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How much snow is lost to sublimation?
How much snow sublimates and is returned to the atmosphere in the form of water vapour? Field measurements, numerical modelling and wind tunnel experiments provide new discoveries.
New knowledge thanks to wind tunnel experiments
An unanswered question in the mass balance of polar regions and the alpine snowpack is: how much snow is returned to the atmosphere in the form of water vapour? The water content of the atmosphere as well as the formation and conservation of powerful ice sheets and glaciers all depend on this evaporation, which is particularly intensive during periods of snow drift.
SLF carries out investigations into this process through field measurements, numerical modelling and wind tunnel experiments. A collaboration of Swiss (SLF) and Japanese scientists provided a first breakthrough in the understanding of the sublimation processes, by quantitatively reproducing them in a wind tunnel experiment. This experiment showed that the influence of snow drift is far larger than described by previous models.
Particularly large quantities of vapour are produced when strongly dentritic (ramified) snow crystals are present, in addition to strong solar radiation periods. Initial model calculations showed that during periods with medium wind speeds (5 ms-1) and/or medium relative humidity (70 %), approx. 1 cm of snow can sublimate per day. Scientists are now trying to improve understanding of the mechanisms involved in the sublimation and hence provide better forecasts on the role of sublimation, particularly in mountainous areas.
Wind tunnel experiments of drifting snow sublimation
Experiments in a cold wind tunnel are used to verify drifting snow sublimation models. A layer of drifting snow particles is formed over a sintered snow surface. Sublimation and drifting snow flux is estimated from two vertically resolved profile measurements separated along the flow path and compared to a simple, one-dimensional diffusion model of the drift and drifting snow sublimation. The experiments show an increase in water vapour content of the air from drifting snow sublimation. The measured drifting snow sublimation effect appeared to be larger than theoretical values found in the model study. Under wind tunnel conditions, particle number density appears to be the controlling factor on the sublimation rate. For experiments with external solar radiative forcing, the increase of the sublimation rate is larger than theoretical predictions. The experiments suggest that irregular snow crystals and solar radiation might increase the sublimation rate more than described by many drifting snow models. For more information see Wever et al. (2009).
Simulations of drifting snow sublimation in mountainous terrain
The introduction of drifting snow sublimation in the snow surface-processes model Alpine3D enabled scientists to estimate how much snow can be lost in mountainous terrain. They performed several simulations for the Wannengrat field site. The model results show that there is a high spatial variability of this process and that only in particular slopes the loss of snow due to drifting snow sublimation is relevant. However, they also show that only in short periods with weather conditions favourable for drifting snow (for example Föhn storms) loss of snow due to sublimation can be up to 10% in these particular slopes. Over a complete season and averaged over the test site, drifting snow sublimation is, according to the Alpine3D simulations, negligibly small compared to the total snow water equivalent of the snowpack.