Near-Surface and Near-Crust Facets

Near-Surface andNear-Surface and Near-Crust Facets Near-Crust FacetsNear-Surface andNear-Surface and

Near-Crust Facets Near-Crust Facets

Photo: Ruby Mt. Helicopter Skiing

What type of temperaturegradient is required?

What type of temperaturegradient is required?

For near-surface faceting

to occur?

For near-surface faceting

to occur?Photo: Ruby Mt. Helicopter Skiing

For near-surface faceting tooccur we need what?

For near-surface faceting tooccur we need what?

Photo: Ruby Mt. Helicopter Skiing

Near-surface facetted grainsNear-surface facetted grains

Snow formed by near-surface vapor pressure gradients caused by strong temp gradients

Usually form within 15cm of the surface The weakest grains form near top of layer

An example of why near-surface An example of why near-surface facets are importantfacets are important

Mechanisms of near-surface facet formation

Mechanisms of near-surface facet formation

• Gradients from dry snow over wet • Gradients from dry snow over wet

• Near-surface gradients from radiation balance• Near-surface gradients from radiation balance

Near-Surface Gradients from Radiation Balance

(Diurnal Changes)

Near-Surface Gradients from Radiation Balance

(Diurnal Changes)

Conditions that Promote Near-Surface Gradients from

Radiation Balance

(Diurnal Changes)

Conditions that Promote Near-Surface Gradients from

Radiation Balance

(Diurnal Changes)

• Clear cold nights following relatively warm days • The cold nights crank up the faceting process • Faceted crystals may get a lot larger if conditions persist for several days • PRODUCT: bi-directional faceted crystals

• Clear cold nights following relatively warm days • The cold nights crank up the faceting process • Faceted crystals may get a lot larger if conditions persist for several days • PRODUCT: bi-directional faceted crystals

14 HOURS OLD14 HOURS OLD14 HOURS OLD14 HOURS OLD

BIRKELAND,JOHNSON,SCHMIDT

24 HOURS OLD24 HOURS OLD24 HOURS OLD24 HOURS OLD

BIRKELAND,JOHNSON,SCHMIDT

Near-Surface Gradients from Radiation Balance

(Extensive LWR loss during the day)

Near-Surface Gradients from Radiation Balance

(Extensive LWR loss during the day)

•Usually found at high altitudes•Occurs in the upper few cm of the snowpack •Southern aspects •Clear sunny days •Short wave radiation absorbed (may melt, certainly warms) • Creates a strong TG in upper few cm •PRODUCT: faceted crystals often over a melt freeze crust

•Usually found at high altitudes•Occurs in the upper few cm of the snowpack •Southern aspects •Clear sunny days •Short wave radiation absorbed (may melt, certainly warms) • Creates a strong TG in upper few cm •PRODUCT: faceted crystals often over a melt freeze crust

Conditions that promote faceting from extensive LWR loss during the day

Conditions that promote faceting from extensive LWR loss during the day

BIRKELAND,JOHNSON,SCHMIDT

24 hours old24 hours old

Dry snow over wet snow facetingDry snow over wet snow faceting

• Occurs in the upper few cm of the snowpack• Melt of snow surface/near surface due to solar radiation (short wave) or rain or wet snow • New cold snow falls • Strong temperature gradient between 0 o C layer cold snow (100-200 oC/m)• PRODUCT: Faceted crystals above the new ice crust

• Occurs in the upper few cm of the snowpack• Melt of snow surface/near surface due to solar radiation (short wave) or rain or wet snow • New cold snow falls • Strong temperature gradient between 0 o C layer cold snow (100-200 oC/m)• PRODUCT: Faceted crystals above the new ice crust

Dry snow over wet snow facetingDry snow over wet snow facetingDry snow over wet snow facetingDry snow over wet snow faceting

Dry snow over wet snow Dry snow over wet snow facetingfaceting

Dry snow over wet snow Dry snow over wet snow facetingfaceting

Dry snow over wet snow Dry snow over wet snow facetingfaceting

Looks sparkly, loose, granular, small-medium sized

Feels like a granular weak layer in the snow

Grows as a result of strong TG between a buried warm/wet old surface and a cold, dry layer of new snow layer on top

Distributed by aspect and altitude Persistence ranges from days to months

Conditions that promoteConditions that promote dry dry snow over wet snow facetingsnow over wet snow faceting

Sunny daysClear daysLow-density new snow at surfaceSubfreezing conditionsWarm precipitation events follow by

cold ones

Again think latent heat…..Again think latent heat…..

• Snow on crusts or wet snow

•Rain or wet snow on snow events (heat record)

•Avalanche debris

Near-Crust FacetingNear-Crust Faceting

Near-Crust FacetingNear-Crust FacetingNear-Crust FacetingNear-Crust Faceting

Crust Crust Temperature Gradient Temperature Gradient

Near-Crust Near-Crust FacetingFacetingNear-Crust Near-Crust FacetingFaceting

CRUSTSTRONG TGSTRONG TG

COLBECK 1991

Near Crust Near Crust FacetingFaceting

Near Crust Near Crust FacetingFaceting

CRUST HIGH DENSITY/LOW POROSITY

INCREASES THE VAPORDENSITY DIFFERENCE ON BOTTOM OF THE CRUST

INCREASES THE VAPORDENSITY DIFFERENCE ON BOTTOM OF THE CRUST

COLBECK 1991

Near Crust Near Crust FacetingFacetingNear Crust Near Crust FacetingFaceting

CRUSTFORMS SIMILARTO MELT LAYERRECRYSTALIZATION

FORMS SIMILARTO MELT LAYERRECRYSTALIZATION

COLBECK AND JAMIESON 2002

NSF AND NCF Wrap UpNSF AND NCF Wrap Up

• Faceting occurs when?• Faceting occurs when?

• Facets occur where?• Facets occur where?

• Is faceting good or bad?• Is faceting good or bad?