highlight any shortcomings, the reduced and much utilised driving stress approximation is also applied and compared. Modelled velocity patterns using the three full stress schemes exhibit consistency with one another and good coincidence with observed velocities for the 1991 summer melt season. Furthermore, these stress patterns indicate that longitudinal stresses are significant and of a similar order of magnitude as the basal shear stress components. However, the driving stress approximation yields erratic fluctuations in the stress and velocity fields which are neither realistic in terms of mass continuity nor agree with observations. Basal decoupling experiments indicate a complex relationship between basal velocity and englacial stresses with considerable dampening of any basal perturbation occurring as it is dissipated towards the surface and transferred throughout the ice mass. The driving stress approximation fails to account at all for any such coupling. Experiments to identify the length scale over which longitudinal effects operate indicate that they are significant even up to 10 ice thicknesses. The implication here is that longitudinal stresses play a significant role in determining glacier dynamics on length scales up to at least 2 km and that the predictive power of models of glacier flow based purely on the driving stress approximation is therefore subject to significant limitations. Inclusion of longitudinal stresses overcomes one of the main limitations imposed on such models and, given the potential ease of incorporation of the schemes described here, this deficiency may readily be resolved.     

Melt–water Accumulation on the Surface of the Greenland Ice Sheet: Effect on Albedo and Mass Balance

Satellite–derived albedo maps of the western part of the Greenland ice sheet (between 64.5 and 70.5 ^∘ N) reveal a north–south extending zone with relatively low albedos at some distance from the ice margin. In the literature it has been hypothesized that this "dark zone" is due to a local maximum in melt–water accumulation on the ice–covered surface. A plausible explanation for this maximum in melt–water accumulation is thatrelative to the situation within the "dark zone", melt–water accumulation is reduced at higher elevations by a smaller melt–water production rate whereas runoff occurs more easily at lower elevations where slopes are generally steeper. For the present paper AVHRR images from eight years (1990–1997) were analysed. The following indications confirming the "melt–water accumulation hypothesis" were found: (1) there is a significant correlation between the annual mean albedo lowering within the "dark zone" and the annual amount of melt as inferred from local mass–balance measurements; and (2) within each summer season the albedo lowering within the "dark zone" seems to respond to the melt–water production rate as inferred from local temperature measurements. The effect of melt–water accumulation on the albedo implies a positive feedback between the albedo and the amount of melt. It is estimated that approximately 40% of the interannual mass–balance variations in the "dark zone" are due to this feedback.     

An analysis of short-term albedo variations at Haut Glacier d'Arolla, Switzerland

Abstract An analysis of ten‐minute albedo variations, recorded on Haut Glacier d'Arolla, Switzerland, over an 11 day period in the 1999 ablation season is presented. Most of the short‐term (<1 day) albedo variability is caused by variations in cloud cover, while solar zenith angle variations in the range 25° to 75° are of minor importance, probably due to the predominantly cloudy conditions during the measurement period. A new method to calculate albedo variation as a function of cloud cover is developed. Short‐term albedo variations are expressed by the ratio of the measured albedo to the daily albedo ‘minimum’, defined as the albedo under cloud‐free conditions when the solar zenith angle is <50°. Variations in cloud cover are quantified by the ratio of the measured incoming shortwave radiation flux to the theoretical direct‐beam shortwave radiation flux. The resulting relationships are successful, explaining 83% and 87–90% of short‐term albedo variation on snow and ice respectively, and may be incorporated into albedo parameterizations already used in numerical energy balance melt models, without the need for additional data. Simulations with a glacier energy balance model suggest that melt rates are overestimated by between 1 and 3 mm water equivalent per day if a correction is not made for the increase in albedo under cloudy conditions. Other causes of albedo variation are identified and evidence is found for the removal of fine debris from the glacier surface by intense rainfall, leading to an albedo increase. The implications for energy balance models and satellite‐derived albedo measurements are discussed.     

Analysis on factors of glacier velocity: A case study of the Qiyi Glacier, Qilian Mountains

Glacier shape factors (area, length, and thickness), climatic factors (annual temperature and precipitation), mass balance, and other influence factors, of the Qiyi glacier velocity and their intensity were analyzed with the application of the path analysis method during 1958–2007. Results indicate that glacier velocity was mainly influenced by glacier shape, followed by mass balance and climatic conditions. Among the influence factors, glacier area and thickness are most significant, and direct and indirect path coefficients are respectively 6.56, 4.71, 19.29 and 13.57. This research provides information for further understanding glacier velocity and its influencing factors.