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Introduction to Snow
Who Cares? - Importance
Precipitation, Distribution, Redistribution
Modified, borrowed, or stolen from, Don ClineMark Williams, Tom Painter, Erin Hood, Denny Hogan, and probably others
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Introduction to Snow
More specifically, seasonal snow cover – glaciers, no firn, no ice sheets (unfortunately)
Def. Develops during winter and ablates by theend of the summer by:melt, sublimation, wind,avalanche
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Cold land areas where water is either seasonally or permanently frozen.Cold land areas where water is either seasonally or permanently frozen.
Terrestrial CryosphereTerrestrial Cryosphere
0.25 m Frost Penetration One Year in Ten0.25 m Frost Penetration One Year in Ten
0C Mean Temperature during Coldest Month0C Mean Temperature during Coldest Month
100 Days of Ice on Navigable Waterways100 Days of Ice on Navigable Waterways
Most Mountain RegionsOver 1000 m in ElevationMost Mountain Regions
Over 1000 m in Elevation
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Spatial extents of frozen and thawed areas vary significantly on daily,Spatial extents of frozen and thawed areas vary significantly on daily,seasonal, and interannual time scales.seasonal, and interannual time scales.
Terrestrial CryosphereTerrestrial Cryosphere
Over 30% of Earth’s land surface has seasonal snow.Over 30% of Earth’s land surface has seasonal snow.
On average, 60% of Northern Hemisphere has snow cover in midwinter.On average, 60% of Northern Hemisphere has snow cover in midwinter.
About 10% of Earth’s land surface is covered permanently by snow and ice.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.Seasonally and permanently frozen soils occur over ~35% of Earth’s land surface.
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Process-OrientedProcess-OrientedState VariablesState Variables
Snow and Freeze/Snow and Freeze/Thaw ProcessesThaw Processes
Snow Water Equivalent(Depth and Density)
Snow Water Equivalent(Depth and Density)
Snow and Frozen SoilInternal Energy
(relative to melting point)
Snow and Frozen SoilInternal Energy
(relative to melting point)
Snow Wetness(Liquid Water Content)
Snow Wetness(Liquid Water Content)
Snow Grain Size, AlbedoSnow Grain Size, Albedo
Snow and Soil Surface Temperature
Snow and Soil Surface Temperature
Soil MoistureSoil Moisture
Cold Land/AtmosphereEnergy Exchanges
Cold Land/AtmosphereEnergy Exchanges
Boundary LayerTurbulence and Stability
Boundary LayerTurbulence and Stability
Liquid Water Movementthrough Snow and Soil
Liquid Water Movementthrough Snow and Soil
Water Vapor Movementthrough Snow and Soil
Water Vapor Movementthrough Snow and Soil
Effects of Clouds onRadiation Energy Fluxes
Effects of Clouds onRadiation Energy Fluxes
Precipitation CharacteristicsPrecipitation Characteristics
FEEDBACKS
Energy Sink
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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?
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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?
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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
Who Cares?
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A tremendous gap exists A tremendous gap exists between the scales of our between the scales of our process-oriented process-oriented understanding, and the scales understanding, and the scales of synoptic weather and of synoptic weather and climate.climate.
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SnowAccumulation/
Ablation
SnowAccumulation/
Ablation
SoilFreeze/ThawTransitions
SoilFreeze/ThawTransitions
Most of our knowledge of Most of our knowledge of cold land hydrologic cold land hydrologic processes is limited to local processes is limited to local and hillslope scales.and hillslope scales.
Snow Energy and Mass ExchangesInfiltration, Unsaturated FlowEvapotranspirationOverland FlowSaturated Flow
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Definition: Snow Water Equivalent
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What factors control the distribution of snow extent and
snow water equivalent?
•Scale Dependent Question
•Continental Scale •Mesoscale •Basin Scale
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Continental Scale
• Latitude • Elevation
• Orography
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MesoScale
• Synoptic scale storms • Elevation
• Topographic configuration
100 km
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Basin Scale
•Wind Redistribution •Avalanche Redistribution •Terrain Configuration
•Vegetation Properties
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Precipitation
• First thing we need to do is get it to snow
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Precipitation Mechanisms
Convergence Frontal Forcing Orographic Forcing Convection (minimal)
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Convergence
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Frontal Effects
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Orographic Effects
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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 ranges: Sierra Nevada, Cascades, Coast Range (B.C.)
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Maritime
Continental
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Wind Redistribution
Free Atmosphere Wind Speed Slope Posed to Wind Surface Roughness Source Volumes
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Wind Speed Profile
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Wind Speed
where U* is the shear velocity (m/s) τo is the shear stress at the surface ρ is the air density
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Snow Transport
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Acceleration and Deceleration
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Eddy Formation
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Wind Redistribution – Examples
Cornice Formation
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Physical Redistribution – Examples
Avalanches
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Slope Dependence
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Mass Redistribution
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Snow Morphology due to Wind Sastrugi
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More Sastrugi
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Snow Morphology due to the WindRiming
Summit, Cerro Torre, Patagonia
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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
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Static nature of distribution drivers
Niwot Ridge, CO Saddle Met. Site
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August 8, 1995
July 1, 1996
July 1, 1997
Mammoth Mountain Ski
Area