How much snow is lost to sublimation?
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 evaporation 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 as well as during strong solar radiation periods. Initial model calculations showed that during periods with medium wind speeds (5ms-1) and/or medium relative humidity (70%), approx. 1 cm of snow can evaporate per day. Scientists are now trying to improve understanding of the mechanisms involved in the evaporation and hence provide better forecasts on the role of evaporation, 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 vapor 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).
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Figure 1: Pattern of
snow sublimation during drift over the Gaudergrat mountain ridge
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Wever, N., Lehning, M., Clifton, A.,
Rüedi, J.-D., Nishimura, K., Yamaguchi, S., Nemoto, M., Sato, A., 2009.
Verification of moisture budgets during drifting snow conditions in a cold wind
tunnel, Water Resources Res., 45, doi:10.1029/2008WR007522.
Lagrange model development to describe drifting
snow and drifting snow sublimation
A
Lagrangian dispersion model is developed to correctly describe the transition
from saltation to suspension (modified saltation) and to allow the
reconstruction of small scale erosion and deposition patterns. The approach is
based on a modified Langevin equation, which can take into account partly
resolved turbulence from a LES model. A new feature of the model will be that
heat- and mass transfer equations will be coupled to the transport equation to
allow the calculation of sublimation during snow transport. This approach will
improve current estimations of sublimation of drifting and blowing snow, which
heavily depends on the particle number density. Using standard models of meteorology
(ARPS) and advanced LES codes, the influence of mean flow and resolved
turbulent eddies on the snow distribution will be studied.
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