Delphine Six

Spectral Bidirectional Reflectance of Antarctic Snow: Measurements and Parameterization

The bidirectional reflectance distribution function (BRDF) of snow was measured from a 32-m tower at Dome C, at latitude 75°S on the East Antarctic Plateau. These measurements were made at 96 solar zenith angles between 51° and 87° and cover wavelengths 350–2400 nm, with 3- to 30-nm resolution, over the full range of viewing geometry. The BRDF at 900 nm had previously been measured at the South Pole; the Dome C measurement at that wavelength is similar. At both locations the natural roughness of the snow surface causes the anisotropy of the BRDF to be less than that of flat snow. The inherent BRDF of the snow is nearly constant in the high-albedo part of the spectrum (350–900 nm), but the angular distribution of reflected radiance becomes more isotropic at the shorter wavelengths because of atmospheric Rayleigh scattering. Parameterizations were developed for the anisotropic reflectance factor using a small number of empirical orthogonal functions. Because the reflectance is more anisotropic at wavelengths at which ice is more absorptive, albedo rather than wavelength is used as a predictor in the near infrared. The parameterizations cover nearly all viewing angles and are applicable to the high parts of the Antarctic Plateau that have small surface roughness and, at viewing zenith angles less than 55°, elsewhere on the plateau, where larger surface roughness affects the BRDF at larger viewing angles. The root-mean-squared error of the parameterized reflectances is between 2% and 4% at wavelengths less than 1400 nm and between 5% and 8% at longer wavelengths.

Glacier thickening and decay analysis from 50 years of glaciological observations performed on Glacier d'Argentiere, Mont Blanc area, France

Abstract Numerous glaciological data have been obtained from measurements carried out on Glacier d’Argentière, Mont Blanc area, France, since the beginning of the 20th century. Moreover, data on annual mass balance, ice-flow velocity, thickness variation and length fluctuation have been obtained from yearly measurements performed since 1975. This dataset provides an excellent opportunity to analyze the relationships between surface mass balance and dynamic response over time periods during which net mass balance changed from positive to negative. Following a positive specific-net-balance period between 1960 and 1981, the ablation zone experienced a large increase in thickness and ice-flow velocities. Conversely, the highly negative specific-net-balance period since 1982 has led to strong thinning, deceleration and retreat of the tongue. The response of these observed dynamics to surface mass balance is analyzed from ice-flux calculations performed on three transverse cross-sections. Our results reveal that the ice fluxes are largely accommodated by ice-flow velocities. Velocity fluctuations are synchronous over the entire area studied. In the largest part of the glacier, no compressing/extending flow change has been observed over the last 30 years and thickness changes are solely driven by surface mass-balance changes. However, on the tongue of the glacier, thickness changes do not depend on surface mass balance but are mainly driven by changes in the longitudinal strain rate.

Atmospheric Temperature Measurement Biases on the Antarctic Plateau

AbstractObservations of atmospheric temperature made on the Antarctic Plateau with thermistors housed in naturally (wind) ventilated radiation shields are shown to be significantly warm biased by solar radiation. High incoming solar flux and high surface albedo result in radiation biases in Gill (multiplate)-styled shields that can occasionally exceed 10°C in summer in cases with low wind speed. Although stronger and more frequent when incoming solar radiation is high, biases exceeding 8°C are found even when solar radiation is less than 200 W m−2. Compared with sonic thermometers, which are not affected by radiation but are too complex to be routinely used for mean temperature monitoring, commercially available aspirated shields are shown to efficiently protect thermistor measurements from solar radiation biases. Most of the available in situ reports of atmospheric temperature on the Antarctic Plateau are from automatic weather stations that use passive shields and are thus likely warm biased in the summ...

Snow/atmosphere coupled simulation at Dome C, Antarctica

Using a snow/atmosphere coupled model, the evolution of the surface and near-surface snow temperature is modeled at Dome C, Antarctica, during the period 20-30 January 2010. Firstly, the detailed multilayer snow model Crocus is run in stand-alone mode, with meteorological input forcing data provided by local meteorological observations. The snow model is able to simulate the evolution of surface temperature with good accuracy. It reproduces the observed downward propagation of the diurnal heatwave into the upper 50 cm of the snowpack reasonably well. Secondly, a fully coupled 3-D snow/atmosphere simulation is performed with the AROME regional meteorological model, for which the standard single-layer snow parameterization is replaced by Crocus. In spite of a poor simulation of clouds, the surface and near-surface snow temperatures are correctly simulated, showing neither significant bias nor drifts during the simulation period. The model reproduces particularly well the average decrease of the diurnal amplitude of air temperature from the surface to the top of the 45 m instrumented tower. This study highlights the potential of snow/atmosphere coupled models over the Antarctic plateau and the need to improve cloud microphysics and data assimilation over polar regions.

Bilans de masse des glaciers alpins et scandinaves, leurs relations avec l'oscillation du climat de l'Atlantique nord

This paper considers the relations between the North Atlantic Oscillation, reflecting the climate variation patterns on the North Atlantic region, and mass balance variations of Alpine and Scandinavian glaciers. This study first enhances the mass balance representativeness of climate variations on the massif scale. However, dealing with glaciers in the North and the South of Europe, the mass balance variations are inversely correlated. Analyses comparing the mass balance variations and the NAO index show that annual trends between these two components are really poor, but at the decadal scale, the two signals are well correlated.

Decadal Region-Wide and Glacier-Wide Mass Balances Derived from Multi-Temporal ASTER Satellite Digital Elevation Models. Validation over the Mont-Blanc Area

Since 2000, a vast archive of stereo-images has been built by the Advanced Spaceborne Thermal Emission and Reflection (ASTER) satellite. Several studies already extracted glacier mass balances from multi-temporal ASTER digital elevation models (DEMs) but they lacked accurate independent data for validation. Here, we apply a linear regression to a time series of 3D-coregistered ASTER DEMs to estimate the rate of surface elevation changes (dh/dtASTER) and geodetic mass balances of Mont-Blanc glaciers (155 km²) between 2000 and 2014. Validation using field and spaceborne geodetic measurements reveals large errors at the individual pixel level (> 1 m a-1) and an accuracy of 0.2-0.3 m a-1 for dh/dtASTER averaged over areas larger than 1 km². For all Mont-Blanc glaciers, the ASTER region-wide mass balance (-1.05±0.37 m water equivalent (w.e.) a-1) agrees remarkably with the one measured using Spot5 and Pleiades DEMs (-1.06±0.23 m w.e. a-1) over their common 2003-2012 period. This multi-temporal ASTER DEM strategy leads to smaller errors than the simple differencing of two ASTER DEMs. By extrapolating dh/dtASTER to mid-February 2000, we infer a mean penetration depth of about 9±3 m for the C-band Shuttle Radar Topographic Mission (SRTM) radar signal, with a strong altitudinal dependency (range 0-12 m). This methodology thus reveals the regional pattern of glacier surface elevation changes and improves our knowledge of the penetration of the radar signal into snow and ice.

Future fluctuations of Mer de Glace, French Alps, assessed using a parameterized model calibrated with past thickness changes

Abstract Simulations of glacier evolution are needed to assess future changes in the runoff regime of mountain catchments. A simplified parameterized model is applied here to simulate future thickness changes and glacier retreat of Mer de Glace, French Alps. A normalized thickness change function describing the spatial distribution of surface-elevation changes as a function of elevation has been determined. The model reveals that under present climatic conditions Mer de Glace will continue to shrink dramatically in the coming decades, retreating by 1200 m between now and 2040. The method has certain limitations related to the uncertainties of the normalized function based on thickness change data. An error of 10% in the normalized function leads to uncertainties of 46%, 30% and 18% in Mer de Glace front, surface area and glacier-wide mass-balance changes respectively in 2040. Because the difference of the normalized function largely exceeds 10% from one glacier to another, even within a given glacier size class and elevation range, it would be very risky to extrapolate the normalized function to unmeasured glaciers. Consequently, the method is applicable only on glaciers where past surface elevation changes are well constrained.

Origin of the outburst flood from Glacier de Tete Rousse in 1892 (Mont Blanc area, France)

Extensive field measurements and historical data have been used to re-analyse the cause of the outburst flood from Glacier de Tete Rousse that devastated the village of Saint-Gervais-Le Fayet, French Alps in 1892, causing 175 fatalities. The origin of this disaster was the rupture of an intraglacial cavity in Glacier de Tete Rousse that released 200 000 m 3 of water and ice. All previous studies have concluded that the intraglacial cavity was formed from a crevasse that was filled and enlarged by meltwater. The re-analysis presented here suggests that the reservoir of the upper cavity did not originate as an enlarging crevasse. The origin of the meltwater reservoir was more likely a supraglacial lake formed before 1878 during a period of negative mass balance. Following a period of positive mass balance after 1878, the lake was hidden until the outburst flood of 1892. This means that such hazards may be detected by checking regularly for the formation of a lake on the surface of the glacier before it is hidden.

Common climatic signal from glaciers in the European Alps over the last 50 years

Conventional glacier-wide mass balances are commonly used to study the effect of climate forcing on glacier melt. Unfortunately, the glacier-wide mass balances are also influenced by the glaciers dynamic response. Investigations on the effects of climate forcing on glaciers can be largely improved by analyzing point mass balances. Using a statistical model, we have found that 52% of the year-to-year deviations in the point mass balances of six glaciers distributed across the entire European Alps can be attributed to a common variability. Point mass balance changes reveal remarkable regional consistencies reaching 80% for glaciers less than 10 km apart. Compared to the steady state conditions of the 1962–1982 period, the surface mass balance changes are −0.85 m water equivalent (w.e.) a−1 for 1983–2002 and −1.63 m w.e. a−1 for 2003–2013. This indicates a clear and regionally consistent acceleration of mass loss over recent decades over the entire European Alps.

Greenland ice-sheet mass-balance distribution: a variance analysis of existing field data

A large number of mass-balance measurements have been carried out on the Greenland ice sheet, but few of the series obtained are well suited for statistical studies. Rather than looking at the global mass-balance value, this paper deals with the spatial and temporal mass-balance variability on the ice sheet. Two sorts of analysis are possible: direct comparisons of the series measured at a given site, or a broader approach involving comparisons between different sites using variance analysis. For glaciers in the southwest ablation area, a significant interannual variability (around 1.0 m w.e.) is found. This variability is spatially consistent. In the accumulation area, the results are more complex. For example, there is consistent evidence of year-to-year variations on the west-east Expeditions Glaciologiques Internationales au Groenland (EGIG) profile, but other closer sites are weakly correlated. These results emphasize the need for a better coverage of measurements over the entire ice sheet, as well as longer and more continuous measurement series.