Snow
Measuring the amount of snowfall is problematic, but for most cases we have the
luxury of simply being concerned with the snowpack on the ground.
Water content in the snowpack is usually referred to as the snowpack’s “water
equivalent”

h  h
s
m
s
w
where hm is the height of water that would be on the ground if the snowpack
melted in place
Ds is the average density of the snowpack
Dw is the average density of the water (assume 1.00 g/cm)
hs is the height of the
snow pack
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Snowpack Metamorphism
Density of new fallen snow is dependant on the configuatation of the flakes, which
in turn is a function of air temperature, degree of supersaturation in the cloud, and
wind speed at surface
When converting snowfall to water equivalent, 0.1 is often used as the density
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Once on the ground, metamorphosis begins through:
1.
- rates increase with increased weight on top and layer temperature
- rates decrease with increasing density
2.
- vapor pressures greater over surfaces with smaller radii of curvature
- points get worn down and ice is redeposited on broadly curved surfaces
- density of new layer of snow increases by about 1% per hour initially by this
process, up to ~0.25 g cm-3
3.
-“sintering” is process where water molecules are deposited at the contacts
between two crystals
-- depth hoar forms because of temperature gradient (cold on top, warm at bottom)
in snow pack. Sublimation in warmer parts of pack and condensation at colder parts
4.
-two processes:
-a) water introduced at the surface through rain or melt sinks and freezes (also
releases latent heat)
b) disappearance of small grains and growth of large grains in presence of liquid
water.
All but formation of depth
hoar (which is isolated and transitory) lead to
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densification
Measurement
Precip (water equivalent)
-gages, radar
Snowfall (depth of snow for a storm)
- direct measurement, or “teasing” of precip measurements
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Snowcover (i.e. over a basin)
- snow tubes and surveys
- most standard
- generally gets precise water equivalent, but can overestimate
- calibration of sites over years, reduces need to measure everywhere
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- snow pillows
- bridging
-
-
- (60Co,137Cs)(not good for very deep snows)
-good areal coverage but precision is low
-need to know a lot about surface conditions (temperature, soil
characteristics, etc.)
- not good for low density snows
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Ablation
1. Water output
- universal gage
2. Ablation
- pans
3. Both
-
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Snowmelt Processes
1.
-mean temperature increases until isothermal
cold content of snowpack is the amount of energy needed to raise its average
temperature to the melting point:


(
T

T
)

c
h
s
m
i
m
CC
w
Q
where: ci is the heat capacity of the ice (2.115 J g-1 oC-1)
Ts is the average temperature of the pack
Tm is the melting point
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2.
- melting occurs but water is retained. A pack is “ripe” when
it is isothermal at freezing and can not hold water any more
h
wret
where:
  ret hs
hwret liquid water retaining capacity of the snow
θret is max volumetric water content of the snow
s
 s2
 ret   0.0735  w  0.267 w
The net energy input required to take a snow pack from isothermal to ripe is:
Qm2  hret  w f  ret hs  w f
where λf is the latent heat
of fusion
attendance,
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3.
- further energy inputs produce water output
amount o energy needed to complete output phase is the amount of energy
needed to melt all snow remaining at the end of the ripening phase:
Qm3  ( hm  hwret ) w f
Snowmelt-Runoff Generation
- Water makes its way to the base of the snow pack and either infiltrates or
collects at the surface depending on soil conditions.
- generally, the conditions at the base of the pack are predetermined during
freeze-up
- if a ground is saturated at freeze-up there will be a lot of overland flow in spring
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Energy Balance
For a block of snow:
S is the net rate of energy fluxes into this block over a time period Δ t and Δ Q
is the change in heat energy absorbed by the snow pack during the same time
The energy exchanges involved in snowmelt are
S is the net rate of energy exchanges
Shortwave (solar) radiation input, K
Longwave radiation exchange, L
Turbulent exchange of sensible heat with the atmosphere, H
Turbulent exchange of latent heat with the atmosphere, LE
Heat input by rain, R
Conductive exchange of sensible heat with the ground, G
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Modeling Snow melt
1.
2. Temperature-Index approach
simplified model which requires only air temperature and empirical estimation
of a melt coefficient for a specific basin that is controlled by latitude, elevation,
slope inclination and aspect, forest cover and time of year
When modeling snowmelt over a large area, for best results make sure to
consider:
1. the phase of precip (often not properly reported)
2. variations in topography, elevation, and land use
3. temporal and spatial variations in the areal extent of snow cover
4.
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