TCD - Latest Articles

What drives basin scale spatial variability of snow water equivalent during two extreme years?

2013-06-18T00:00:00+02:00

What drives basin scale spatial variability of snow water equivalent during two extreme years?

The Cryosphere Discussions, 7, 2943-2977, 2013

Author(s): G. A. Sexstone and S. R. Fassnacht

This study uses a combination of field measurements and Natural Resource Conservation Service (NRCS) operational snow data to understand the drivers of snow water equivalent (SWE) spatial variability at the basin scale. Historic snow course snowpack density observations were analyzed within a multiple linear regression snow density model to estimate SWE directly from snow depth measurements. Snow surveys were completed on or about 1 April 2011 and 2012 and combined with NRCS operational measurements to investigate the spatial variability of SWE. Bivariate relations and multiple linear regression models were developed to understand the relation of SWE with terrain and canopy variables (derived using a geographic information system (GIS)). Calculation of SWE directly from snow depth measurement using the snow density model has strong statistical performance and model validation suggests the model is transferable to independent data within the bounds of the original dataset. This pathway of estimating SWE directly from snow depth measurement is useful when evaluating snowpack properties at the basin scale, where many time consuming measurements of SWE are often not feasible. During both water year (WY) 2011 and 2012, elevation and location (UTM Easting and UTM Northing) were the most important model variables, suggesting that orographic precipitation and storm track patterns are likely consistent drivers of basin scale SWE variability. Terrain characteristics, such as slope, aspect, and curvature, were also shown to be important variables, but to a lesser extent at the scale of interest.

2001–2010 glacier changes in the Central Karakoram National Park: a contribution to evaluate the magnitude and rate of the "Karakoram anomaly"

2013-06-18T00:00:00+02:00

2001–2010 glacier changes in the Central Karakoram National Park: a contribution to evaluate the magnitude and rate of the "Karakoram anomaly"

The Cryosphere Discussions, 7, 2891-2941, 2013

Author(s): U. Minora, D. Bocchiola, C. D'Agata, D. Maragno, C. Mayer, A. Lambrecht, B. Mosconi, E. Vuillermoz, A. Senese, C. Compostella, C. Smiraglia, and G. Diolaiuti

Karakoram is one of the most glacierized region worldwide, and glaciers therein are the main water resource of Pakistan. The attention paid to this area is increasing, because the evolution of its glaciers recently depicted a situation of general stability, known as "Karakoram Anomaly", in contrast to glacier retreat worldwide. Here we focused our attention upon the glacier evolution within the Central Karakoram National Park (CKNP, a newborn park of this region, ca. 12 162 km² in area) to assess the magnitude and rate of such anomaly. By means of Remote Sensing data (i.e.: Landsat images), we analyzed a sample of more than 700 glaciers, and we found out their area change between 2001 and 2010 is not significant (+27 km² ± 42 km²), thus confirming their stationarity. We analyzed climate data, snow coverage from MODIS, and supraglacial debris presence, as well as potential (con-) causes. We found a slight decrease of summer temperatures (down to −1.5 °C during 1980–2009) and an increase of wet days during winter (up +3.3 days yr⁻¹ during 1980–2009), possibly increasing snow cover duration, consistently with MODIS data. We further detected considerable supra-glacial debris coverage (ca. 20% of the glacier area which rose up to 31% considering only the ablation area), which could have reduced buried ice melting during the last decade. These results provide further ground to uphold the existence of the Karakoram Anomaly, and present an useful template for assessment of water availability within the glaciers of the CKNP.

Parameter and state estimation with a time-dependent adjoint marine ice sheet model

2013-06-17T00:00:00+02:00

Parameter and state estimation with a time-dependent adjoint marine ice sheet model

The Cryosphere Discussions, 7, 2845-2890, 2013

Author(s): D. N. Goldberg and P. Heimbach

To date, assimilation of observations into large-scale ice models has consisted predominantly of time-independent inversions of surface velocities for basal traction, bed elevation, or ice stiffness, and has relied primarily on analytically-derived adjoints of diagnostic ice velocity models. To overcome limitations of such "snapshot" inversions, i.e. their inability to assimilate time-dependent data, or to produce initial states with minimum artificial drift and suitable for time-dependent simulations, we have developed an adjoint of a time-dependent parallel glaciological flow model. The model implements a hybrid shallow shelf-shallow ice stress balance, involves a prognostic equation for ice thickness evolution, and can represent the floating, fast-sliding, and frozen bed regimes of a marine ice sheet. The adjoint is generated by a combination of analytic methods and the use of algorithmic differentiation (AD) software. Several experiments are carried out with idealized geometries and synthetic observations, including inversion of time-dependent surface elevations for past thicknesses, and simultaneous retrieval of basal traction and topography from surface data. Flexible generation of the adjoint for a range of independent uncertain variables is exemplified through sensitivity calculations of grounded ice volume to changes in basal melting of floating and basal sliding of grounded ice. The results are encouraging and suggest the feasibility, using real observations, of improved ice sheet state estimation and comprehensive transient sensitivity assessments.

Grain shape influence on light extinction in snow

2013-06-17T00:00:00+02:00

Grain shape influence on light extinction in snow

The Cryosphere Discussions, 7, 2801-2843, 2013

Author(s): Q. Libois, G. Picard, J. L. France, L. Arnaud, M. Dumont, C. M. Carmagnola, and M. D. King

The energy budget and the photochemistry of a snowpack greatly depend on the penetration of solar radiation into the snowpack. While representing snow by a collection of spherical particles has been a successful option in the numerical computation of the albedo, such models poorly reproduce light extinction measurements. Here, we explore the limits of the spherical representation by using numerical tools and experimental data. For this, we investigate the influence of grain shape on light extinction in the visible and near-infrared (NIR) ranges. To compute light extinction, we developed a multi-layer radiative transfer model based on the δ-Eddington approximation and analytical expressions of the albedo, α, and the asymptotic flux extinction coefficient (AFEC), k_e. The snowpack is characterized by the profiles of density, specific surface area (SSA) and two parameters (B and g^G) depending only on the grain shape. The aim of the paper is to estimate the values of B and g^G and to understand how they impact macroscopic optical properties of snow. First, the values of B and g^G are deduced from simulations with ray tracing models for a variety of simple geometric shapes. The results show that spherical grains propagate light deeper into snow than the other shapes we have investigated, in agreement with theoretical and experimental studies from the literature. Then we present an experimental method to retrieve B for natural snow using optical measurements. Analytical expressions of albedo and AFEC demonstrate that B can be retrieved from simultaneous measurements of albedo and AFEC of a snow layer, or similarly from vertical profiles of reflectance and light intensity in a snowpack. Such measurements were performed in Antarctica and in the Alps and led to values of B between 0.8 and 2.0, which significantly differs from the theoretical value for spherical grains: B = 1.25. In addition, values of B were estimated from data in the literature. This led to a wider range of values (1.0–9.9) which may be partially explained by the accuracy of the data. We demonstrate that grain shape has a significant influence on AFEC in natural snow. It highlights the large variety of natural snow microstructure and the importance of considering grain shape for snow optics questions. It experimentally demonstrates that spherical grains are inappropriate to model light extinction in snow, an important result that should be considered in further studies dedicated to subsurface absorption of shortwave radiation and snow photochemistry.

Feedbacks and mechanisms affecting the global sensitivity of glaciers to climate change

2013-06-17T00:00:00+02:00

Feedbacks and mechanisms affecting the global sensitivity of glaciers to climate change

The Cryosphere Discussions, 7, 2761-2800, 2013

Author(s): B. Marzeion, A. H. Jarosch, and J. M. Gregory

Mass loss by glaciers has been an important contributor to sea level rise in the past and is projected to contribute a substantial fraction of total sea level rise during the 21st century. Here, we use a model of the world's glaciers in order to quantify equilibrium sensitivities of global glacier mass to climate change, and to investigate the role of changes in glacier hypsometry for long term mass changes. We find that 21st century glacier mass loss to a~large degree is governed by the glaciers responding to 20th century climate change. This limits the influence of 21st century climate change on glacier mass loss, and explains why there are relatively small differences in glacier mass loss under greatly different scenarios of climate change. Because of the geographic distribution of glaciers, both temperature and precipitation anomalies experienced by glaciers are vastly stronger than on global average. The projected increase in precipitation partly compensates for the mass loss caused by warming, but this compensation is negligible at higher temperature anomalies since an increasing fraction of precipitation at the glacier sites it liquid. Loss of low-lying glacier area, and more importantly, eventual complete disappearance of glaciers, strongly limit the projected sea level contribution from glaciers in coming centuries. The adjustment of glacier hypsometry to changes in the forcing reduces the sensitivity of global glacier mass to changes in global mean temperature by a factor of two to three. This result is a second reason for the relatively weak dependence of glacier mass loss on future climate scenario, and helps explain why glacier mass loss in the first half of the 20th century was of the same order of magnitude as in the second half of the 20th century, even though the rate of warming was considerably smaller.

Seasonal evolution of snow permeability under equi-temperature and temperature-gradient conditions

2013-06-17T00:00:00+02:00

Seasonal evolution of snow permeability under equi-temperature and temperature-gradient conditions

The Cryosphere Discussions, 7, 2725-2759, 2013

Author(s): F. Domine, S. Morin, E. Brun, and M. Lafaysse

The permeability K of snow to air flow affects the transfer of energy, water vapor and chemical species between the snow and the atmosphere. Yet today little is known of the temporal evolution of snow permeability as a function of metamorphic regime. Furthermore, our ability to simulate snow permeability over the seasonal evolution of a snowpack has not been tested. Here we have measured the evolution of snow permeability in a subarctic snowpack subject to high temperature-gradient (TG) metamorphism. We have also measured the evolution of the same snowpack deposited over tables so that it evolved in the equi-temperature (ET) regime. Permeability varies in the range 31 × 10^–10 (ET regime) to 650 × 10^–10 m² (TG regime). Permeability increases over time in TG conditions and decreases under ET conditions. Using measurements of density ρ and of specific surface area (SSA), from which the equivalent sphere radius r is determined, we show that the equation linking SSA, density ρ and permeability, K = 3.0 r² e^{(–0.013 ρ)} (with K in m², r in m and ρ in kg m⁻³) obtained in a previous study adequately predicts permeability values. The detailed snowpack model Crocus is used to simulate the physical properties of the TG and ET snowpacks. For the most part, all variables are well reproduced. Simulated permeabilities are up to a factor of two greater than measurements for depth hoar layers, which we attribute to snow microstructure, as the aerodynamic properties of hollow depth hoar crystals are different from those of spheres. Finally, the large difference in permeabilities between ET and TG metamorphic regimes will impact atmosphere-snow energy and mass exchanges and these effects deserve consideration in predicting the effect of climate change on snow properties and snow-atmosphere interactions.

Modeling surface response of the Greenland Ice Sheet to interglacial climate

2013-06-14T00:00:00+02:00

Modeling surface response of the Greenland Ice Sheet to interglacial climate

The Cryosphere Discussions, 7, 2703-2723, 2013

Author(s): D. Rau and I. Rogozhina

This study presents a~new approach to parameterizing surface mass balance (SMB) of the Greenland Ice Sheet (GIS) under interglacial climate validated against recent satellite observations and the results of a high-resolution model on a regional scale. Based on detailed analysis of the modeled SMB, we conclude that existing parameterizations fail to capture either spatial pattern or amplitude of the observed surface responses of the GIS. This is due to multiple simplifying assumptions adopted by the majority of modeling studies within the framework of a positive degree-day method. Modeled surface melting is found to be highly sensitive to a choice of daily temperature standard deviation (SD), which is generally assumed to have uniform distribution across Greenland. The range of commonly used SD values does not however receive support from climate datasets available. In this region, SD distribution is highly inhomogeneous and characterized by low values during summer months in areas where most surface melting occurs. Our approach is to make use of spatially variable SD and here we show that this leads to significant improvements in the modeled SMB over the instrumental record. Our findings necessitate evaluating potential consequences of the simplified SMB treatment for assessment of the history and future of glaciation on Earth.

A note on the water budget of temperate glaciers

2013-06-14T00:00:00+02:00

A note on the water budget of temperate glaciers

The Cryosphere Discussions, 7, 2679-2702, 2013

Author(s): J. Oerlemans

In this note the total dissipative melting in temperate glaciers is studied. The analysis is based on the notion that the dissipation is determined by the loss of potential energy, due to the downward motion of mass (ice, snow, meltwater and rain). A mathematical formulation of the dissipation is developed and applied to a simple glacier geometry. In a next step, meltwater production resulting from enhanced ice motion during a glacier surge is calculated. The amount of melt energy available follows directly from the lowering of the centre of gravity of the glacier.

To illustrate the concept, schematic calculations are presented for a number of glaciers with different geometric characteristics. Typical dissipative melt rates, expressed as water-layer depth averaged over the glacier, range from a few cm per year for smaller glaciers to half a meter per year for Franz-Josef Glacier, one of the most active glaciers in the world (in terms of mass turnover).

The total generation of meltwater during a surge is typically half a meter. For Variegated Glacier a value of 70 cm is found, for Kongsvegen 20 cm. These values refer to water layer depth averaged over the entire glacier. The melt rate depends on the duration of the surge. It is generally an order of magnitude larger than the water production by "normal" dissipation. On the other hand, the additional basal melt rate during a surge is comparable in magnitude to the water input from meltwater and precipitation. This suggests that enhanced melting during a surge does not grossly change the total water budget of a glacier. Basal water generated by enhanced sliding is an important ingredient of many theories of glacier surges. It provides a positive feedback mechanism that actually makes the surge happen. The results found here suggest that this can only work if water generated by enhanced sliding is accumulating in a part of the glacier base where surface meltwater and rain has no or very limited access. This finding seems compatible with the fact that on many glaciers surges are initiated in the lower accumulation zone.

A wavelet melt detection algorithm applied to enhanced resolution scatterometer data over Antarctica (2000–2009)

2013-06-14T00:00:00+02:00

A wavelet melt detection algorithm applied to enhanced resolution scatterometer data over Antarctica (2000–2009)

The Cryosphere Discussions, 7, 2635-2678, 2013

Author(s): N. Steiner and M. Tedesco

Melting is mapped over Antarctica at a high spatial resolution using a novel melt-detection algorithm based on wavelets and multi-scale analysis. The method is applied to Ku band (13.4 GHz) normalized backscattering measured by SeaWinds on QuikSCAT and spatially enhanced on a 5 km grid over the operational life of the sensor (1999–2009). Wavelet-based estimates of melt spatial extent and duration are compared with those obtained by means of threshold-based detection methods, where melting is detected when the measured backscattering is 3 dB below the preceding winter mean value. Results from both methods are assessed by means of Automatic Weather Station (AWS) air surface temperature records. The yearly melting index, the product of melted area and melting duration, found using a fixed threshold and wavelet-based melt algorithm are found to have a relative difference within 7% for all years. The majority of the difference between melting records determined from QuikSCAT are related to short-duration backscatter changes identified as melting using the threshold methodology but not the wavelet-based method. Compared with AWS records both methods show a relative accuracy to within 10% based on estimated melt conditions using air temperatures. Melting maps obtained with the wavelet-based approach are also compared with those obtained from spaceborne brightness temperatures recorded by the Special Sensor Microwave/Image (SSMI). With respect to passive microwave records, we find a higher degree of agreement (9% relative difference) for the melting index using the wavelet-based approach than threshold-based methods (11% relative difference). Additionally, linkages between melting variability and the Southern Annular Mode (SAM), an important large-scale climate driver for Antarctica, are suggested by the results using wavelet based methods that are not found using threshold-based methods.

Comparison of automatic segmentation of full polarimetric SAR sea ice images with manually drawn ice charts

2013-06-13T00:00:00+02:00

Comparison of automatic segmentation of full polarimetric SAR sea ice images with manually drawn ice charts

The Cryosphere Discussions, 7, 2595-2634, 2013

Author(s): M.-A. N. Moen, A. P. Doulgeris, S. N. Anfinsen, A. H. H. Renner, N. Hughes, S. Gerland, and T. Eltoft

In this paper we investigate the performance of an algorithm for automatic segmentation of full polarimetric, synthetic aperture radar (SAR) sea ice scenes. The algorithm uses statistical and polarimetric properties of the backscattered radar signals to segment the SAR image into a specified number of classes. This number was determined in advance from visual inspection of the SAR image and by available in-situ measurements. The segmentation result was then compared to ice charts drawn by ice service analysts. The comparison revealed big discrepancies between the charts of the analysts, and between the manual and the automatic segmentations. In the succeeding analysis, the automatic segmentation chart was labeled into ice types by sea ice experts, and the SAR features used in the segmentation were interpreted in terms of physical sea ice properties.

Studies of automatic and robust estimation of the number of ice classes in SAR sea ice scenes will be highly relevant for future work.

The sensitivity of flowline models of tidewater glaciers to parameter uncertainty

2013-06-13T00:00:00+02:00

The sensitivity of flowline models of tidewater glaciers to parameter uncertainty

The Cryosphere Discussions, 7, 2567-2593, 2013

Author(s): E. M. Enderlin, I. M. Howat, and A. Vieli

Depth-integrated (1-D) flowline models have been widely used to simulate fast-flowing tidewater glaciers and predict future change because their computational efficiency allows for continuous grounding line tracking, high horizontal resolution, and a physically-based calving criterion, which are all essential to realistic modeling of tidewater glaciers. As with all models, the values for parameters describing ice rheology and basal friction must be assumed and/or tuned based on observations. For prognostic studies, these parameters are typically tuned so that the glacier matches observed thickness and speeds at an initial state, to which a perturbation is applied. While it is well know that ice flow models are sensitive to these parameters, the sensitivity of tidewater glacier models has not been systematically investigated. Here we investigate the sensitivity of such flowline models of outlet glacier dynamics to uncertainty in three key parameters that influence a glacier's resistive stress components. We find that, within typical observational uncertainty, similar initial (i.e. steady-state) glacier configurations can be produced with substantially different combinations of parameter values, leading to differing transient responses after a perturbation is applied. In cases where the glacier is initially grounded near flotation across a basal overdeepening, as typically observed for rapidly changing glaciers, these differences can be dramatic owing to the threshold of stability imposed by the flotation criterion. The simulated transient response is particularly sensitive to the parameterization of ice rheology: differences in ice temperature of ∼ 2 °C can determine whether the glaciers thin to flotation and retreat unstably or remain grounded on a marine shoal. Due the highly non-linear dependence of tidewater glaciers on model parameters, we recommend that their predictions are accompanied by sensitivity tests that take parameter uncertainty into account.

Impact of physical properties and accumulation rate on pore close-off in layered firn

2013-06-13T00:00:00+02:00

Impact of physical properties and accumulation rate on pore close-off in layered firn

The Cryosphere Discussions, 7, 2533-2566, 2013

Author(s): S. A. Gregory, M. R. Albert, and I. Baker

Investigations into the physical characteristics of deep firn near the lock-in zone through pore close-off are needed to improve understanding of ice core records of past atmospheric concentrations. Specifically, the permeability and microstructure profiles of the firn through the diffusive column influence the entrapment of air into bubbles and thus the ice age-gas age difference. The purpose of this study is to examine the nature of pore closure processes at two polar sites with very different local temperatures and accumulation rates. Density, permeability, and microstructure measurements were made on firn cores from WAIS Divide in West Antarctica and Megadunes in East Antarctica. We found that the open pore structure plays a more important role than density in predicting gas transport properties, through the porous firn matrix. For both WAIS Divide and Megadunes, fine grained layers experience close-off shallower in the firn column than do coarse grained layers, regardless of which grain sized layer is the more dense layer at depth. Pore close-off occurs at an open porosity that is accumulation rate dependent. Low accumulation sites, with coarse grains, close-off at lower open porosities (< 10%) than the open porosity (> 10%) of high accumulation sites with finer grains. The depth and length of the lock-in zone is primarily dependent upon accumulation rate and microstructural variability due to differences in grain size and pore structure, rather than the density variability of the layers.

Decadal changes from a multi-temporal glacier inventory of Svalbard

2013-06-10T00:00:00+02:00

Decadal changes from a multi-temporal glacier inventory of Svalbard

The Cryosphere Discussions, 7, 2489-2532, 2013

Author(s): C. Nuth, J. Kohler, M. König, A. von Deschwanden, J. O. Hagen, A. Kääb, G. Moholdt, and R. Pettersson

We present a multi-temporal digital inventory of Svalbard glaciers with the most recent from the late 2000s containing 33 775 km² of glaciers, or 57% of the total land area of the archipelago. At present, 68% of the glaciated area of Svalbard drains through tidewater glaciers that have a summed terminus width of ~ 740 km. The glaciated area over the entire archipelago has decreased by an average of 80 km² a⁻¹ over the past ~ 30 yr, representing a reduction of 7%. For a sample of ~ 400 glaciers (10 000 km²) in the south and west of Spitsbergen, three digital inventories are available from 1930/60s, 1990 and 2007 from which we calculate average changes during 2 epochs. In the more recent epoch, the terminus retreat was larger than in the earlier epoch while area shrinkage was smaller. The contrasting pattern may be explained by the decreased lateral wastage of the glacier tongues. Temporal retreat rates for individual glaciers show a mix of accelerating and decelerating trends, reflecting the large spatial variability of glacier types and climatic/dynamic response times in Svalbard. Last, retreat rates estimated by dividing glacier area changes by the tongue width are larger than centerline retreat due to a more encompassing frontal change estimate with inclusion of lateral area loss.

The physical basis for gas transport through polar firn: a case study at Summit, Greenland

2013-06-07T00:00:00+02:00

The physical basis for gas transport through polar firn: a case study at Summit, Greenland

The Cryosphere Discussions, 7, 2455-2487, 2013

Author(s): A. C. Adolph and M. R. Albert

Compared to other natural porous materials, relatively little is known about the physical nature of polar firn. This intricate network of ice and pore space that comprises the top 60–100 m of the polar ice sheets is the framework that forms the natural archive of past climate information. Despite the many implications for ice core interpretation, direct measurements of physical properties throughout the firn column are limited. Models of gas transport through firn are used to interpret in-situ chemical data which is retrieved to analyze past atmospheric composition. These traditional models treat the firn as a "black box," with gas transport parameters tuned to match gas concentrations with depth to known atmospheric histories. Though this method has been largely successful and provided very useful insights, there are still many questions and uncertainties to be addressed. This work seeks to understand the impact of firn structure on gas transport in firn from a first principles standpoint through direct measurements of permeability, gas diffusivity and microstructure. The relationships between gas transport properties and microstructure will be characterized and compared to existing relationships for general porous media. Direct measurements of gas diffusivity are compared to diffusivities deduced from models based on firn air chemical sampling. Our comparison illuminates the primary importance of including microstructural parameters, beyond just porosity or density, in mass transport modeling, and it provides insights about the nature of gas transport throughout the firn column. Guidance is provided for development of next-generation firn air transport models.

Spatial debris-cover effect on the maritime glaciers of Mount Gongga, south-eastern Tibetan Plateau

2013-06-06T00:00:00+02:00

Spatial debris-cover effect on the maritime glaciers of Mount Gongga, south-eastern Tibetan Plateau

The Cryosphere Discussions, 7, 2413-2453, 2013

Author(s): Y. Zhang, Y. Hirabayashi, K. Fujita, S. Liu, and Q. Liu

The Tibetan Plateau and surroundings contain a large number of debris-covered glaciers, on which debris cover affects glacier response to climate change by altering ice melting rates and spatial patterns of mass loss. Insufficient spatial distribution of debris thickness data makes it difficult to analyze regional debris-cover effects. Mount Gongga glaciers, maritime glaciers in the south-eastern Tibetan Plateau, are characterized by a substantial reduction in glacier length and ice mass in recent decades. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)-derived thermal property of the debris layer reveals that 68% of the glaciers have extensive mantles of supraglacial debris in their ablation zones, in which the proportion of debris cover to total glacier area varies from 1.74% to 53.0%. Using a surface energy-mass balance model accounting for the debris-cover effect applied at a regional scale, we find that although the presence of supraglacial debris has a significant insulating effect on heavily debris-covered glaciers, it accelerates ice melting on ~ 10.2% of the total ablation area and produces rapid wastage of ~ 25% of the debris-covered glaciers, resulting in the similar mass losses between debris-covered and debris-free glaciers. Widespread debris cover also facilitates the development of active terminus regions. Regional differences in the debris-cover effect are apparent, highlighting the importance of debris cover for understanding glacier status and hydrology in both the Tibetan Plateau and other mountain ranges around the world.

Solving Richards Equation for snow improves snowpack meltwater runoff estimations

2013-06-05T00:00:00+02:00

Solving Richards Equation for snow improves snowpack meltwater runoff estimations

The Cryosphere Discussions, 7, 2373-2412, 2013

Author(s): N. Wever, C. Fierz, C. Mitterer, H. Hirashima, and M. Lehning

The runoff from the snow cover during spring snow melt or rain-on-snow events is an important factor in the hydrological cycle. In this study, water transport schemes for a 1-dimensional physical based snowpack model are compared to 14 yr of lysimeter measurements at a high alpine site. The schemes include a simple bucket-type approach, an approximation of Richards Equation (RE), and the full RE. The results show that daily sums of runoff are strongly related to a positive energy balance of the snow cover and therefore, all water transport schemes show very similar performance in terms of Nash–Sutcliffe efficiency (NSE) coefficients (around 0.59) and r² values (around 0.77). Timing of the arrival of meltwater in spring at the bottom of the snowpack showed differences between the schemes, where especially in the bucket-type and approximated RE approach, meltwater release is slower than in the measurements. Overall, solving RE for the snow cover yields the best agreement between modelled and measured runoff. On sub-daily time scales, the water transport schemes behave very differently. Also here, solving RE provides the highest agreement between modelled and measured runoff in terms of NSE coefficient (0.48), where other water transport schemes loose any predictive power. This appears to be mainly due to bad timing of meltwater release during the day. Accordingly, solving RE for the snow cover improves several aspects of modelling snow cover runoff. The additional computational cost was found to be in the order of a factor of 1.5.

Evaluation of the snow regime in dynamic vegetation land surface models using field measurements

2013-06-05T00:00:00+02:00

Evaluation of the snow regime in dynamic vegetation land surface models using field measurements

The Cryosphere Discussions, 7, 2333-2372, 2013

Author(s): E. Kantzas, M. Lomas, S. Quegan, and E. Zakharova

An increasing number of studies have demonstrated the significant climatic and ecological changes occurring in the northern latitudes over the past decades. As coupled, earth-system models attempt to describe and simulate the dynamics and complex feedbacks of the Arctic environment, it is important to reduce their uncertainties in short-term predictions by improving the description of both the systems processes and its initial state. This study focuses on snow-related variables and extensively utilizes a historical data set (1966–1996) of field snow measurements acquired across the extend of the Former Soviet Union (FSU) to evaluate a range of simulated snow metrics produced by a variety of land surface models, most of them embedded in IPCC-standard climate models. We reveal model-specific issues in simulating snow dynamics such as magnitude and timings of SWE as well as evolution of snow density. We further employ the field snow measurements alongside novel and model-independent methodologies to extract for the first time (i) a fresh snow density value (57–117 kg m^–3) for the region and (ii) mean monthly snowpack sublimation estimates across a grassland-dominated western (November–February) [9.2, 6.1, 9.15, 15.25] mm and forested eastern sub-sector (November–March) [1.53, 1.52, 3.05, 3.80, 12.20] mm; we subsequently use the retrieved values to assess relevant model outputs. The discussion session consists of two parts. The first describes a sensitivity study where field data of snow depth and snow density are forced directly into the surface heat exchange formulation of a land surface model to evaluate how inaccuracies in simulating snow metrics affect important modeled variables and carbon fluxes such as soil temperature, thaw depth and soil carbon decomposition. The second part showcases how the field data can be assimilated with ready-available optimization techniques to pinpoint model issues and improve their performance.

An upper-bound estimate for the accuracy of volume-area scaling

2013-06-05T00:00:00+02:00

An upper-bound estimate for the accuracy of volume-area scaling

The Cryosphere Discussions, 7, 2293-2331, 2013

Author(s): D. Farinotti and M. Huss

Volume-area scaling is the most popular method for estimating the ice volume of large glacier samples. Here, a series of resampling experiments based on different sets of synthetic data are presented in order to derive an upper-bound estimate (i.e. a level achieved only with ideal conditions) for the accuracy of its application. We also quantify the maximum accuracy expected when scaling is used for determining the glacier volume change, and area change of a given glacier population. A comprehensive set of measured glacier areas, volumes, area and volume changes is evaluated to investigate the impact of real-world data quality on the so assessed accuracies. For populations larger than a few thousand glaciers, the total ice volume can be recovered within 30% if all measurements available worldwide are used for estimating the scaling coefficients. Assuming no systematic biases in ice volume measurements, their uncertainty is of secondary importance. Knowing the individual areas of a glacier sample for two points in time allows recovering the corresponding ice volume change within 40% for populations larger than a few hundred glaciers, both for steady-state and transient geometries. If ice volume changes can be estimated without bias, glacier area changes derived from volume-area scaling show similar uncertainties as for the volume changes. This paper does not aim at making a final judgement about the suitability of volume-area scaling, but provides the means for assessing the accuracy expected from its application.

Changes in glacier Equilibrium-Line Altitude (ELA) in the western Alps over the 1984–2010 period: evaluation by remote sensing and modeling of the morpho-topographic and climate controls

2013-06-03T00:00:00+02:00

Changes in glacier Equilibrium-Line Altitude (ELA) in the western Alps over the 1984–2010 period: evaluation by remote sensing and modeling of the morpho-topographic and climate controls

The Cryosphere Discussions, 7, 2247-2291, 2013

Author(s): A. Rabatel, A. Letréguilly, J.-P. Dedieu, and N. Eckert

We present time series of equilibrium-line altitude (ELA) measured from the end-of-summer snowline altitude computed using satellite images, for 43 glaciers in the western Alps over the 1984–2010 period. More than 120 satellite images acquired by Landsat, SPOT and ASTER were used. In parallel, changes in climate parameters (summer cumulative positive degree days, CPDD, and winter precipitation) were analyzed over the same time period using 22 weather stations located inside and around the study area. Assuming a continuous linear trend over the study period: (1) the average ELA of the 43 glaciers increased by about 170 m; (2) summer CPDD increased by about 150 PDD at 3000 m a.s.l.; and (3) winter precipitation remained rather stationary. Summer CPDD showed homogeneous spatial and temporal variability; winter precipitation showed homogeneous temporal variability, but some stations showed a slightly different spatial pattern. Regarding ELAs, temporal variability between the 43 glaciers was also homogeneous, but spatially, glaciers in the southern part of the study area differed from glaciers in the northern part, mainly due to a different precipitation pattern. A sensitivity analysis of the ELAs to climate and morpho-topographic parameters (elevation, aspect, latitude) highlighted the following: (1) the average ELA over the study period of each glacier is strongly controlled by morpho-topographic parameters; and (2) the interannual variability of the ELA is strongly controlled by climate parameters, with the observed increasing trend mainly driven by increasing temperatures, even if significant nonlinear low frequency fluctuations appear to be driven by winter precipitation anomalies. Finally, we used an expansion of Lliboutry's approach to reconstruct fluctuations in the ELA of any glacier of the study area with respect to morpho-topographic and climate parameters, by quantifying their respective weight and the related uncertainties in a consistent manner within a hierarchical Bayesian framework. This method was tested and validated using the ELA measured on the satellite images.

Simulation of wind-induced snow transport in alpine terrain using a fully coupled snowpack/atmosphere model

2013-06-03T00:00:00+02:00

Simulation of wind-induced snow transport in alpine terrain using a fully coupled snowpack/atmosphere model

The Cryosphere Discussions, 7, 2191-2245, 2013

Author(s): V. Vionnet, E. Martin, V. Masson, G. Guyomarc'h, F. Naaim-Bouvet, A. Prokop, Y. Durand, and C. Lac

In alpine regions, wind-induced snow transport strongly influences the spatio-temporal evolution of the snow cover throughout the winter season. To gain understanding on the complex processes that drive the redistribution of snow, a new numerical model is developed. It couples directly the detailed snowpack model Crocus with the atmospheric model Meso-NH. Meso-NH/Crocus simulates snow transport in saltation and in turbulent suspension and includes the sublimation of suspended snow particles. A detailed representation of the first meters of the atmosphere allows a fine reproduction of the erosion and deposition process. The coupled model is evaluated against data collected around the experimental site of Col du Lac Blanc (2720 m a.s.l., French Alps). For this purpose, a blowing snow event without concurrent snowfall has been selected and simulated. Results show that the model captures the main structures of atmospheric flow in alpine terrain, the vertical profile of wind speed and the snow particles fluxes near the surface. However, the horizontal resolution of 50 m is found to be insufficient to simulate the location of areas of snow erosion and deposition observed by terrestrial laser scanning. When activated, the sublimation of suspended snow particles causes a reduction in deposition of 5.3%. Total sublimation (surface + blowing snow) is three times higher than surface sublimation in a simulation neglecting blowing snow sublimation.