sorry for the hiatus - but I'm just back from Red Rocks.
Adam:
Well an interesting topic that should be near to everyone's heart, why the end of the "POW"? I am grad student that does a bit of snow energy balance modeling and a few things come to mind when spring melt is about to set in. First spring storms typically have warmer temps, the snowfall at the warmer spring temperatures (usually near zero) take little energy to metamorphosis or melt. These warmer temps would also lead to larger crystals. This is especially important in the near infrared wavelengths. This increases the net energy input to the snowpack, promoting surface metamorphosis and melt. Second, spring storms typically fall on crust layers. These crust layers would impede percolation of free water in the upper layer of the snowpack, hence making the powder, heavier. Also the underlying snowpack is near or at isothermal, so the lower snowpack would exchange energy to the new snow, typically warming it.
I agree with what you are saying here, Adam. One of the problems is also insulation of the deeper isothermal layer. But, even when the crust re-freezes before dry new snow falls and the surface temperature of the dry new snow stays sub-freezing .....i.e. dry on it's surface, unless the sky is clear on a north slope the new snow rapidly seems to turn to junk (partially cloudy skies or higher humidity). Most recently this happened at hurricane ridge a few dsays before this posting. The new snow was dry at the surface, but got progressively wetter at depth as the day went on. The surface dry snow stayed dry, but underneath - yuck! - I like the term "elephant snot". Now, in as much as as there was about 2', 60cm, of new snow and studies (I've seen) seem to indicate UV radiation is unable to penetrate more than about 50cm - 4 hours exposure as I recall - the effect deeper in the new snow layer shouldn't be UV penetration. So how about long wave radiation, Adam? Any feel about the depth that longwave radiation can penetrate new snow? The effect I'm curious about is a high humidity/cloudy day effect.
In relation to sun angle, the lower the solar zenith angle the lower the albedo, increasing the net energy input. So our longer days give us solar radiation at lower angles.
Definitely.
Sensible heat exchange from warmer air temps can also not be neglected, however these exchanges will be small if there is little air movement. But the larger the temperature gradient the more sensible heat exchange between the air and the snow.
That's interesting, Adam. But I guess that makes sense. But, here I'm talking about a situation that often starts at the bottom interface, and works upward (dry surface snow - so sub-freezing surface temperature)
The one EB component that one may glance over is the thermal radiation from the atmosphere. If atmosperhic temps are high (in spring) than more moisture can be held. The more moisture and higher temps up there the more thermal radiation down here. So if we have longer days, warmer temps, we also get more thermal radiation. Solar angle does not affect the thermal radiation, only how much sky a slope sees. So find a slope that is north facing and has a limited skyview...that's where the powder is..
Good insight.
Hey, I've skied lot's of days with good dry snow past March....over the years. I recall fantastic powder in April a number of times (several times with you Lowell) , once on May 5th, once on May 18th, and June 20th and even July 8th. Heck, even in the Sierra on April 25th and May 3rd. But the skies are almost always clear when this happens. Certainly it is more likely to become cloudy in the spring - especially on the west side of the crest - because there is more solar radiation and with a westerly flow, more convection. East of the crest, cloud cover from convective build-up is somewhat less common. There is also higher humidity west of the crest which absorbs and re-emits a greater amount of long wave radiation to the snow surface, warming it.
But what I'm proposing here is likely related to long wave radiation, but can there also be a critical sun angle for penetration of cloud cover? Bear with me, suppose you develope a cloud deck (likely broken), but for simplicities' sake - model it as thin, solid-white, and completely planar on it's upper surface (with clear skies above). Is it possible that there is a non-linear solar reflectivity (penetration = 100% -reflectivity and scatter within the cloud layer) such that up to a certain solar angle of incidence with the top of the cloud deck, most radiation would reflect back to space, but that as the sun reaches a certain angle (around March 20th) that the function describing radiation penetration becomes dramatically non-linear?
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