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

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

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

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.

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

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

Who Cares?

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.

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

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 ranges: Sierra Nevada, Cascades, Coast Range (B.C.)

Maritime

Continental

Wind Redistribution

Free Atmosphere Wind Speed Slope Posed to Wind Surface Roughness Source Volumes

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 WindRiming

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

August 8, 1995

July 1, 1996

July 1, 1997

Mammoth Mountain Ski

Area