Effects of the North Atlantic Oscillation (NAO) on combined temperature and precipitation winter modes in the Mediterranean mountains: Observed relationships and projections for the 21st century

. Light-absorbing impurities (LAIs) decrease snow albedo, increasing the amount of solar energy absorbed by the snowpack. Its most intuitive and direct impact is to accelerate snowmelt. Enhanced energy absorption in snow also modifies snow metamorphism, which can indirectly drive further variations of snow albedo in the near-infrared part of the solar spectrum because of the evolution of the near-surface snow microstructure. New capabilities have been implemented in the detailed snowpack model SURFEX/ISBA-Crocus (referred to as Crocus) to account for impurities' deposition and evolution within the snowpack and their direct and indirect impacts. Once deposited, the model computes impurities' mass evolution until snow melts out, accounting for scavenging by meltwater. Taking advantage of the recent inclusion of the spectral radiative transfer model TARTES (Two-stream Analytical Radiative TransfEr in Snow model) in Crocus, the model explicitly represents the radiative impacts of light-absorbing impurities in snow. The model was evaluated at the Col de Porte experimental site (French Alps) during the 2013–2014 snow season against in situ standard snow measurements and spectral albedo measurements. In situ meteorological measurements were used to drive the snowpack model, except for aerosol deposition fluxes. Black carbon (BC) and dust deposition fluxes used to drive the model were extracted from simulations of the atmospheric model ALADIN-Climate. The model simulates snowpack evolution reasonably, providing similar performances to our reference Crocus version in terms of snow depth, snow water equivalent (SWE), near-surface specific surface area (SSA) and shortwave albedo. Since the reference empirical albedo scheme was calibrated at the Col de Porte, improvements were not expected to be significant in this study. We show that the deposition fluxes from the ALADIN-Climate model provide a reasonable estimate of the amount of light-absorbing impurities deposited on the snowpack except for extreme deposition events which are greatly underestimated. For this particular season, the simulated melt-out date advances by 6 to 9 days due to the presence of light-absorbing impurities. The model makes it possible to apportion the relative importance of direct and indirect impacts of light-absorbing impurities on energy absorption in snow. For the snow season considered, the direct impact in the visible part of the solar spectrum accounts for 85 % of the total impact, while the indirect impact related to accelerated snow metamorphism decreasing near-surface specific surface area and thus decreasing near-infrared albedo accounts for 15 % of the total impact. Our model results demonstrate that these relative proportions vary with time during the season, with potentially significant impacts for snowmelt and avalanche prediction.

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A multilayer physically based snowpack model simulating direct and indirect radiative impacts of light-absorbing impurities in snow

The Extended Column Test (ECT) is a new stability test that aims to assess the fracture propagation potential across a 0.90 m wide isolated column. This paper: 1) describes the test procedure and presents new recording standards for the test, 2) uses two independent datasets (each consisting of over 300 tests) to assess the effectiveness of the test, 3) looks at the spatial variability of ECT results from several test grids, and 4) compares adjacent results between the ECT and the Propagation Saw Test (PST) on stable and unstable slopes. Our results indicate that the ECT is an effective stability test, with a false-stability rate less than other standard snow stability tests. Results are sometimes quite spatially uniform, though occasionally slopes may exhibit variable ECT results. In comparison to the PST, our data suggest that the ECT has a lower false-stability rate, but a higher false instability rate. Overall, the ECT is better at discriminating between stable and unstable slopes in our dataset. No test is perfect and all tests must be used in conjunction with additional data, but our results show that the ECT is valuable additional tool for assessing snow stability.

/pdf/the-extended-column-test-test-effectiveness-spatial-4c9r1wzghq.pdf

The Extended Column Test: Test Effectiveness, Spatial Variability, and Comparison with the Propagation Saw Test

http://www.issw2008.com/papers/P__8190.pdf

The Extended Column Test: Test effectiveness, spatial variability, and comparison with the Propagation Saw Test

Information on snowpack instability is crucial for assessing avalanche risk in backcountry operations as well as for operational forecasting of the regional avalanche danger. Since slab avalanche release requires both fracture initiation and fracture propagation in a weak snowpack layer, field observations should ideally provide reliable information on the probability or propensity of both fracture processes. Even simple field observations that do not require digging a snow pit can provide useful information. Traditional snowpack tests include the shovel shear test, the shear frame test, the compression test (CT) and the rutschblock test (RB). Interpretation of the test results for the CT and RB has been improved by considering the appearance or type of the fracture in addition to the score. More recently, two tests have been developed that focus on fracture propagation rather than initiation: the extended column test (ECT) and the propagation saw test (PST). We compare the sensitivity, specificity and unweighted average accuracy of various stability tests. Comparative studies indicate that the RB, ECT and PST have comparable accuracy. For most test methods the unweighted average accuracy of a single test was 70-90% depending on the dataset. Test methods such as the RB, ECT and PST, which fracture an area large enough to include fracture propagation, are generally more accurate than test methods that fracture smaller areas (e.g. the CT). The threshold-sum method was also less accurate. Even with very experienced observers for the RB, ECT and PST an error rate of at least about 5-10% has to be expected. Performing a second, adjacent test on the same slope improves test reliability.

/pdf/snowpack-tests-for-assessing-snow-slope-instability-tima73q4t0.pdf

Snowpack tests for assessing snow-slope instability

Major avalanches occurrence at regional scale and related atmospheric circulation patterns in the Eastern Pyrenees

Field methods for stability evaluation were recently developed and validated on the basis of skier-triggered slopes data from Switzerland and North America. In this paper, some of these methods (structural stability indexes, shear quality, and extended column test) were tested and adapted when necessary to the snow conditions of the Eastern Pyrenees. This was done by analyzing a dataset of 86 snow profiles and stability tests (from the last 8 winter seasons) observed next to skier-triggered slabs (including remotely triggered slab avalanches and whumpfs) and on skier-tested slopes that did not release a slab avalanche. ECT and shear quality were validated, whereas the profile field analysis methods had to be revised and new thresholds which provide a better adjustment for this part of the Pyrenees were established.

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Application of the Field Stability Evaluation Methods to the Snow Conditions of the Eastern Pyrenees

/pdf/integrating-the-ates-into-the-avalanche-information-in-aran-7v2gddinsy.pdf

Integrating the ATES into the avalanche information in Aran Valley

During the 2011-12 winter season, the Aran Avalanche Center established an Ava- lanche Terrain Exposure Scale (ATES) pilot area in Aran Valley. The ATES was developed by Parks Canada and the Canadian Avalanche Center in order to provide users with information on the terrain. Having tested the method and its acceptance by potential users, last season 33.000 ha were mapped, corresponding to the areas more heavily used in winter. The maps were made available to the users via web ( www.lauegi.conselharan.org , kmz format) and by means of panels in the main trail heads. In addition, an analysis of the accidents occurred in Aran Valley and in the Aiguestortes National Park (southern side of the range) during the last 25 years was performed. It was found that the majori- ty of the accidents occur in Complex (55%) or Challenging (26%) terrain when the danger is Consid- erable (3). During the 2012-13 season, the ATES was included in the daily Avalanche Report through Avaluator TM . The implementation of the ATES and the use of Avaluator TM in the Avalanche Reports have resulted in a better information about terrain, a better integration of terrain in decision making and, moreover, in a better danger rating. The good results obtained will encourage the spreading of the method to other areas of the Pyrenees

/pdf/integrating-the-ates-into-the-avalanche-information-in-aran-3t459u58s4.pdf

Integrating the ATES into the avalanche information in Aran Valley (Central Pyrenees)

Avalanche size is a key parameter in avalanche danger rating as well as in the communication between technicians. In 2009 the European Avalanche Warning Services (EAWS) adopted the Canadian Destructive Avalanche Size Scale (Perla 1980) but with some changes: for example, the numerical rating used in Canada and the US was substituted by the descriptive terms 'sluff', 'small', 'medium', 'large' and 'very large'. In 2010 an additional column was introduced in order to include the characteristics of the avalanche runout. To evaluate the uniformity in the use of the said scale, a survey questionnaire was sent out to all the EAWS avalanche centers and to different avalanche services in Canada. It comprised 18 avalanche cases with pictures, maps and basic morphological data. At the moment of performing this analysis, 70 surveys have been received back, 61 of which from 10 different European countries and 9 from Canada. Basic statistical description is hereby performed, including Mode, Mean and Standard Deviation. The results of the survey show a lack of uniformity in the classification of avalanche sizes within and between EAWS centers. In comparison, Canadian results are much more uniform. The cause of this lack of standardization in the European centers seems to be the absence of guidelines on how to use the scale as well as the additional parameters that were introduced in the European scale. The results of this survey are currently being used to improve the avalanche size scale used by the EAWS.

/pdf/how-big-is-big-results-of-the-avalanche-size-classification-4ccmlerhzk.pdf

Ivan Moner

Papers

Major avalanches occurrence at regional scale and related atmospheric circulation patterns in the Eastern Pyrenees

Application of the Field Stability Evaluation Methods to the Snow Conditions of the Eastern Pyrenees

Integrating the ATES into the avalanche information in Aran Valley

Integrating the ATES into the avalanche information in Aran Valley (Central Pyrenees)

How big is big: results of the avalanche size classification survey