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 Measuring

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

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 placeDs is the average density of the snowpackDw is the average density of the water (assume 1.00 g/cm)hs is the height of the snow pack

ms

w

sh h


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


Once on the ground, metamorphosis begins through:1. - rates increase with increased weight on top and layer temperature- rates decrease with increasing density2. - 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-33. -“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


Measurement

Precip (water equivalent)-gages, radar

Snowfall (depth of snow for a storm)- direct measurement, or “teasing” of precip measurements


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


- 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


Ablation 1. Water output

- - universal gage

2. Ablation- pans

3. Both- -


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: ci is the heat capacity of the ice (2.115 J g-1 oC-1)Ts is the average temperature of the packTm is the melting point

CC i w m s mQ c h T T ( )


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 snowθret 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

w re t re t sh h

re ts

w

s

w

0 0735 0 2672

. .

Q h hm re t w f re t s w f2


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:

Q h hm m w re t w f3 ( )

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


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 exchangesShortwave (solar) radiation input, KLongwave radiation exchange, LTurbulent exchange of sensible heat with the atmosphere, HTurbulent exchange of latent heat with the atmosphere, LEHeat input by rain, RConductive exchange of sensible heat with the ground, G


Modeling Snow melt

1. 2. Temperature-Index approachsimplified 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 use3. temporal and spatial variations in the areal extent of snow cover4.

Documents

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 Measuring