Results and discussion

Simulations of the current and future potential snow depths and duration have been carried out over the four winter-types from December 1st to April 15th. The first part of the analysis concerns the snow pattern on the whole in different tourist regions of Switzerland. The second part considers the current and projected situation of snow in selected ski resorts themselves. The possible future shifts in the timing of both the beginning and end of the snow season has not been investigated in the present paper because of its large interannual fluctuation.

2.3.1. Snow patterns in Swiss regions

Figure 2.4a illustrates the current mean snow depth by region and winter-type. The graph shows a large spatiotemporal variability: it ranges from a value of 8 cm of snow depth in the Bernese Alps to almost 220 cm in the Eastern Prealps. It is not necessarily always the same type of winter that implies the greatest snow amount and conversely. But on average, it is the cold winter that is the sparsest season in terms of snow since cold air is usually much drier, whereas the warm one is the snow-richest, owing to more moisture potentially leading to more abundant precipitation. Nevertheless, the Jura receives proportionally more snow during cold winters as snow cover at lower altitude is much more sensitive to even a small change in temperature (Beniston, 1997). Conversely, the snow depth is thicker for the Eastern Prealps during a warm winter because most of the precipitation falls as snow at these heights. However, a direct correlation between the quantities of snow and the mean elevation of an area (according to the height of the weather stations) is not systematic.

In a warmer climate, the situation would be much different. As seen in figure 2.4b, the simulated snow pack is reduced for every region. Moreover, some part of the country – especially the Jura, the Bernese Alps and the Western Prealps – could suffer from an almost total loss of their snow cover, particularly during a warm winter. These first three regions that already currently have little snow are the ones that are the most vulnerable in a warmer climate. No such trend can be inferred for the other areas. It is during a cold winter that the reduction of snow pack is the smallest, which seems coherent, while the contrary applies to warm winters. In comparison, the difference in projected snow amount between humid and dry winters is much less pronounced. This suggests that air temperature is a more dominant factor than air moisture for the accumulation of snow.

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Figure 2.4. Simulation of current mean snow depths by winter-type (a) and simulated reduction of mean snow depths compared to current mean snow depths over an average winter, projections for the years 2071-2100 (b) by region from December 1^st to April 15^th.

The snow patterns simulated by GRENBLS generally match other studies (Beniston et al., 2003; Beniston et al., 2004; Keller and Goyette, 2005; OECD, 2006). The few differences observed with respect to the results of previous investigations probably derive essentially from the small number of weather stations considered here; sometimes one single station represents an entire region. Nevertheless, this first general analysis enables to reproduce the global pattern of snow depth and gives confidence in the model’s ability in reproducing both current and potential future snow depths and duration at the very local level of ski resorts.

2.3.2. Snow patterns in Swiss ski resorts

To have a more precise picture of snow depth and duration in ski resorts, GRENBLS has been driven with the height-corrected input data (see section 2.2.1 for more details). Figure 2.5a shows the results obtained for the current mean snow depth at lower runs, upper runs and critical altitude. The dashed line indicates the 30 cm threshold of natural snow generally considered necessary for an economically-viable skiing industry (Abegg and Froesch, 1994). The ski resorts are classified according to their mean altitude. Except for the stations of CHA and GSB where snow depths seems a little under- and over-estimated by the model, respectively, this figure gives a more accurate view of the current distribution of precipitation in Switzerland. Actually, the first ten stations, belonging to the western part of the country, the Sudbunden and the Eastern Prealps are rather dry and/or low-lying areas where the snow cover is fairly thin, whereas the reverse is true in the other ski regions. The fact that the amount of snow in a resort is, naturally, dependant not only on its altitude but also on its location in the country is highlighted on this figure. For instance, the slopes of ZER have the highest average altitude but are not the snow-richest runs. Topographical characteristics such as exposure or inclination of slopes may also have an influence.

Currently, the snow layer is sufficient on average for skiing in almost every resort on upper runs and in 75% of the stations at the critical altitude. In contrast, it is notable that even at present, snow reaches the 30 cm limit on lower runs during a mean winter only in four resorts. However the snow conditions for skiing seem reasonably good on average in Switzerland, fact which has been confirmed by other studies (OECD, 2006, Müller et al, 2007).

In a simulated warmer climate, the projected mean snow depths compared to the current snow pattern show a decrease in every station and at every altitude (Fig. 2.5b). We can also observe that the resorts with little snow at present are also the most affected in the future.

This is, however, not true for snow at the lower runs where the reduction of snow cover is large everywhere. DAV and ZER are also exceptions: it snows there at present relatively little or seldom but thanks to the high elevation, the gradient of slopes and runs that are well oriented, the projected reduction of snow depths is also attenuated. For the opposite reasons but despite their high altitude, SAM and COV suffer from a larger decrease in snow pack than DAV. In addition, at the critical altitude for 13 of the selected stations, the 30 cm limit is no longer guaranteed during an average winter.

If we consider the current and projected percentage of days with more than 30 cm of snow during a mean winter season at the critical altitude (Fig. 2.6), a clear threshold can be observed between MLS and HIR.

Reduction of snow depth [%]

Lower runs Upper runs Critical altitude 30 cm guaranteed30 cm not guaranteed

Mean snow depth [cm]

Figure 2.5. Simulation of current mean snow depths (a) and simulated reduction of mean snow depths compared to current mean snow depths over an average winter, projections for the years 2071-2100 (b) at lower runs, upper runs and critical altitude.

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Percentage of snow days (>30 cm) during a mean winter [%]

CDF CHASAE FRE

Figure 2.6. Simulated current and projected percentage of days with more than 30 cm of snow on the ground at the critical altitude for each ski resort during a mean winter season.

Indeed, the first six resorts are at the moment those that exhibit the least snow and, at the same time, are the ones most threatened by a warmer climate. Some of them do not encounter a single day with 30 cm of snow, and duration of the snow season is reduced in every resort. The situation also becomes difficult for some resorts in the Bernese Alps and in the Sudbunden, as the percentage of days where skiing is possible is reduced to a great extent. In contrast, the conditions of snow cover are not much affected in HIR, ENG, WFJ or MVE.