Overview of observations

The snow depth distribution at the end of the accumulation period of two successive winters 2007/08 and 2008/09 is shown in Figure 4.2. Both dates correspond very well with the date of maximum snow depth (HSmax situation) at the snow station, although one month later observed in 2007/08.

The snow depth distribution in the three slopes investigated showed different characteristics, which can consistently be observed in both winters: a homogeneous distribution in the lee slope (C), an inhomogeneous distribution in the cross-loaded slope (B) and less snow in the the windward slope (A). This impression is confirmed regarding simple statistics as mean (µ) and standard deviation (σ) as summarised in Table 4.1.

In more detail, the cross-loaded slope (B) was dominated by two accumulation zones with snow depth over 6 m, while mostly no snow between these zones and in the upper area was observed. These accumulation zones developed behind two cross-slope ridges of summer terrain, which are orientated mostly normal to assumed wind direction from NW. In the lee slope, maxima of snow depth were found in thin parallel bands orientated along the largest relief, which corresponds to depressions in summer terrain. The mean snow depth was very similar to the snow depth measured at the sheltered snow station. In the third slope investigated, the windward slope (A), maxima of

4.3 Results

Figure 4.2: Snow depth distribution at the end of the accumulation period in two winters, (a) 26.04.2008 and (b) 27.03.2009.

Table 4.2: Mean µ and standard deviation σ calculated for snow depth change due to the NW storm shown in Figure 4.3d

17.03.2009 [m]

Area µ σ

Windward slope (A) -0.04 0.12

Cross-loaded slope (B) 0.17 0.32

Lee slope (C) 0.63 0.32

snow depth were found in the lower part of the slope, where wind speed should be less than in the upper part, and in a depression next to the marked hiking trail.

The snow depth distributions presented in Figure 4.2 suggest a large interannual consistency be- tween both winters. However, the years differed in average snow amount (see also Table 4.1). In the cross-loaded slope half a meter more snow on average was deposited in the first year, while the variance was comparable. The obvious differences in snow depth distribution in the lee slope were caused by a large avalanche in the second year.

Snow depth changes (dHS) caused by individual NW storms are shown in Figure 4.3. These events produce deposition patterns comparable to each other and to the patterns discussed for the HSmax

situation: a variable cross-loaded slope, in which the previously mentioned accumulation zones are noticeable, homogeneous loading in the lee slope and erosion in the windward slope (Table 4.2). In the upper parts of the cross-loaded and in most parts of the windward slope erosion dominated during NW storms. Snow was deposited in these areas during precipitation periods of different characteristics, which was eroded during NW storms. The snow depth loss in the lee slope of the period shown in Figure 4.3b was the result of the large avalanche mentioned previously with a starting zone width of 600 m. The homogeneous loading of that slope during NW storms might favour the existence of a widespread failure layer, which allowed fracture propagation over such large distances.

The similarity of NW storms and the HSmax distribution for both years indicates that the distri-

bution at the end of the accumulation period is dominantly influenced by one typical event. The processes during these NW storms seem to be the major driving factors influencing the snow depth distribution at the end of the accumulation season. Consistency between years was also recognised by Deems et al. (2008) for two study areas. They concluded that these major driving factors are consistent between years. Moreover, the similarity of typical NW storms indicates that processes influencing snow distribution remain similar throughout an accumulation season. Differences in wind distribution, changing snowpack properties and altered surface structure due to snow deposi- tion seem to be underpart.

4.3 Results

Figure 4.3: Snow depth change of NW storm periods between (a) 20.11.2008 and 27.11.2008, (b) 04.02.2009 and 04.03.2009, (c) 04.03.2009 and 17.03.2009, (d) 17.03.2009 and 27.03.2009.

Figure 4.4:Snow depth change of (a) a SE storm between 28.01.2009 and 04.02.2009, and of (b) a homogeneous snow fall with low wind speeds between 14.01.2009 and 22.01.2009.

observed during the accumulation season 2008/09. Figure 4.4 shows two of those events, (a) a SE storm without precipitation and (b) a snowfall during low wind speed conditions. During the SE storm snow depth changes were small and close to the accuracy of the laser scanner. Most erosion was observed in the slope (A), which was consistent with observations of snow drifts as dunes and sastrugi only in this slope. These observations were confirmed by measurements of another SE storm (not shown). During low wind speed conditions (Figure 4.4b), a rather homogeneous snow distribution was observed in comparison to NW storms.

Figure 4.5 shows additional measurements restricted to the cross-loaded slope. In (a) the summer terrain is presented and in (b) and (c) the first snowfall events in two different years. In both cases the large influence of the rough summer terrain is recognisable. However, both events show a very different behaviour. While the first snowfall in 2008/09 (c) is an example for homogeneous loading during low wind speed conditions, the first snowfall in 2009/10 (b) revealed characteristics of a NW storm. The period shown in Figure 4.5e seems to be largely negatively correlated to observations during typical NW storms and to the preceding period shown in (d). As discussed later, possible explanations for the negative values in snow depth change are erosion, sublimation or settling.

4.3 Results

Figure 4.5:(a) Hillshade picture of summer terrain in cross-loaded slope, (b) first snow fall in winter 2009/2010, (c) first snow fall in winter 2008/2009, snow depth change between (d) 27.11.2008 and 23.12.2008, and between (e) 23.12.2008 and 14.01.2009. Same colour coding as in Figure 4.3 and 4.4.