Both orography and wind strongly affect the spatial distribution of snow depth in mountainous regions. Fohn (1977, 1985) addressed the question of representative new snow depth measurements in mountainous terrain. Clearly, crests, wind- and leeward slopes as well as depressions have to be avoided. Even at the most suited level sites, the catch efficiency of precipitation gauges (60-80%) does not closely match the reliability of a snow board laid on the snow surface. Even so, Fohn (1977, 1985) states that precipitation gauges located in surrounding valley floors may help to extrapolate new snow depths over larger areas in wintertime; very often one has to rely on available weather forecast outputs as well. By combining the latter with statistical and climatic approaches, promising results have been obtained (Durand etal., 1993; Raderschall, 1999).
Yet for many applications redistribution by wind on leeward slopes has to be taken into account. Fohn and Hachler (1978) give an empirical relation for a 35° steep leeward slope as:
where HNw is the mean additional new snow (in m) deposited on the leeward slope over a time period of 24 h, c2 = 0.8 x 10-4 (s3 m-2) is an empirical coefficient and Mcrest is the mean wind speed measured on the crest. Despite the complexity of both modeling and measurements, blowing and drifting snow in heterogeneous terrain is currently a topic of strong interest (Pomeroy, 1991; Liston and Sturm, 1998; Pomeroy etal., 1999c; Durand etal., 2001; Michaux etal., 2001). Modeling blowing snow over a steep alpine ridge, Gauer (2001) obtained a power relation for the simulated steady-state flux, decreasing from approximately 4 to 2 with increasing wind speed. The mass flux decrease with increasing wind speed further indicates that a saturation of snow transport occurs. Combining a saltation model (Doorschot and Lehning, 2002) with an analytical wind profile taking into account speed-up effects over a ridge, Doorschot et al. (2001) observed a similar trend. The latter authors also included preferential deposition of precipitation as a third mode of snow transport to the lee slope. Preferential deposition does not require any threshold value to be exceeded to occur and may thus be predominant under certain conditions in steep Alpine terrain.
Finally, on the scale of a few hundred to one thousand square kilometers, Fohn (1992)as well as Martin etal. (1994) studied the impact of climate on snow depth in alpine regions. Such compilations provide valuable information for governmental agencies, hydropower facilities, winter tourism, and flood mitigation.
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