More proof that rain doesn't really melt snowpack

Here is a simple estimate of the amount of snow melted by cooling rainwater.

The ratio of the heat of fusion of water and the specific heat of liquid water is approx. 80 celcius deg. (from tables on the wikipedia pages for these quantities) so that an ice-water mixture (wet snowpack) will need to melt ~1/8 kg of ice to cool 1 kg of rainwater 10 celcius deg. If we assume the density of 0.5 for the snow as suggested by Amar above, each cm of rain 10 degrees warmer than the snow will melt at most 1/4 cm of snow. Smaller temperature differences lead to less melting.

In general I disagree that rainfall doesn't have enough energy to melt snow. The rain is providing energy to the snowpack. However, the amount of energy certainly varies signficantly by event. Both the amount of energy the rain has when it hits the snowpack (temperature driven) and the snowpack structure play important roles.

OK, not sure why you disagree with something which is just verifiable fact. The amount of energy in the rain may vary, but it varies only from totally negligible to very small, and not much above that. So it doesn't matter how much of that minimal energy actually gets transferred to the snow.

Let's assume the maximum situation where 100% of the energy in the rain does get transferred to the snow, that all of the rain fully cools to near the freezing point and dumps all of its heat as it flows through. If the rain is at 1 °C (34 °F), those 8" of rain which fell can only melt 0.1" of SWE from the snowpack (too small to measure). If the rain is at 5 °C (41 °F), it would melt 0.5" of SWE (about 1" of settled snowdepth), and even if it falls at 10 °C (50 °F) which is about the maximum temperature for rain on mountain snowpack, it would only melt 1" of SWE (2" of settled depth).

And this is with an absurd amount of rain, 8" is highly unusual. Given more normal amounts of heavy rain (say 1-2", which is still very heavy rain), it's going to melt only a fraction of an inch of snow regardless of its temperature (within normal bounds). Other agents are much more effective in melting snow, strong warm winds being the worst, but rain just can not do it effectively.

Here is a simple estimate of the amount of snow melted by cooling rainwater.

The ratio of the heat of fusion of water and the specific heat of liquid water is approx. 80 celcius deg. (from tables on the wikipedia pages for these quantities) so that an ice-water mixture (wet snowpack) will need to melt ~1/8 kg of ice to cool 1 kg of rainwater 10 celcius deg. If we assume the density of 0.5 for the snow as suggested by Amar above, each cm of rain 10 degrees warmer than the snow will melt at most 1/4 cm of snow. Smaller temperature differences lead to less melting.

Then, too. Much of the water probably doesn't make it down through the snowpack except near trees or rocks that quickly. What happens to a lot of that water is that it descends in a meltwater column until it hits a mostly impermeable layer it can't penetrate or a weak, porous layer (surface hoar or facets most likely) that it then saturates. So, it flows laterally for a while at various depths and begins to refreeze as it encounters snow that is subfreezing (normal winter snowpack). In the process of refreezing, it gives off heat (the latent heat Micah is talking about) which warms the surrounding snowpack to temperatures nearer 0 degrees Celsius. This refrozen snow (usually the top surface thereof) is hardened in this process and for a while inhibits the vertical flow of meltwater. This process repeats. So, in a sub-freezing snowpack most of the heat of the rain is used up in simply warming the deeper snowpack. To melt it, as Micah points out, takes a lot more meltwater and a warmer snowpack - and even then, the effect is limited.

The structure of the end product of meltwater flow and refreezing is a bunch of vertical meltwater columns connected to horizontal (slope parallel) knife hard ice lenses which exist at different depths in different places (except near porous layers or ice layers where the end product is more uniform).

So, it flows laterally for a while at various depths and begins to refreeze as it encounters snow that is subfreezing (normal winter snowpack).

From what you just said, it sounds like rain around now would mostly just add to the SWE of the snowpack, instead of draining out. But Amar suggested it was obvious that rain would not add much to the SWE (implying it would drain out).

Well there's another thing that can happen too: those knife-hard ice lenses at various depths in the snowpack that Gary mentions can effectively bridge across the antifreeze-filled pillow which measures SWE ( www.wcc.nrcs.usda.gov/factpub/sect_4b.html ). This can affect the reading of any additional weight added to the snowpack above the bridging layer (whether the new weight is snow, or water held in the snowpack). This can delay the recording of new snowfall by several days sometimes, you can see the SWE slowly rising towards an equilibrium value for days after a snowstorm has ended in some cases.

So it could be any or all or some combo of these various effects (rapid draining, bridging, etc) which results in only 0.8" of SWE at Buckinghorse. In addition, the daily precip total of 12+" is almost certainly too high, since 7+" was recorded in only 4-5 hours. See the other thread for an explanation of such glitches in the precip readings at SNOTEL sites.

www.callatg.com/~nsps/weather.htm

Down from 124" just a few days ago.



Fortunately pleanty of base for future corn crops