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Near-Surface and Near-Crust Facets. Photo: Ruby Mt. Helicopter Skiing. What type of temperature gradient is required?. For near-surface faceting to occur?. Photo: Ruby Mt. Helicopter Skiing. For near-surface faceting to occur we need what?. Photo: Ruby Mt. Helicopter Skiing. - PowerPoint PPT Presentation
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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?
