Surface wind-direction mapping

The first step in the generation of an ASM is the mapping of the surface wind directions inferred from the snowforms, and thus the production of surface wind direction maps. The observation of these maps allows visualization of the interaction between the wind and the topographical features.

We have observed that wind episodes with similar wind directions generate aeolian snowforms at almost identical locations and with similar orientations. Therefore the surface wind-direction maps of these episodes resemble each other. Differences of around 20° in the wind direction of two different wind episodes are well reflected on the aeolian snowforms and thus also in surface wind-direction maps.

Spatial snow-distribution mapping

In this second type of information, and through observation of the aeolian forms and mapping of the snowdrifts, we set the boundaries of the prevailing accumulation and erosion zones. Erosion and accumulation processes may coexist, and what is considered in the mapping is the dominant process. We also observe minor forms (principally ripples and small sastrugi) coexisting with major structures, so sastrugi are often found at the windward front of a dune or a barchan, and ripples leeward of sastrugi. Consequently, when mapping aeolian forms we have to be selective and avoid mixing structures of different scales, especially when considering which is the dominant process that formed them. Minor structures respond to minor processes and to very local eddies, so expert discernment is required.

In our study area, erosion is the dominant process, which implies that sastrugi are the most frequent forms measured. However, depositional forms, principally dunes, are also found in some areas.

Sastrugi and dunes are stable during wind episodes that have similar wind directions. These aeolian snowforms are always found in the same places even if their orientations are slightly different (Reference FontFont, 1999). Thus, their distribution permits the boundaries of erosion and accumulation zones to be set according to the different forms mapped. These boundaries are not always well defined, and the transition zones may be in the order of meters (<10m). Although these boundaries are dynamic and move with wind intensity, their perimeters are well defined for similar wind episodes.

Local topography, particularly slope ruptures, is the main factor controlling the distribution of erosion and denudation zones. This control was observed in all the surveys made after each wind episode. The importance of slope angle in controlling the distribution of erosion and depositional zones was also observed at Stillberg, Swiss Alps (Reference Mases, Buisson, Good and VilaplanaMases and others, 1995).

In Figure 1 we show three topographic profiles alongside wind direction. We observe that small changes in the slope angles, of 3–7°, in a topographic profile alongside the wind direction may produce a change in wind behaviour in the snow cover. In consequence, if the wind blows uphill, as in Figure 1, a convexity in the topographic profile (decrease in slope angle) generates a prevailing accumulation zone immediately upwind of the slope change. On the other hand, a concavity in the topographic profile (increase in slope angle) accelerates the flow that generates a prevailing erosion zone.

Fig. 1

Topographic profiles showing the relation between snow erosion and deposition with slope-angle variations.

ASMs synthesize information about the surface wind direction and erosion and accumulation zones. They are representative of a specific wind direction (Fig. 2).

Fig. 2.

Diagram summarizing the main steps in the generation of the ASMs.

Methodology and results

An exhaustive follow-up of the meteorological conditions was done from mid-December 1998 until May 1999. Four ASMs were drawn up during this period. The density of the sampling affects the accuracy of the final map, and depends on the aims of the study. In this case we mapped around two aeolian snowforms per 25 m2, depending on the existence of the forms, their preservation, and so on. The first step, as mentioned above, was the mapping of the surface wind-direction maps. Simultaneously, we mapped the snowdrifts, which allowed us to set out the boundaries of the erosion and accumulation areas.

Of the four ASMs, two are of northerly winds and the other two of northwesterly and westerly winds. The dominant winds in the area are northerly, northwesterly and westerly (Reference Font, Mases and VilaplanaFont and others, 1998). In this paper we present two of these four maps, one for the northerly winds and the other for the northwesterly winds (third and second episodes in Table 1) (Figs 3 and 4).

Fig. 3.

Example of the ASM result of the drifting-snow episode of 10–12 February 1999. Application of the methodology of the aeolian snowforms and spatial distribution of snow mapping in the redesign of Dues Estacions ski slope, La Molina.

Fig. 4.

Example of the ASM result of the drifting-snow episode of 1–2 January 1999. Application of the methodology of the aeolian snowforms and spatial distribution of snow mapping in the redesign of Dues Estacions ski slope.

Table 1.

Main features of the wind episodes that produced the aeolian snowforms mappedfor the study of the Tosa d’Alp improvements

The wind data were obtained at an automatic weather station (Reference Font, Mases and VilaplanaFont and others, 1998). We define the snow available for drifting as the upper layer of the snow cover liable to be eroded by the wind. These maps show the accumulation zones, and we regard the other areas as erosion zones. We group the maps in function of the wind directions that generated the mapped forms and snowdrifts. Therefore the December and February maps are very similar, as are the April and January maps.

North-wind ASMs

Comparing the north-wind maps for December and February, we observed that the snowdrifts were much smaller in December than in February. This is attributed mainly to the availability of snow to be drifted, which was much less in December than in February. In both maps the snowdrifts formed at almost identical spots. The directions inferred from the aeolian snowforms mapped are also very similar. Combining both maps, we are able to set out the boundaries of the problematic zones in the studied area for the northerly winds. As an example of these maps, in Figure 3 we present the map resulting from the drifting-snow episode of 10–12 February 1999.

Northwesterly- and westerly-wind ASMs

The April and January maps show the existence of an accumulation zone in the upper area of the ski slopes, above 2315 m (Fig. 4). These areas make up a small basin, situated leeward of the westerly winds, which accumulates significant amounts of snow during westerly-wind drifting-snow episodes. Below this altitude the westerly winds affect only the ski slopes. The main differences between the snowdrifts are in their size rather than their exact location. The accumulation areas are more extensive in January than in April, which is attributed to the availability of snow to be drifted. By combining the northwesterly- and westerly-wind maps we obtain a good delimitation of the erosion/accumulation zones and also a good representation of the surface wind directions for the northwesterly and westerly winds. As an example of these maps, in Figure 4 we present the map resulting from the drifting-snow episode of 10–12 February 1999.

We have determined the accumulation/erosion zones for a specific wind, we divide the ski slope into sectors according to the problems posed by this wind and analyze in detail the wind effects in each sector. Then, knowing the wind adaptation to topography (surface wind directions), we can take action to minimize the wind effects. A series of steps is proposed for avoiding or taking advantage of the wind effects.

In the example presented in this work, we must consider the dominant winds of the region in order to improve the design of the ski slope. When applying the method to the specific problematics of redesigning the ski slope, we must consider as much information as possible. Therefore, we propose a synthesized map for the three principal wind directions, northwest, west and north, summarizing all the information obtained in the previous maps (Fig. 5).

Fig. 5.

ASM for the Dues Estacions area (Tosa d’Mp) for northerly, northwesterly and westerly winds. Application of the methodology of the aeolian snow forms and spatial distribution of snow mapping in the redesign of Dues Estacions ski slope.

Once the ASM is produced, we can propose some solutions to the drifting-snow problem in the area. In this example, focused on improving a ski slope, we propose the relocation of old snow fences and the implantation of new ones.

In Figure 5 we show the boundaries of the erosion/accumulation zones, the average surface wind directions, the proposed new snow fences and the relocation of some of the old ones. The map in Figure 5 is the result of combining the four ASMs and synthesizing the surface wind directions measured from the aeolian snowforms.

Further investigations are needed to improve our knowledge of the effects of the pre-drifting snow surface condition on the final forms and snowdrifts and their spatial distribution. However, the use of these maps by the managers of La Molina ski resort has decisively affected the redesign of Dues Estacions ski slope in the Tosa dAlp sector.