On the mechanisms of sensible heat transfer between snow and a cold atmosphere
Abstract
The mechanisms of heat exchange between the atmosphere and the snowpack during cold, stable periods are imperfectly understood. Air flow over smooth snow surfaces under weak synoptic forcing creates weak or intermittently turbulent mixing conditions, strong thermal stratification, interaction of non-turbulent motions, and the possible decoupling of the boundary layer from the surface. These conditions pose serious challenges for modeling snow thermodynamics (including melt) and snow-atmosphere interactions. Numerous modellers have discovered that realistic energy balance simulations are extremely sensitive to the nature of the stability corrections used, and in many cases have found it necessary to include a windless exchange coefficient in order to maintain some sensible heat transfer to the snowpack during periods of low mechanical mixing. Snow energetics under cold atmospheric conditions in mid-winter were investigated by making direct measurement of all the snow mass and energy balance components at a homogeneous, flat, and open snow field near Saskatoon, Saskatchewan, Canada. The energy balance was dominated by the net radiation flux, which was usually negative due to longwave radiation losses during clear sky conditions. Owing to the aerodynamically smooth surface and often stable conditions, the measured turbulent heat fluxes were very small (much smaller than the net radiation flux). Surface temperatures declined dramatically under strongly negative net radiation conditions and were poorly coupled to the atmospheric temperature, however the near-surface internal temperatures of the snowpack were not observed to cool significantly, but rather they tracked near to the air temperature. In order to maintain the observed thermal conditions of the snowpack, the true sensible heat flux to the snowpack would have to be much larger than that which was measured by eddy correlation technique and could be expected to closely mirror the net longwave radiation. The observed coupling between the atmosphere and the snowpack interior clearly indicates a sensible heat transfer mechanism which is responsive to the radiation forcing, but can operate under very weakly turbulent conditions. This mechanism is neither described by current turbulent transfer formulations nor measureable with eddy correlation systems. A new hypothesis of sensible heat exchange between the atmosphere and a permeable snowpack is presented, in which the conditions of the interfacial layer of the porous snowpack are theoretically considered. In combination, penetration of air flow into the snowpack, an inherently large surface area for heat exchange, and thermal non-equilibrium conditions all play a dynamic role in allowing the radiation balance to control the sensible heat flux. Well-developed turbulent conditions are not required to support this exchange. Approaches to calculating these processes are compared to observations of snowpack thermodynamics and atmospheric boundary layer fluxes and states.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.C33B0494H
- Keywords:
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- 0736 CRYOSPHERE / Snow