Snow Accumulation and Ablation
on Boards of Different Sizes and Shapes
Rolf Pfister1 and Martin Schneebeli1
Snow accumulation and ablation on trees play a major role for the structure of the snow cover in
snow hydrology and for protective effects of boreal and subalpine forests against avalanches. To
better estimate this impact we examined the processes of snow accumulation and ablation on boards
of different sizes and shapes. Boards were selected because real branches are too difficult to measure
and model the complicated processes of interception. For this purpose we exposed the boards to
natural and manmade snowfalls. After each snowfall, we measured the accumulated snow water
equivalent on the boards as well as different snow characteristics and meteorological conditions. To
investigate the ablation processes, we exposed the boards in an environmental chamber that allowed
us to control the temperature and the short- and longwave radiation.
The observed snow interception efficiency increased with board width for snowfalls at a mean air
temperature below 3C. This is explained by the decreasing rebound of the snow crystals near the
border. At temperatures above 3C the snow interception efficiency was independent of the board
width. The bridging of the snow between boards increased strongly for snowfalls at higher temp-
eratures close to the melting point. For wet snowfalls bridging occurred even at distances of 10 cm
between boards. The inclination and the shapes of the boards had a significant influence on the
amount of accumulated snow. Based on the measured accumulation and meteorological data, we
derived statistical models for the accumulation of snow. In addition to precipitation, the maximal
wind speed exerts a significant influence on the snow accumulation.
In the environmental chamber we determined the sublimation rate in relation to different temp-
erature and radiation regimes. We calculated sublimation with a physical model based on the subli-
mation rate of a single ice sphere and a model of the exposed numbers of ice crystals. Under action
of a light source that emits short-wave radiation similar to the sun, we observed an increased subli-
mation rate compared to the model. The shortwave radiation transmitted through the snow and the
subsequent absorption by the boards caused a substantial heating of them and accelerated the subli-
mation rate.
Our investigations showed that size and shape of branches are essential for snow interception, espe-
cially for snowfalls at cold air temperatures. The next step will be to transfer the accumulation
models to real branches and trees, considering the particularities of bending branches and the wind
influence. The incident solar radiation is an important energy source of ablation processes and con-
sequently has an important influence for the snowpack and hydrology in forested areas.
1 Swiss Federal Institute for Snow and Avalanche Research, Fluelastrasse 11, CH-7260 Davos Dorf, Switzer-
land
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