3.1. Early winter conditions

The first snowfalls at low altitudes in mid-November marked the beginning of winter in the Mont Blanc massif. Largely negative air temperatures, corresponding to a 0°C surface isotherm that was generally below 1200 ma.s.l., were recorded until 13 December (Fig. 2). Over this period, sparse precipitations resulted in snow depths slightly lower than seasonal means, particularly below 1500 m a.s.l. and on southern exposures (Reference VillecroseVillecrose, 1999). Such conditions of cold and thin snowpack often favor strong temperature gradients within the snowpack and thus the growth of faceted crystals of weak mechanical cohesion (e.g. Reference HöllerHöller, 2009).

Fig. 2.

Meteorological conditions in the Mont Blanc massif during the 1998/99 winter, with the altitude of the 08C surface isotherm (curve) and the daily precipitation at 1800ma.s.l. (bars) analyzed by SAFRAN, as well as records of daily precipitation (dots) at the Chamonix observation station (1035ma.s.l.; see Fig. 1b).

During the first 3 weeks of January, successive northwesterly meteorological storms resulted in snowfalls often accompanied or followed by strong winds. The resulting snowpack was very sensitive to accidental overloading, leading to 15 people being victims of accidental avalanches in the northern French Alps from 16 to 17 January (Reference VillecroseVillecrose, 1999). The week from 20 to 26 January corresponds to a period of clear skies and mild weather, with the altitude of the 0°C surface isotherm showing marked daily fluctuations of maximums ~2000m a.s.l. (Fig. 2). These conditions favored partial snowpack stabilization, especially in the southern exposures. This period was followed by the first significant precipitation of the winter, from 27 to 30 January, with a mean accumulation of 1.2 m of cold snow at 1800 m a.s.l. and intensities reaching 0.7 m d^-1 at Le Tour. Several avalanches were observed, but these caused limited damage, probably because snow depths were low below 2200 m a.s.l. (Reference VillecroseVillecrose, 1999). From 29 January to 5 February, the weather was sunny, windy and cold, with a 0°C surface isotherm barely climbing above 0m a.s.l. (Fig. 2).

Figure 3 shows a snow pit dug at Megeve station on 2 February on a northwestern flank at 2000 m a.s.l., a location characteristic of the departure zone of the avalanches observed later in February. This snow pit displays a hard base of partially refrozen depth hoar overlaid by layers of loose faceted crystals and fine grains of low volumetric weight 0.3-0.7 m above the ground. Such weak layers of faceted crystals are representative of the thin snowpack and cold conditions of the beginning of winter, as mentioned above. The fine grains are characteristic of the recent snowfall at the end of January, which had probably gained, at least at the beginning of the episode, a sintering cohesion due to the effect of the wind. The Megeve snow pit also shows a typical slab structure with the lower part of the fine grain layer, of hardness 3, overlying the weakest snow, of hardness 2 (Fig. 3). Other snow pits made throughout the winter (not shown) display similar qualitative features. The snow-cover stability at the end of January was nevertheless difficult to assess, as no stability tests were carried out.

Fig. 3.

Snow pit dug at 2000ma.s.l. on a northwestern exposure at the Mege`ve station on 2 February 1999. Plot of height as a function of ram resistance and temperature, and observations from the pit (table) according to the international classification for seasonal snow on the ground (Reference FierzFierz and others, 2009).

3.2. Synoptic situation in February 1999

In February 1999, synoptic atmospheric conditions typical of winter prevailed, with a front of low pressure extending from Scandinavia to Eastern Europe and an area of high pressure centered on the Azores (Fig. 4). However, the Azores anticyclone stretched to unusually high latitudes, up to the Atlantic coast of Spain, yielding particularly large pressure gradients over northern Europe (Fig. 4a and c). As a result, fast altitude streams conveyed cold and wet air from the Arctic regions to the northern Alps (Fig. 4b and d). The frontal activity of these streams displayed a rapid circulation of storms in a general westerly/northwesterly flow (Fig. 4a and c), which were strengthened locally over the Alps by topographical effects, particularly on 9 February (Fig. 4a). Such active northwesterly-oriented frontal zones are often associated with intense precipitation and major avalanche release in the northern Alps (Holler, 2009). In February 1999, they produced exceptional snow accumulations in the northern French Alpine massifs, with up to 5.6 m of snowfall at 1800 ma.s.l. from 26 January to 25 February in the Mont Blanc massif and northern Haute-Tarentaise, twice as much as in the southern massifs of Oisans and Grandes Rousses (Fig. 1a).

Fig. 4.

Synoptic weather charts from (a,b) 9 February 1999 and (c,d) 22 February 1999 at 1200 UTC, showing (a,c) the pressure in hPa at the mean sea level and (b,d) the geopotential heights (solid curve, in ma.s.l.) and temperature (dashed curve, in 8C) at the 500 hPa isobar.

3.3. February 1999 in the Mont Blanc massif

In February 1999, contrasting snow and weather conditions were recorded in the Mont Blanc massif according to exposures and altitudes. From 6 to 11 February, massive snowfalls and low air temperatures were measured at low altitudes, with 1.4 m of fresh-snow accumulation in the center of Chamonix (1050 m a.s.l.), a once in 40 years event. Observations of snow blowing on the summits, from the west, were reported on 7 February at Le Tour station (location in Fig. 1b). A short lull occurred on 8 February, but no significant snowpack humidification was observed. At high altitudes, fresh and light snow was transported by winds of moderate velocity, forming large accumulations during these 6days (Reference VillecroseVillecrose, 1999).

The meteorological conditions of early February resulted in a highly unstable snowpac at all altitudes, so that on 6 February the local meteorological center posted a first alert of maximum avalanche danger, according to the standard five-level European avalanche-hazard scale. This alert was maintained until 10 February (Fig. 5). During this period, 26 major avalanches were reported (Fig. 5), some of them showing unusual paths and 17 reaching the bottom of the Chamonix valley. Among them, on the afternoon of 9 February, the avalanche of Montroc broke away at ~2500 ma.s.l. over an area of 30ha on a northwestern flank above Le Tour village, revealing a 1.5 m high fracture in the triggering zone. The exceptional snow volumes moved by this aerosol crossed the valley bottom and climbed up ~200m on the opposite flank, eventually sweeping away part of the hamlet of Montroc situated at 1350 m a.s.l.

Fig. 5.

Daily number of observed avalanches (blue bars, Cemagref database) and avalanche-hazard level forecasted by Météo-France (white bars) in the Mont Blanc massif in February 1999, according to the standard five-level European avalanche-hazard scale. Such forecasts, mainly based on the SCM output, may also result from the empirical knowledge of the local forecaster and additional local observations.

After this series of exceptional avalanches, 5 days of dry weather allowed significant settling of the recent snow below 2000m a.s.l. and superficial snowpack humidification in the southern exposures, up to ~2200 ma.s.l. (Reference VillecroseVillecrose, 1999). Sustained precipitation was recorded from 17 to 25 February, with average intensities of 0.3 m d^-1 above 1800ma.s.l. and maximum values of 0.75 md^-1 at 1800 m a.s.l. (Fig. 2), resulting in large snow accumulations. The temperature increased simultaneously and progressively so that the 0°C surface isotherm and thus the rain-snow limit reached higher altitudes on average than during the first precipitation episode of February (Fig. 2). These meteorological conditions resulted in moderate fresh-snow accumulations and partial snowpack stabilization at low altitudes, in contrast to early February. As a result of these snowpack characteristics, at low altitude the avalanches that occurred from 20 to 25 February, with a maximum activity on 22 February (Fig. 5), were generally of wet snow and small in extent.

4.1. Numerical models

This study was carried out with the SCM numerical chain of Meteo-France (Reference Durand, Giraud, Brun, Mérindol and MartinDurand and others, 1999). The model suite provides diagnosed or forecasted avalanche hazards in the French Alps at the massif scale (400-500 km² on average) for various altitudes, exposures and slopes. The models have been run since the early 1990s and validated on all French massifs on seasonal and climatological timescales (Reference Durand, Giraud, Brun, Mérindol and MartinDurand and others, 1999, 2009). We used the latest version of the models, which include an optional treatment of snowdrift, instead of the version available in 1999.

SAFRAN (Reference Durand, Brun, Merindol, Guyomarc’h, Lesaffre and MartinDurand and others, 1993, 1998, 1999) provides hourly outputs of air temperature, precipitation and their phase, humidity, cloudiness, horizontal wind speed as well as direct and indirect solar radiation. These data are available at the massif scale for elevations 900-3600 m a.s.l. at intervals of 300 m and six orientations (N, S, E, W, SE and SW). SAFRAN can be run in analysis or in forecast mode. In both cases, the first estimate is provided by mesoscale atmospheric models and several climatological fields. The analyses are made every 6hours using all available observations from various networks. In the forecast mode, the model exploits a database of analogous weather situations. The simulation mode (analysis or forecast) determines the type of the final SCM outputs, i.e. avalanche-hazard analysis or forecast.

Crocus (Reference Brun, Martin, Simon, Gendre and ColeouBrun and others, 1989, 1992) is a snow model that can use SAFRAN output to describe the energy and mass evolution of the snow cover. Values of temperature, density, liquid-water content and the stratigraphy of the snowpack are calculated each hour for the same elevations and orientations as those in SAFRAN and for three slopes (flat, 20° and 40°). Crocus is commonly initialized with a snow-free ground on 1 August. Simulations are subsequently run until complete snowpack melting without any intermediate reinitialization.

MÉPRA is an expert system (Reference Giraud and ArmstrongGiraud, 1993) which then estimates an avalanche hazard for the massif every 3 hours. This system deduces additional mechanical characteristics of the snow cover (shear stress, rammsonde resistance) from the snow properties simulated by Crocus. The natural mechanical stability of the snowpack is characterized by a natural stability index for each snow layer and for the same elevations, exposures and slopes as those in Crocus. This index can be written as follows for each snow layer, l, following Reference FöhnFöhn (1987), using the subscripts a for altitude range, o for orientation, s for slope and t for time-step:

where C _laost is the shear strength of the snow layer, l, and τ_laost is the shear stress resulting from the overburden snow depth. In Equation (1), 1 < l < L, 1 < a < A, 1 < o < O, 1 < s < S and 1 < t < T, with L, A, O, S and T, respectively, the number of layers, altitude ranges, orientations, slopes and time-steps. According to the magnitude and temporal evolution of S _laost, and further based on concepts of fuzzy logic and empirical threshold values, a natural avalanche hazard is deduced for each snow profile on a six-level scale by MÉPRA (very low, moderate increasing, moderate decreasing, high or very high; Table 1, left column). Values ranging from 0 to 8, called M _aost (Table 1, right column), can be attributed to each of these natural avalanche hazard levels to build snow stability indices (e.g. Reference Martin, Giraud, Lejeune and BoudartMartin and others, 2001; section 4.3). In MÉPRA, the natural hazard estimations are completed by a classification of different avalanche types (fresh dry, fresh wet, fresh mixed, surface slab, surface wet, bottom wet). In a similar way, MÉPRA can also diagnose accidental avalanche hazard due to skier overloading. This type of hazard is not considered in this study.

Table 1.

Natural avalanche hazard classes and corresponding M _aost values from ME´PRA

Basic SCM runs do not treat snow accumulation or wind erosion. However, a routine called SYTRON can be included in the SCM chain to account for such effects (Durand and others, 2001). In such SAFRAN/SYTRON/Crocus/MÉPRA (SSCM) simulations, SYTRON uses both SAFRAN and Crocus output to compute, at a massif scale, snow transport rates by creep, saltation, suspension and sublimation, as a function of a friction wind and characteristics of the near-surface snow layers. Layers of drifted snow are subsequently integrated into the simulated snow profiles used by MEPRA to diagnose the avalanche hazard.

4.2. Sensitivity tests

The analysis of snow and weather conditions in section 3 raises several questions about the factors involved in this extreme avalanche situation:

1. 1. Are the large and rapid surface cold-snow accumulations from 6 to 11 February a sufficient factor to explain the exceptional avalanche cycle?

2. 2. Or did the mechanical characteristics of the underlying snowpack play a role in the large instability? In particular, what was the influence of the loose faceted crystals and recent snow in the upper snowpack on the avalanche trigger mechanisms?

3. 3. What was the influence of the wind during the snowfalls of the end of January and early February? Did the wind-transported fresh snow enhance the snowpack instability?

To investigate these questions and explore the sensitivity of the avalanche hazard to various snow and weather factors, we carried out several numerical experiments in the first step of this study.

In a first simulation, all the snow and weather observations available from winter 1999 in the Mont Blanc massif are analyzed at the massif scale by the SCM models. This run, which used maximum input observations and thus minimized the simulation errors, can be considered as a reference experiment (REF). This simulation was initialized with a snow-free ground on 1 October 1998 and run until 28 February 1999.

The sensitivity to the mechanical features of the late January snowpack (i.e. large amounts of loose snow lying on a dense base) was tested in a scenario called HTN0. In this experiment, the SCM chain was run in the analysis mode as in the REF experiment, but the test was started on 1 February, with a ground free of snow cover. In this way, we removed the influence of the mechanical properties of the snowpack existing before the 6-11 February precipitation on the modeled instability.

The sensitivity to the amount of fresh snow accumulated in early February was tested in a scenario called RR/2. In this scenario, which was run from 1 October 1998 until 28 February 1999 as in the REF experiment, precipitation intensities were halved after 1 February. Therefore, the snowpack simulated in February shows a dense base covered by loose old snow layers and fresh surface snow accumulations that are roughly half those obtained in the REF run.

In a fourth experiment (SYT), the effects of snow transport by wind were investigated with SYTRON, i.e. using the SSCM chain, from 1 October 1998 to 28 February 1999.

In the second step of this work, we undertook the validation of the avalanche hazard forecasted by the current SCM chain on the exceptional avalanche cycles of February 1999. With this aim, a fifth experiment (PREVI) was undertaken in which the Crocus/MEPRA suite was initialized with SAFRAN analyses from 1 October 1998 to 8 February 1999 at 0600 UTC and subsequently fed each day by the 1 day range weather forecasts from SAFRAN until 28 February 1999.

4.3. Snow instability indices

To compare the results of the numerical tests, we used an index previously employed in a snow-cover stability study (Reference Martin, Giraud, Lejeune and BoudartMartin and others, 2001). We also built a new index to take into account extreme instability levels.

The first index, hereafter called IG, has been proposed by Reference Martin, Giraud, Lejeune and BoudartMartin and others (2001) to summarize at a daily and massif scale the avalanche hazards simulated at a 3 hour time-step for each snow layer, exposure and elevation by the SCM chain. To compute IG, the M _aost estimations (Table 1, right column) are weighted by the altitude and exposure of the snow profile they describe. The daily maximum of these values is subsequently selected. Thus, with α_a being an altitude weighting function, IG is given by

where 1 < a < A, 1 < o < O, 1 < s < S and 1 < t <T. This index spatially integrates the effects of altitude and orientation and advantageously accounts for the fact that natural avalanche release more likely occurs for maximum daily instability. However, by construction, IG ranges from 0 to 8 (Reference Martin, Giraud, Lejeune and BoudartMartin and others, 2001). Thus, this index may be insufficiently adapted to describe the evolution of large instability, particularly when it reaches its maximum value.

The new index developed for the needs of this study is based on a stability criterion applied to each snow layer of a profile. The most unstable layers, i.e. of shear strength less than twice the shear stress applied by the overlying layers, are sorted out. The raw index, S _laost (Equation (1)), of each of these layers is subsequently weighted by the overlying snow depth, H _laost, and the maximum of these values is selected. The new instability index is thus

where l ranges from 1 to the number of unstable snow layers within a profile, 1 < a < A, 1 < o < O, 1 < s < S and 1 < t < T. As with the M _aost values, the ILP _aost index characterizes the snowpack stability as a function of altitude, orientation and slope at a 3 hour time interval. This index accounts furthermore for the snow amount potentially drained in case of avalanche through the value of H _laost. It can be noticed that, as S _laost is linked linearly to H _laost (Reference FöhnFöhn, 1987), ILP _aost is quadratically dependent on this parameter. This power, not representative of any physical property, has been built according to a normalization scaling process and in order to obtain a more discriminating index, not saturated at high instability levels.

To compare the properties of ILP _aost, available for each altitude, orientation and slope, to those of IG, available at the massif scale, we considered a spatial synthesis of the ILP _aost values. These values are averaged over three successive downward elevation ranges, i.e. a height difference of 900 m representative of a large avalanche path. Finally, the maximum over the massif is selected at 0600 and 1200 UTC. The resulting synthetic index is thus

where 3 < a < A, 1 < o < O, 1 < s < S and t = 0600 or 1200 UTC. This index, which has a null minimum value and no upper limit, describes the snowpack instability on the massif twice a day. As this index quantifies the snow instability over large altitude differences, it also provides a qualitative indication of potential avalanche magnitudes.

5.1. SCM analysis (REF experiment)

Figure 6 shows air temperatures, wind speeds and precipitation intensities analyzed at 1800 and 3000ma.s.l. by SAFRAN in February. The altitudinal trend in the precipitation intensities obtained over this period is in accordance with orographic effects which enhance precipitation at altitude. Negative altitudinal gradients of temperature dominate except during several periods of temperature inversion in early and mid-February (Fig. 6a), which correspond to air soundings performed in Lyon located 150 km east of Chamonix (not shown). Moderate wind velocities prevailed throughout the month (Fig. 6b).

Fig. 6.

Weather conditions analyzed at 1800 (black) and 3000m a.s.l. (gray) in February 1999. (a) Air temperature, (b) wind speeds and (c) precipitation intensity.

The meteorological analyses display two main events of precipitation, from 5 February in the evening to 10 February and from 17 to 25 February, respectively (Fig. 6c). The first episode is characterized by large precipitation intensities that reach a climax on 9 February and are accompanied by strongly negative air temperatures and moderate winds. A short interruption in the precipitation on 8 February coincides with a brief temperature increase and weakening wind (Fig. 6). The second precipitation episode is characterized by globally milder temperatures, stronger winds and lower precipitation intensities. During this episode, maximums of precipitation intensities and wind speeds are reached on 22 February, while the temperature simultaneously decreases (Fig. 6a and c). For both episodes, the weather analyses qualitatively agree with the observed weather conditions described in section 3.3.

The snow-cover evolution analyzed by Crocus at middle altitude in the Mont Blanc massif in winter 1998/99 is shown in Figure 7. The simulated snowpack displays a basal layer of depth hoar throughout the winter, especially below 2100 m a.s.l., and relatively small snow depths until late January. Subsequently, the snow depths increase rapidly due to the precipitation events occurring from late January to late February. At about 11 February, particularly large fresh-snow accumulations form at the surface of the snowpack. Other simulated snow profiles (not shown) reveal similar layering features, with maximum snow depths simulated in western and northern orientations. These characteristics, which probably reflect the combined effects of the early winter snow and weather conditions, the intense January and February precipitations and the main direction of synoptic winds, correspond to snow-pit observations made in Megeve on 2 February (Fig. 3).

Fig. 7.

Snow-cover structure simulated on a flat exposure at 1800, 2100, 2400 and 2700 ma.s.l. from 24 October 1998 to 28 February 1999, with colors indicating the snow types (Reference Brun, David, Sudul and BrunotBrun and others, 1992). Note the layers of faceted crystals (light blue) overlying a partially refrozen base (red), and large fresh surface snow accumulations (green), from the end of January to the end of February.

5.2. Information from the instability indices in the REF experiment

The respective properties of the IG index and the new indices (IL and ILP _aost) used in this study are investigated based on the results of the REF experiment at the massif scale (Fig. 8, blue curves) and the projection maps of ILP _aost (Fig. 9).

Fig. 8.

Daily avalanche hazard simulated in the sensitivity experiments and summarized at the massif scale by (a) IG and (b) IL indices.

Fig. 9.

Maps of daily avalanche hazard showing ILP _aost index values obtained for the reference scenario from (a–f) 6 to 11 February 1999, respectively, at 1200 UTC in the Chamonix valley (see box in Fig. 1).

Figure 8 illustrates the avalanche hazard synthesized at the massif scale by IG and IL (Equations (2) and (4), respectively) for all the experiments. In all experiments, the results reflect the two periods of large snow instability of February, with particularly large index values that are reached at about 7-13 and 20-24 February, in agreement with the peaks of observed avalanche activity (Figs 5 and 8). In the REF experiment, the maximum value of IG is attained during both episodes (Fig. 8a, blue curves), in contrast to IL, which reaches an absolute maximum on 9 February, roughly twice as large as the second peak value on 22 and 23 February (Fig. 8b, blue curves). More generally, IL, which is calculated twice a day, seems to be more sensitive to weather fluctuations than daily IG values (Fig. 8, blue curves). As such, IL decreases temporally during the weather lull of 8 February, whereas IG increases monotonically. Moreover, IL decreases immediately after reaching a maximum value on 9 February, whereas IG still exhibits extremely large instability for the following 2 days.

Figure 9 shows daily projections of IL on a digital elevation model (DEM) during the first avalanche cycle. These maps show that the avalanche hazard started to increase on 6 February as a result of the first snowfalls, primarily above 3000ma.s.l. (Fig. 9a and b). From 7 to 11 February, snow instability was constantly large above 2500 m a.s.l. and closely related to weather variations at lower elevations. In particular, the partial snow stabilization displayed by decreasing IL values on 8 February (Fig. 8) seemed to occur preferentially at low altitudes (Fig. 9c). On 9 February, which corresponds to the date of the Montroc avalanche, high avalanche hazards reached the valley bottom (Fig. 9d). Subsequently, as the storm dispersed, there was a general and slow decrease of the instability levels, more pronounced at low altitudes (Fig. 9e and f). These results indicate that for the first cycle of February the variations of the hazard at the massif scale (Fig. 8) mostly reflect the snow-cover evolution at middle altitudes.

The good correlation between the snow stability indices and the avalanche occurrence is in agreement with previous works (Reference Jamieson, Zeidler and BrownJamieson and others, 2007), which also suggest that such correlation increases in case of snowpacks showing widespread weak layers. In this study, weak layers were identified, mainly in the loose snow of the upper snowpack (section 3.1). The occurrence frequency table of IG and IL in the Mont Blanc massif calculated over the period 19582004 indicates that their maximum values in February 1999 are reached only exceptionally (Table 2), so that Figure 8 underlines the extreme magnitude of the snow instability throughout February 1999. As it has no upper limit, IL furthermore provides fine insights into the evolution of the snowpack instability throughout February, suggesting that the avalanche situation was more critical at the beginning than at the end of the month. Moreover, the high sensitivity of this index to weather conditions is of primary interest for avalanche-hazard forecasting. Indeed, it is empirically known that, for new-snow avalanches, instability indices built on the ratio of shear strength to load following Fohn (1987) (Equation (1)) reflect the ratio of bonding between ice particles to shear stress so that IL trends are likely to provide information on the densification process within the snow- pack. Therefore, this index, completed by ILP _aost instability maps (Fig. 9), may be a helpful tool to anticipate the magnitude, occurrence and location of large avalanche cycles in an operational context.

Table 2.

Occurrence frequency (%) of IG and IL index values in the Mont Blanc massif computed on days with snow-covered ground, from 1958 to 2004

5.3. Avalanche hazard sensitivity and forecast