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Boundary-layer dynamics and snow melt

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What happens at the boundary between the snowpack and the near-surface layers of air? How does the energy balance of a thinning snowpack change? With a view to answering questions such as these, we are using high-resolution turbulence measurements to examine the interaction between the snowpack and the atmosphere.

 

Diverse and complex types of interaction take place between the snowpack and the atmosphere. In particular when the snowpack is melting and the ground is no longer completely covered with snow, local processes are initiated that can either assist or impede melting.

Exchange between snowpack and atmosphere in wintertime

Freshly fallen snow reflects a large proportion of the incoming shortwave radiation (as much as 95 %)  back into atmosphere. No other natural surface has such a strong albedo effect. Even though the reflected proportions decrease as the snow ages, its high albedo continues to maintain low air temperatures above the snow. At the same time, snow also emits longwave radiation very effectively. On clear nights, this cools down the snowpack surface to very low temperatures since there are no clouds to reflect the released energy back to earth.

The comparatively low temperatures at the snow surface have an indirect effect. As a rule, the turbulent flux of latent heat (energy trapped in the air by water vapour) above snow is low and often changes sign as the day progresses. During daytime the direction of flux is mostly upward, away from the snow, and the surface is thus cooled by sublimation. During the night the direction of flux is frequently reversed, from atmosphere to the surface. If the air is sufficiently humid, cooling causes the water vapour to deposit directly into surface hoar crystals.

An important exception occurs when warm, saturated air masses gather above a snow surface, typically when rain is falling on the snow. In such situations large quantities of additional moisture condense on the relatively cold (0 °C) snow surface and thus provide more melt energy.

At nighttime in particular, when the snow surface cools substantially, a positive temperature gradient exists above the snowpack, i.e. the air is warmer than the snow, which triggers a turbulent sensible heat flux in the direction of the snowpack. The temperature of the snowpack increases thereby, and the cooling effect of the outgoing radiation is diminished. Since the turbulent fluxes are a function of average wind speed, the snowpack warming is especially pronounced when the wind is strong.

Particularities of the snowpack energy balance during the melting phase

When air temperatures rise above freezing in the spring and the surface temperature reaches melting point, stable atmospheric stratification is generated above the snow. In case of low wind speeds, the near-ground layers within the stable stratification can become detached from the main current, which has the effect of significantly impeding the turbulent heat exchange close to the surface. We have concluded from initial studies that such phenomena are the reason for locally diminished snow melt and give rise to the endurance of individual patches of snow in the spring and even year-round snow patches in the high Alpine regions.

Generally speaking, radiation dominates the energy balance above snow but, in areas exposed to the wind, turbulent sensible heat fluxes can contribute significantly to snow melt. When the blanket of snow gradually thins and exposes bare ground in the spring, the energy balance changes as well. The warming pattern differs in bare and snow-covered areas. The transfer of warm air from bare to snow-covered areas triggers a local increase in heat transfer to the snowpack and therefore higher melt rates at the edges of the snowy areas facing the wind.

Large patches of snow can also have an impact on the immediately local wind system. We used an atmosphere model to show that relatively large snow-covered areas substantially reduce the temperature of the near-ground layers of air when there is no wind or only light wind. The cooling effect is so pronounced that localised katabatic wind systems can develop (Mott et al., 2015). The near-ground wind speed increases as a consequence, and the turbulent sensible heat exchange increases as long as such weather conditions prevail. If the extent of the snow cover decreases, the influence of katabatic wind systems on the mean heat exchange to the snowpack is likewise appreciably diminished.

In the context of the Dischma experiment we are measuring and quantifying the processes described above. In order to enhance our understanding of the snowpack's interaction with the adjacent atmosphere, we are addressing the following research questions:

  • How much energy, in the form of sensible heat, is transported from bare areas to the snow? How does this quantity change, depending on the snow cover?
  • How does the turbulent heat flux above the snow change as a consequence of atmospheric stability?
  • What complex wind systems arise from the simultaneous occurrence of downslope and upslope winds, and how do these influence the energy balance of the snowpack?
 

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