CNRS Laboratory for Glaciology and Environmental Geophysics

Grenoble, France

CNRS Laboratory for Glaciology and Environmental Geophysics

Grenoble, France
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Weiss J.,CNRS Institute of Earth Sciences | Dansereau V.,CNRS Laboratory for Glaciology and Environmental Geophysics
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences | Year: 2017

Mechanics plays a key role in the evolution of the sea ice cover through its control on drift, on momentum and thermal energy exchanges between the polar oceans and the atmosphere along cracks and faults, and on ice thickness distribution through opening and ridging processes. At the local scale, a significant variability of the mechanical strength is associated with the microstructural heterogeneity of saline ice, however characterized by a small correlation length, below the ice thickness scale. Conversely, the sea ice mechanical fields (velocity, strain and stress) are characterized by long-ranged (more than 1000 km) and long-lasting (approx. few months) correlations. The associated space and time scaling laws are the signature of the brittle character of sea ice mechanics, with deformation resulting from a multiscale accumulation of episodic fracturing and faulting events. To translate the short-range-correlated disorder on strength into long-range-correlated mechanical fields, several key ingredients are identified: long-ranged elastic interactions, slow driving conditions, a slow viscous-like relaxation of elastic stresses and a restoring/healing mechanism. These ingredients constrained the development of a new continuum mechanics modelling framework for the sea ice cover, called Maxwell-elastobrittle. Idealized simulations without advection demonstrate that this rheological framework reproduces the main characteristics of sea ice mechanics, including anisotropy, spatial localization and intermittency, as well as the associated scaling laws. © 2016 The Author(s) Published by the Royal Society. All rights reserved.

Gardelle J.,CNRS Laboratory for Glaciology and Environmental Geophysics | Berthier E.,French National Center for Scientific Research | Arnaud Y.,CNRS Laboratory for Glaciology and Environmental Geophysics
Nature Geoscience | Year: 2012

Assessments of the state of health of Hindu-Kush-Karakoram-Himalaya glaciers and their contribution to regional hydrology and global sea-level rise suffer from a severe lack of observations. The globally averaged mass balance of glaciers and ice caps is negative. An anomalous gain of mass has been suggested for the Karakoram glaciers, but was not confirmed by recent estimates of mass balance. Furthermore, numerous glacier surges in the region that lead to changes in glacier length and velocity complicate the interpretation of the available observations. Here, we calculate the regional mass balance of glaciers in the central Karakoram between 1999 and 2008, based on the difference between two digital elevation models. We find a highly heterogeneous spatial pattern of changes in glacier elevation, which shows that ice thinning and ablation at high rates can occur on debris-covered glacier tongues. The regional mass balance is just positive at +0.11±0.22myr -1 water equivalent and in agreement with the observed reduction of river runoff that originates in this area. Our measurements confirm an anomalous mass balance in the Karakoram region and indicate that the contribution of Karakoram glaciers to sea-level rise was -0.01mmyr -1 for the period from 1999 to 2008, 0.05mmyr -1 lower than suggested before. © 2012 Macmillan Publishers Limited. All rights reserved.

Berthier E.,French National Center for Scientific Research | Vincent C.,CNRS Laboratory for Glaciology and Environmental Geophysics
Journal of Glaciology | Year: 2012

By subtracting surface topographies from 1979, 1994, 2000 and 2008, we measured icethinning rates increasing from 1ma-1 (1979-94) to >4ma-1 (2000-08) on the tongue of Mer de Glace, French Alps. The relative contributions of changes in surface mass balance and ice fluxes to this acceleration in the thinning are estimated using field and remote-sensing measurements. Between 1979-94 and 2000-08, surface mass balance diminished by 1.2mw.e. a-1, mainly because of atmospheric warming. Mass-balance changes induced by the growing debris-covered area and the evolving glacier hypsometry compensated each other. Meanwhile, Mer de Glace slowed down and the ice fluxes through two cross sections at 2200 and 2050ma.s.l. decreased by 60%. Between 1979-94 and 2000-08, two-thirds of the increase in the thinning rates was caused by reduced ice fluxes and onethird by rising surface ablation. However, these numbers need to be interpreted cautiously given our inability to respect mass conservation below our upper cross section. An important implication is that large errors would occur if one term of the continuity equation (e.g. surface mass balance) were deduced from the two others (e.g. elevation and ice-flux changes).

Girard L.,University of Zürich | Weiss J.,CNRS Laboratory for Glaciology and Environmental Geophysics | Amitrano D.,CNRS Institute of Earth Sciences
Physical Review Letters | Year: 2012

We investigate compressive failure of heterogeneous materials on the basis of a continuous progressive-damage model. The model explicitly accounts for tensile and shear local damage and reproduces the main features of compressive failure of brittle materials like rocks or ice. We show that the size distribution of damage clusters, as well as the evolution of an order parameter-the size of the largest damage cluster-argue for a critical interpretation of fracture. The compressive failure strength follows a normal distribution with a very small size effect on the mean strength, in good agreement with experiments. © 2012 American Physical Society.

Gallee H.,CNRS Laboratory for Glaciology and Environmental Geophysics | Gorodetskaya I.V.,CNRS Laboratory for Glaciology and Environmental Geophysics
Climate Dynamics | Year: 2010

The limited area model MAR (Modèle Atmosphérique Régional) is validated over the Antarctic Plateau for the period 2004-2006, focussing on Dome C during the cold season. MAR simulations are made by initializing the model once and by forcing it through its lateral and top boundaries by the ECMWF operational analyses. Model outputs compare favourably with observations from automatic weather station (AWS), radiometers and atmospheric soundings. MAR is able to simulate the succession of cold and warm events which occur at Dome C during winter. Larger longwave downwelling fluxes (LWD) are responsible for higher surface air temperatures and weaker surface inversions during winter. Warm events are better simulated when the small Antarctic precipitating snow particles are taken into account in radiative transfer computations. MAR stratosphere cools during the cold season, with the coldest temperatures occurring in conjunction with warm events at the surface. The decrease of saturation specific humidity associated with these coldest temperatures is responsible for the formation of polar stratospheric clouds (PSCs) especially in August-September. PSCs then contribute to the surface warming by increasing the surface downwelling longwave flux. © 2008 Springer-Verlag.

Brankart J.-M.,CNRS Laboratory for Glaciology and Environmental Geophysics
Ocean Modelling | Year: 2013

In this study, it is shown (i) that, as a result of the nonlinearity of the seawater equation of state, unresolved scales represent a major source of uncertainties in the computation of the large-scale horizontal density gradient from the large-scale temperature and salinity fields, and (ii) that the effect of these uncertainties can be simulated using random processes to represent unresolved temperature and salinity fluctuations. The results of experiments performed with a low resolution global ocean model show that this parameterization has a considerable effect on the average large-scale circulation of the ocean, especially in the regions of intense mesoscale activity. The large-scale flow is less geostrophic, with more intense associated vertical velocities, and the average geographical position of the main temperature and salinity fronts is more consistent with observations. In particular, the simulations suggest that the stochastic effect of the unresolved temperature and salinity fluctuations on the large-scale density field may be sufficient to explain why the Gulf Stream pathway systematically overshoots in non-stochastic low resolution ocean models. © 2013 Elsevier Ltd.

Thibert E.,IRSTEA | Eckert N.,IRSTEA | Vincent C.,CNRS Laboratory for Glaciology and Environmental Geophysics
Cryosphere | Year: 2013

Refined temporal signals extracted from a winter and summer mass balance series recorded at Glacier de Sarennes (French Alps) using variance decomposition are related to local meteorological data and large-scale North Atlantic Oscillation (NAO) anomalies in terms of interannual variability, trends of the low-frequency signals, and breaks in the time series. The winter balance has increased by +23% since 1976 due to more precipitation in early and late winter. The summer balance has decreased since 1982 due to a 43% increase in snow and ice melt. A 24-day lengthening of the ablation period-mainly due to longer ice ablation-is the main component in the overall increase in ablation. In addition, the last 25 yr have seen increases in ablation rates of 14 and 10% for snow and ice, respectively. A simple degree-day analysis can account for both the snow/ice melt rate rise and the lengthening of the ablation period as a function of higher air temperatures. From the same analysis, the equilibrium-line altitude of this 45° N latitude south-facing glacier has a sensitivity to temperature of ±93 m °C-1 around its mean elevation of 3100 m a.s.l. over 6 decades. The sensitivity of summer balance to temperature is 0.62 m w.e. yr-1 C-1 for a typical 125-day-long ablation season. Finally, the correlation of winter and summer mass balance terms with NAO anomalies is investigated. Singularly, highest values are obtained between winter NAO anomalies and summer balance. Winter NAO anomalies and winter balance and precipitation are almost disconnected. However, these results strongly depend on how the NAO signal is smoothed, so that the link between Sarennes mass balance seasonal terms and NAO signal remains tenuous and hard to interpret. © Author(s) 2013.

Marcq S.,CNRS Laboratory for Glaciology and Environmental Geophysics | Weiss J.,CNRS Laboratory for Glaciology and Environmental Geophysics
Cryosphere | Year: 2012

Leads are linear-like structures of open water within the sea ice cover that develop as the result of fracturing due to divergence or shear. Through leads, air and water come into contact and directly exchange latent and sensible heat through convective processes driven by the large temperature and moisture differences between them. In the central Arctic, leads only cover 1 to 2% of the ocean during winter, but account for more than 70% of the upward heat fluxes. Furthermore, narrow leads (several meters) are more than twice as efficient at transmitting turbulent heat than larger ones (several hundreds of meters). We show that lead widths are power law distributed, P(X)∼X-a with a>1, down to very small spatial scales (20 m or below). This implies that the open water fraction is by far dominated by very small leads. Using two classical formulations, which provide first order turbulence closure for the fetch-dependence of heat fluxes, we find that the mean heat fluxes (sensible and latent) over open water are up to 55% larger when considering the lead-width distribution obtained from a SPOT satellite image of the ice cover, compared to the situation where the open water fraction constitutes one unique large lead and the rest of the area is covered by ice, as it is usually considered in climate models at the grid scale. This difference may be even larger if we assume that the power law scaling of lead widths extends down to smaller (∼1 m) scales. Such estimations may be a first step towards a subgrid scale parameterization of the spatial distribution of open water for heat fluxes calculations in ocean/sea ice coupled models. © 2010 Author(s).

Brutel-Vuilmet C.,CNRS Laboratory for Glaciology and Environmental Geophysics | Menegoz M.,CNRS Laboratory for Glaciology and Environmental Geophysics | Krinner G.,CNRS Laboratory for Glaciology and Environmental Geophysics
Cryosphere | Year: 2013

The 20th century seasonal Northern Hemisphere (NH) land snow cover as simulated by available CMIP5 model output is compared to observations. On average, the models reproduce the observed snow cover extent very well, but the significant trend towards a reduced spring snow cover extent over the 1979-2005 period is underestimated (observed: (-3.4 ± 1.1)% per decade; simulated: (-1.0 ± 0.3)% per decade). We show that this is linked to the simulated Northern Hemisphere extratropical spring land warming trend over the same period, which is also underestimated, although the models, on average, correctly capture the observed global warming trend. There is a good linear correlation between the extent of hemispheric seasonal spring snow cover and boreal large-scale spring surface air temperature in the models, supported by available observations. This relationship also persists in the future and is independent of the particular anthropogenic climate forcing scenario. Similarly, the simulated linear relationship between the hemispheric seasonal spring snow cover extent and global mean annual mean surface air temperature is stable in time. However, the slope of this relationship is underestimated at present (observed: (-11.8 ± 2.7)% C-1; simulated: (-5.1 ± 3.0)% C-1) because the trend towards lower snow cover extent is underestimated, while the recent global warming trend is correctly represented. © 2013 Author(s).

Gilbert A.,CNRS Laboratory for Glaciology and Environmental Geophysics | Vincent C.,CNRS Laboratory for Glaciology and Environmental Geophysics
Geophysical Research Letters | Year: 2013

Given the paucity of observations, a great deal of uncertainty remains concerning temperature changes at very high altitudes over the last century. Englacial temperature measurements performed in boreholes provide a very good indicator of atmospheric temperatures for very high elevations although they are not directly related to air temperatures. Temperature profiles from seven deep boreholes drilled at three different sites between 4240 and 4300 m above sea level in the Mont Blanc area (French Alps) have been analyzed using a heat flow model and a Bayesian inverse modeling approach. Atmospheric temperature changes over the last century were estimated by simultaneous inversion of these temperature profiles. A mean warming rate of 0.14°C/decade between 1900 and 2004 was found. This is similar to the observed regional low altitude trend in the northwestern Alps, suggesting that air temperature trends are not altitude dependent. © 2013 American Geophysical Union. All Rights Reserved.

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