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Author Topic: Arctic sea ice OPTICS  (Read 1205 times)

Nightvid Cole

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Arctic sea ice OPTICS
« on: July 24, 2015, 08:00:07 PM »
Even snow-covered sea ice is not a Lambertian reflector, since at shallower incidence angles not only is the illumination nearer the surface and thus more photons escape without absorption, but also the individual snow crystals scatter preferentially in the forward direction and thus at shallow incidence angles you can get more single-scatter-escaping photons. For these reasons, light at shallow incidence angles has a higher % reflected.

When snow melts and leaves bare ice, there is still a surface scattering layer, though it has a lower air/ice ratio than snow does, but the same phenomena occur.

When you replace it with melt ponds, albedo drops, but a water surface also has higher albedo at shallower incidence angles (see https://en.wikipedia.org/wiki/Fresnel_equations).

Finally, when you have a field of low-concentration but irregular ice, shallow-incidence-angle light is almost certain to hit the peak of a floe before reaching the liquid water, while steep incidence angle light has a good shot at actually hitting liquid surface.

There are several consequences of this. One interesting consequence is that when the sun is low in the sky, clouds may actually *enhance* the absorbed insolation rather than reducing it, because more light is coming from higher in the sky (steeper incidence angles), particularly when the ice is still covered by snow and thus the total illumination doesn't decrease much on account of multiple reflections between surface and clouds.

However, when the sun is at higher angles, clouds of course reduce it.

Another consequence of this is that while the north pole may be getting more insolation than the peripheral Arctic near the Solstice, less energy is absorbed, because almost all of it is coming in at shallow incidence angles. At lower latitudes such as 70N or 75N, the time of day with the most insolation also has the lowest albedo, so the net effect is larger absorbed energy.

A truly accurate understanding of energy balance of sea ice must use proper optics - a single "albedo" figure will never be physically reasonable.
« Last Edit: July 24, 2015, 08:09:37 PM by Nightvid Cole »