Steve Markstrom: Hello.
Continuing on with the PRMS training video series, talking about the snowcomp module
here.
This satisfies the snow process and this is the only module, so there's no choice.
This module develops the snow pack, simulates the accumulation, depletion process and computes
the energy for melting the snow, and that's what we're going to go through.
Properties of snow -- snow is a porous media.
Snow does change through time.
The albedo changes with time.
The density increases with time, and snow does have the ability to store melted water,
free water.
In PRMS, we model the snow pack as a two-layered system.
Since PRMS runs on a daily time step, we divide the day into two twelve-hour periods, basically
night and day, the warm period and the cold period.
The snow pack exchanges heat between the two layers, and also, mass, basically in the form
of liquid water, moves down through the top to the bottom.
The temperature of this surface layer is based on the air temperature.
It's either the air temperature or zero, if the air temperature is above zero.
Then the temperature in the pack, the majority of the snow down here is actually computed
with a heat balance.
If you took this whole depth of snow and melted it, that would be the pack water equivalent
depth.
What are the steps, the computational steps for the snow pack on each HRU?
Basically the first thing that happens is any new precipitation is added to the snow
pack, then the snow-covered area is computed based on the pack water equivalent, albedo
is computed, the depth -- and that's the actual depth -- the pack density is computed, the
energy for the night, the energy for the day, then sublimation, and finally, any melted
ice is computed.
Precipitation falling on a snow pack - If snow falls and a pack doesn't exist yet because
no snow has fallen or whatever snow there was has melted, then a new pack is initiated,
and if rain falls, it's added as melted water to the pack.
You can see there's a few parameters, freeh2o.
This example here, you can see we've got some liquid water inside of our snow pack.
All right, depletion curves, these are pretty important to PRMS.
Along the x-axis, we've got the fractional amount of snow water equivalent, and then
on the y-axis we've got the snow-covered area.
If you really are concerned about this once the pack moves into the melt phase, during
the melt season, or anytime the pack is melting, basically you start out here and you move
in towards having less pack water equivalent and less amount of snow-covered area as the
pack melts off.
Really, the idea here is to describe the driftiness and the patchiness of the snow.
If you think about it, if an inch of snow falls on a parking lot and the sun comes out,
it's all going to melt off in one day.
But if that same amount of snow gets plowed up into a big snow bank, that could last weeks
or months, the same volume.
It's just this relationship between snow water equivalent and snow covered area is different
and it really determines how things melt off.
So here's an example of a couple different shapes of depletion curves.
Typically if you have a forested area, your snow-covered area can go all the way up to
100 percent cover because the trees can keep the snow from drifting.
In the case of Alpine, above Timberline, a lot of times no matter how much snow you'll
get, you still have patchiness because the wind will redistribute the snow into snow
banks.
Another important aspect of the snow module in PRMS is this idea of albedo, which is basically
the reflectiveness of the snow.
The whiter and the more reflective the snow is, the less solar radiation, short wave radiation,
the less effect that energy is going to have on the snow pack because it's reflected away.
You see a real difference here.
This plot is showing that the reflectiveness is much lower during the melt period than
it is when the snow is fresh, white and finer crystals.
Once the snow starts melting, it melts a lot faster because it absorbs more energy.
You can see the idea, how PRMS is doing this.
Number of days since the last time it snowed, basically, once you're in the melt phase or
if you're in the accumulation phase up here.
Here's some equations from the PRMS users' manual.
You can see these bold, den init and den max, those are parameters, and what we're trying
to do is compute the actual pack depth.
It's not the water equivalent, but that's the actual depth of the snow.
This describes basically how the snow is settling through time.
If you've got an aging snow pack, that's what this delta pack depth is.
So PRMS is computing the actual depth of the snow.
We're getting ready to look at some of the energy equations in PRMS.
You can see all of the different terms we take into account in the energy balance.
This is kind of the full energy equation.
I'm not going to go through all this.
But you can see there's all these different sources of heat that can be putting heat into
the snow pack for our melting snow.
If these terms are negative, then it's going to be heat moving out of the snow pack and
the snow pack actually getting colder.
In some form or another, you have to kind of keep track of all this stuff.
When we're looking at the heat through conduction -- that was one of the terms in the previous
equation -- we're using this formulation from Anderson.
It's a temperature index.
Basically if you give it the temperature, it'll tell you the heat being conducted into
or out of the snow pack according to this.
All this stuff is the thermal conductivity.
If we look at these terms, here's the basic form of the energy balance when the pack is
in melt or getting ready to melt some snow.
This first term here is the short-wave radiation.
We've got this equation, which the DD or CC solrad, this albedo term, depending on how
long it's been since it snowed, and then here's our parameter here, rad transmission coefficient.
Solve this.
This gives you this heat short wave term.
The heat long-wave term is this down here, you can see.
It's kind of a black-body type idea where the air and the vegetation and the snow itself
is emitting long-wave radiation, and that's where those different terms come from.
This is condensation and evaporation terms.
There's different forms depending on if you've got a lot of moisture in the air, based on
the fact if it's precipitating or not, the amount of heat that's in the precip.
Basically, if it's raining on a snow pack we assume the rain is coming down at the temperature
of the air.
This final term is the conduction from the snow pack to the ground.
In PRMS, we assume the snow pack and the ground are at the same temperature, which is usually
a pretty good assumption.
If you're really interested in this kind of stuff, I suggest you go read the users' manual
where these things are really spelled out in a lot more detail.
Once you've got all your heat figured out, that's this term here, t-cal, the number of
calories going into the snow pack, and here you can see snow-covered area.
We're only melting snow where there is snow, so that's what this equation is about here.
Sublimation - here's the snow evap equation from the PRMS users' manual.
Here's a parameter, potet_sublim, the sublimation parameter.
These other terms are being computed either by other modules or further up the chain in
this module.
That's the end.