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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”

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Snow

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”

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

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages


Snow

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

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages


Snow

  • 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 densification

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages


Snow

Measurement

Precip (water equivalent)

-gages, radar

Snowfall (depth of snow for a storm)

- direct measurement, or “teasing” of precip measurements

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages


Snow

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

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages


Snow

- 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

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages


Snow

Ablation

1. Water output

-

- universal gage

2. Ablation

- pans

3. Both

-

-

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages


Snow

  • 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:

  • where:ciis the heat capacity of the ice (2.115 J g-1oC-1)

  • Ts is the average temperature of the pack

  • Tm is the melting point

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages


Snow

  • 2. - melting occurs but water is retained. A pack is “ripe” when it is isothermal at freezing and can not hold water any more

    • where: hwret liquid water retaining capacity of the snow

  • θret is max volumetric water content of the snow

  • The net energy input required to take a snow pack from isothermal to ripe is:

  • where λf is the latent heat of fusion

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages


Snow

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:

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

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages


Snow

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

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages


Snow

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.

These notes are provided to help you pay attention IN class. If I notice poor attendance, fewer notes will begin to appear on these pages


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