~ Mid April top centimeter of compact snow takes about 5 days to vaporize
~ Top 1 cm may have wind polished very thin ice only seen by sun reflections
~ Winter, importance of inversions
A mid April day top of snow crust hardened by many days of high winds causing substantial sublimation:
The great snow cover of 2017 made it difficult to measure sublimation rates, while windy it snowed as well, the fields of snow changed shape daily:
There can be several layers of snow crusts in one column, each crust can be 500 to 600 kg/m3 dense as opposed to the entire column being 400 kg/m3. Within this top denser crust, the snow temperature was always colder than standard 2 meter air because of sublimation, this so happens as as long as there is a snow cover until the sun really is high in the sky. The solid small snow crystals to water vapor process requires a great deal of energy to happen, this energy is detected by the temperature drop within or on top of the skin and by conduction on the air immediately above it. Unfortunately, sublimation can only be measured accurately with lab conditions, the Arctic outside is loaded with varying weather, making sublimation appear different with each possible weather scene, when in fact it is rather a continuous process.
Briefly by the numbers,
A 1 cm top of snow column has a density of 500 kg/m3, there is 5 kg in that layer, it takes
3013 (latent heat of sublimation) w/gr X 5000 gr = 15.1 million Watts to sublimate it.
Given a solar constant 1360 w/m2 and given that the atmosphere absorbs 23% , albedo on a thick and dense snow layer varies between 80 to 90%,
on a perfect clear April 24 sky day: 5.68 MW/m2 can be absorbed by the top 1 cm skin
But not all of is absorbed, as with figure 2.23:
A typical 74.5 degrees latitude North High Arctic day top 1 cm snow may absorbs about 2.84 MWatts per meter square. Therefore it would take 5.3 days for the top cm to evaporate by direct sun radiation alone, which has been observed as such, but sublimation heat comes from potentially many other sources, from the warmer snow, the warmer air, back scatter from clouds, heat from ground or sea ice, by winds drawing out the heat within the snow or ground column. It is also very difficult to measure temperature at the surface to air interface due to UV affecting thermistors (coming essay).
Sublimation is one of the main contributors for near ground or sea ice permanent winter Inversions
To maintain a loss of temperature of 1 degrees C within the same top 1 cm of dense crystalline snow, 10,000 watts per square meter would be required, this is clearly not happening. I have observed more like a permanent cooling of .1 to .3 C of the air immediately off top of thick snow column, this means the thickness of snow absorbing heat, rather sublimating, is very shallow, vaporization is actually happening in more like terms smaller than a millimeter, like the shrinking size of the crystals themselves with their micro-surface and total entity vaporize, if we consider 1 mm surface, meaning 500 grams per square meter, it would still take about 90 watts per meter square of energy to drop the surface temperature by 0.1 C. This is what is likely more realistic.
During spring time, when the ground or ice surface becomes much warmed, the top of snow sublimation should be stronger. Thermally speaking this sublimation cooling is eventually overtaken by strong sunshine as observed at the sea ice horizon, sublimation occurs but there is more external solar forcing masking its signature optical effect. During the long night, absent of solar effects, with no shortwave radiation, it would be sensible to believe that top of snow or sea ice temperature would be greater than air right above on most occasions, especially absent warm air advection, since the only source of greater heat is from the covered by ice much warmer ocean, that is not the case, in fact as soon as sea ice covers sea water completely the entire ocean horizon sustains a higher height than Astronomical Horizon (A.H.) until "first Melt Day", till well into spring following the long Arctic night :
Low surface thermal inversions have a huge impact over weather, they stop surface moisture from rising reducing cloudiness causing more over all cooling to space, they create, no, they are the reason for winter to exist. When they vanish it is a sign of summer. When there is a lot of snow on top of the ground or sea ice, this literally further cools temperatures, with solar heat already reflected back up by very white snow albedo, also made further colder by snow sublimation. Although exact temperature numbers about this subject are very difficult to be precise with, a small temperature drop on the surface may implicate a very large over all cooling. WD April 28, 2017