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

0 E C 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/ Thaw Processes Energy Sink Process-Oriented State Variables 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

•

ground temperatures

- stay depressed over snow. - 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 Energy and Mass Exchanges Infiltration, Unsaturated Flow Evapotranspiration Overland Flow Saturated Flow Snow Accumulation/ Ablation Soil Freeze/Thaw Transitions

Definition:

Snow Water Equivalent

What factors control the distribution of snow extent and snow water equivalent?

•Scale Dependent Question •Continental Scale •Mesoscale •Basin Scale

• Latitude • Elevation • Orography Continental Scale

• Synoptic scale storms • Elevation • Topographic configuration MesoScale 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/m 3 new snow density)  Moderate air temperatures (-1.3 o C)  Low temperature/vapor pressure gradient (< 10 o C/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

*

τ o

is the shear velocity (m/s) 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