Monday, December 13, 2010

Cloud processes and skiing: Part II

In the previous post, we discussed how yesterday's altostratus deck produced a rime crust in the upper elevations.  Here, we're going to take a look at how the altostratus deck led to widespread melting of the snow below cloud based and ultimately the formation of a melt-freeze crust in the afternoon.  This is a tough post, so post a clarification if you feel it is warranted.

I have a personal "rule of thumb" that from mid November to late January the powder will persist on aspects on the north side of the compass even if the free atmosphere temperature climbs above freezing (even several degrees above freezing).  This rule failed miserably yesterday as melting occurred even on mid-elevation north-facing aspects and wet sluffs and rollerballs were found at unusual elevations and aspects for December.

Wet sluffs, Upper White Pine. Photo: Trevor Alcott
Life-threatening rollerball. Photo: Trevor Alcott
The melting of the snow occurred while we were beneath the altostratus deck.  As soon as that deck dissipated, the snow surface refroze, despite little change in air temperature.  Why?  

Radiation typically provides most of the energy used to melt snow (Sicart et al. 2006).  Of concern are two types of radiation.  The first type is shortwave radiation, which comes primarily from the sun.  The second type is longwave radiation, which comes primarily from just about everything else (e.g., clouds, surrounding topography, trees, greenhouse gases like water vapor, etc.).  Energy is available to warm or melt the snow if the short and longwave radiation absorbed by the snow exceeds that emitted.  It turns out that the direct transfer of heat from the atmosphere to the snowpack is not unimportant, but the energy balance of the snow surface is usually dominated by radiation.  This is why you can melt snow in the afternoon on a south-facing aspect even when the temperature is well below 0C.

Longwave radiation is particularly important for melting snow at times when the amount of shortwave energy absorbed by the snow is low, such as when the sun angle is low and he snow is fresh (so it reflects rather than absorbs a greater fraction of incident sunlight).  This was the case yesterday as we were near the winter solstice when the sun angle is low and had just received a coat of fresh snow on Friday and Friday night.

The altostratus deck that blanketed the Wasatch on Sunday morning had a cloud-base temperature of -1 C.  The longwave radiation received from such a cloud is sufficient to warm the snow surface to temperatures near the melting point.  During the morning, the solar radiation that penetrated through the cloud then provided just enough additional energy for melting to occur and ruin the snow even on north aspects.  Longwave radiation is an equal opportunity offender and does not discriminate based on aspect like the sun.

As soon as the altostratus deck dissipated, however, the amount of longwave radiation received by the snow surface dropped precipitously.  As a result, refreeze occurred even though air temperatures were a couple degrees higher than they were during the morning melt.

I suspect that if the morning had dawned with clear skies, then the skiing below the rime crust would have been much better than it was as there would have been insufficient energy to drive snowmelt on aspects on the north side of the compass.  Blame it all on altostratus.

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