Near-Surface and Near-Crust Facets

1. Near-Surface and Near-Crust Facets Photo: Ruby Mt. Helicopter Skiing

2. What type of temperature gradient is required? For near-surface faceting to occur? Photo: Ruby Mt. Helicopter Skiing

3. For near-surface faceting to occur we need what? Photo: Ruby Mt. Helicopter Skiing

4. Near-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

5. An example of why near-surface facets are important

6. Mechanisms of near-surface facet formation • Near-surface gradients from radiation balance • Gradients from dry snow over wet

7. Near-Surface Gradients from Radiation Balance • (Diurnal Changes)

8. 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

9. 14 HOURS OLD 24 HOURS OLD BIRKELAND,JOHNSON,SCHMIDT BIRKELAND,JOHNSON,SCHMIDT

10. Near-Surface Gradients from Radiation Balance (Extensive LWR loss during the day)

11. 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

12. 24 hours old BIRKELAND,JOHNSON,SCHMIDT

13. Dry snow over wet snow faceting

14. 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 oC/m) • PRODUCT: Faceted crystals above the new • ice crust

15. Dry snow over wet snow faceting

16. Dry snow over wet snow faceting • 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

17. Conditions that promotedry snow over wet snow faceting • Sunny days • Clear days • Low-density new snow at surface • Subfreezing conditions • Warm precipitation events follow by cold ones

18. Again think latent heat….. • Snow on crusts or wet snow • Rain or wet snow on snow events (heat record) • Avalanche debris

19. Near-Crust Faceting

20. Near-Crust Faceting Temperature Gradient Crust

21. Near-Crust Faceting CRUST STRONG TG COLBECK 1991

22. Near Crust Faceting INCREASES THE VAPOR DENSITY DIFFERENCE ON BOTTOM OF THE CRUST HIGH DENSITY/ LOW POROSITY CRUST COLBECK 1991

23. Near Crust Faceting CRUST FORMS SIMILAR TO MELT LAYER RECRYSTALIZATION COLBECK AND JAMIESON 2002

26. NSF AND NCF Wrap Up • Faceting occurs when? • Facets occur where? • Is faceting good or bad?