NearSurface and NearCrust Facets Photo Ruby Mt Helicopter
Near-Surface and Near-Crust Facets Photo: Ruby Mt. Helicopter Skiing
What type of temperature gradient is required? For near-surfaceting to occur? Photo: Ruby Mt. Helicopter Skiing
For near-surfaceting to occur we need what? Photo: Ruby Mt. Helicopter Skiing
Near-surfacetted grains l Snow formed by near-surface vapor pressure gradients caused by strong temp gradients l Usually form within 15 cm of the surface l The weakest grains form near top of layer
An example of why near-surfacets are important
Mechanisms of near-surfacet formation • Near-surface gradients from radiation balance • Gradients from dry snow over wet
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
14 HOURS OLD BIRKELAND, JOHNSON, SCHMIDT 24 HOURS OLD BIRKELAND, JOHNSON, SCHMIDT
Near-Surface Gradients from Radiation Balance (Extensive LWR loss during the day)
Conditions that promote faceting from 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
24 hours old BIRKELAND, JOHNSON, SCHMIDT
Dry snow over wet snow faceting
Dry 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 o. C/m) • PRODUCT: Faceted crystals above the new ice crust
Dry snow over wet snow faceting
Dry snow over wet snow faceting l l l Looks sparkly, loose, granular, smallmedium 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 promote dry snow over wet snow faceting l Sunny days l Clear days l Low-density new snow at surface l Subfreezing conditions l Warm precipitation events follow by cold ones
Again think latent heat…. . • Snow on crusts or wet snow • Rain or wet snow on snow events (heat record) • Avalanche debris
Near-Crust Faceting
Near-Crust Faceting Temperature Gradient Crust
Near-Crust Faceting STRONG TG CRUST COLBECK 1991
Near Crust Faceting INCREASES THE VAPOR DENSITY DIFFERENCE ON BOTTOM OF THE CRUST HIGH DENSITY/ LOW POROSITY COLBECK 1991
Near Crust Faceting FORMS SIMILAR TO MELT LAYER RECRYSTALIZATION CRUST COLBECK AND JAMIESON 2002
NSF AND NCF Wrap Up • Faceting occurs when? • Facets occur where? • Is faceting good or bad?
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