Introduction to Snow
Who Cares? - Importance
Precipitation, Distribution, Redistribution
Modified, borrowed, or stolen from, Don Cline
Mark Williams, Tom Painter, Erin Hood,
Denny Hogan, and probably others
Introduction to Snow
More specifically, seasonal snow cover –
glaciers, no firn, no ice sheets (unfortunately)
Def. Develops during winter and ablates by the
end of the summer by:melt, sublimation, wind,
avalanche
Terrestrial Cryosphere
0EC Mean Temperature during Coldest Month
0.25 m Frost Penetration One Year in Ten
100 Days of Ice on Navigable Waterways
Most Mountain Regions
Over 1000 m in Elevation
Cold land areas where water is either seasonally or permanently frozen.
Terrestrial Cryosphere
On average, 60% of Northern Hemisphere has snow cover in midwinter.
Over 30% of Earth’s land surface has seasonal snow.
About 10% of Earth’s land surface is covered permanently by snow and ice.
Seasonally and permanently frozen soils occur over ~35% of Earth’s land surface.
Spatial extents of frozen and thawed areas vary significantly on daily,
seasonal, and interannual time scales.
Snow and Freeze/ Energy Sink Process-Oriented
State Variables
Thaw Processes
Cold Land/Atmosphere
Energy Exchanges
Boundary Layer
Turbulence and Stability
Effects of Clouds on
Radiation Energy Fluxes
Precipitation Characteristics
Liquid Water Movement
through Snow and Soil
Water Vapor Movement
through Snow and Soil
F
E
E
D
B
A
C
K
S
Snow Water Equivalent
(Depth and Density)
Snow and Frozen Soil
Internal Energy
(relative to melting point)
Snow and Soil
Surface Temperature
Snow Wetness
(Liquid Water Content)
Snow Grain Size, Albedo
Soil Moisture
Who Cares?
A. Effects of Snow Cover on climate
•albedo - overhead 2 reflectivity (new snow ~ 0.8-9,
older snow ~ 0.5-6). Compare this to ice 0.3-4, forest 0.03-.2
and water 0.05-.3. Result: Blocks incoming radiation from
heating the surface so solar energy is returned to space
instead of being retained as heat in the atmosphere.
Strong climate effects:
•surface temperatures - stay depressed over snow.
•ground temperatures - with low thermal
conductivity, snow serves as an insulator and keeps vast areas
of soil unfrozen
•also strong climate feedback effects: more snow
decreases temp which causes more snow which will then
persist longer. Ex. winter 0f 92-3 after Mt Pinatubo was colder
and longer because of ash in the air
Who Cares?
B. Importance in Water Resources
•Controls the hydrologic cycle - water is stored over
winter and released in a pulse during spring melt. Presents
difficulties for water managers, reason for existance of resevoirs
• frozen water = 80% of fresh water on earth
• Major contributor to river and ground water in mid/high
latitudes. ex. Seasonal Snowcovers - W. US: Calif. 80% of
water, Colo. 70% of water
Who Cares?
C. Avalanches - impacts on backcountry users & mountain
residents:
•Estimated to be 100,000 in the US on average, ~10,000
reported and 100 or 1% cause problems - property damage
and injury
•Damage to buildings and structures - only $1/2 million
per year in US but much higher in Europe because of dense
population
A tremendous gap exists between
the scales of our process-oriented
understanding, and the scales of
synoptic weather and climate.
Most of our knowledge of cold
land hydrologic processes is
limited to local and hillslope
scales.
Snow
Accumulation/
Ablation
Soil
Freeze/Thaw
Transitions
Snow Energy and Mass Exchanges
Infiltration, Unsaturated Flow
Evapotranspiration
Overland Flow
Saturated Flow
Definition: Snow Water Equivalent
What factors control the
distribution of snow extent and
snow water equivalent?
•Scale Dependent Question
•Continental Scale
•Mesoscale
•Basin Scale
Continental Scale
• Latitude
• Elevation
• Orography
MesoScale
• Synoptic scale storms
• Elevation
• Topographic configuration
100 km
Basin Scale
•Wind Redistribution
•Avalanche Redistribution
•Terrain Configuration
•Vegetation Properties
Precipitation
• First thing we need to do is get it to snow
Precipitation Mechanisms
 Convergence
 Frontal
Forcing
 Orographic Forcing
 Convection (minimal)
Convergence
Frontal Effects
Orographic Effects
Maritime Snowpacks
Deep snowpack (15-25 m annual snowfall)
 High density (120 kg/m3 new snow density)
 Moderate air temperatures (-1.3 oC)
 Low temperature/vapor pressure gradient (< 10 oC/m)
 Avalanches during or immediately after storms

Example
(B.C.)
ranges: Sierra Nevada, Cascades, Coast Range
Maritime
Continental
Wind Redistribution
 Free Atmosphere Wind
 Slope
Posed to Wind
 Surface Roughness
 Source Volumes
Speed
Wind Speed Profile
Wind Speed
where U* is the shear velocity (m/s)
τo is the shear stress at the surface
ρ is the air density
Snow Transport
Acceleration and Deceleration
Eddy Formation
Wind Redistribution – Examples
Cornice Formation
Physical Redistribution – Examples
Avalanches
Slope Dependence
Mass Redistribution
Snow Morphology due to Wind
Sastrugi
More Sastrugi
Snow Morphology due to the Wind
Riming
Summit, Cerro Torre, Patagonia
Static nature of distribution drivers
Wind Direction (all U)
Wind Direction (U > 5 m/s)
Mammoth Mountain Met Site
10/1/1999-3/24/2000
Static nature of distribution drivers
Niwot Ridge, CO
Saddle Met. Site
Mammoth
Mountain Ski
Area
August 8, 1995
July 1, 1996
July 1, 1997