By Vivian Underhill Winter Ecology, Spring 2011 Mountain Research Station University of Colorado, Boulder.

Effects of Aspect on Faceting in the Rocky Mountain Snowpack

by Vivian UnderhillWinter Ecology, Spring 2011Mountain Research Station University of Colorado, Boulder

Brief Overview: Faceted Snow

Large temperature gradient 10 degrees/m (Marchand 1996)

Depends on temperature and snow depth

Vapor particles sublimate upCreate hexagonal pyramid crystals

http://www2.gi.alaska.edu/alison/ALISON_Science_Snowmetatg.html

Near-Surface Faceting

•Occurs with large diurnal swings in temperature

•Warm days, cold nights

•Warm days create negative gradient

•Causes downward faceting

•Cold nights create positive gradient

•Causes upward faceting

•Large, bidirectional facets

Birkeland et al 1998

Temperature Profiles Throughout the Day

Why Do We Care?

Subnivean Organisms Tunneling in depth

hoar Insulative capacity

Avalanches Facets form weak

layers “ball bearings”

http://library.thinkquest.org/03oct/01027/avalanchepic.jpg

Research Question

How does aspect (and therefore a difference in solar radiation) affect

temperature-gradient metamorphism in the snowpack?

http://www.fsavalanche.org/Encyclopedia/i-faceted/header_pic_faceted.jpg

2-Sided Hypothesis

1. North Aspect = less solar radiation = colder = more daytime temperature gradient = more faceting

-or-2. South Aspect = more solar radiation

= more daytime melting and greater diurnal temperature swings = greater faceting

-or a combination of both?-

Methods

Sites

-Dug two snowpits on north and south aspects of Berthoud Pass. -Snowpits were within treeline but not under trees.

Procedure

-Density and temperature profiles through entire depths of both pits (about 200 cm).-Identified most obvious layering and observed crystal shape, size.-Later changed temperature to vapor pressure using Goff-Gratch equation.(Birkeland et al 1998)

Method Limitations

-Only one trial

-Did profiles during the day, while almost all faceting occurs at night

-Possible packing of snow by skiers

-Process of the profiles is inherently inexact; involves many opportunities for human error

Snowpack Profiles

NORTH ASPECT SOUTH ASPECT

Pressure and Temperature Gradients, Compared

TEMPERATURE GRADIENTS VAPOR PRESSURE GRADIENTS

This Season’s Snow Story

•Early snowfall, followed by warm temperatures

•South aspect probably melted away•North probably melted but then refroze in late afternoons

•Made ice layer on the north aspect but not on the south.

This Season’s Snow Story

•Subsequent layers on both aspects experienced faceting

•South definitely exceeded 10 degree C/m threshold•North did as well, because the pack was thin enough to create a strong gradient

•Strong depth hoar formation on both aspects

This Season’s Snow Story

• In higher layers, effects of aspect became more pronounced again

•South side had large diurnal temperature swings, large bidirectional facets•North side had deeper snow, and its constant conditions weren’t enough to create a strong gradient for faceting

Discussion

Differences in metamorphosis probably don’t affect subnivean organisms

Large difference in terms of avalanche danger. More compacted north aspect is probably

safer, because it has fewer possible points of failure

South aspect is probably more dangerous, especially after consecutive warm days and cool nights.

Discussion

Future Research Do multiple trials with the same setup,

to make sure any patterns seen weren’t just anomalies

Permanent vertical thermocouple array Compare to weather station data on

precipitation and temperature

Summary

Aspect affects very shallow snow (0-10 cm)

Middle (depth hoar layers) are not affected by aspect (10-50 cm)

In higher layers, differences in aspect have a much larger effect (60 cm-surface)

Supports the hypothesis that the increased solar radiation of south aspects leads to increased near-surface faceting.

Literature Cited

Birkeland, K; Johnson, R; Schmidt, D. “Near-Surface Faceted Crystals Formed by Diurnal Recrystallization: A Case Study of Weak Layer Formation in the Mountain Snowpack and Its Contribution to Snow Avalanches.” Arctic and Alpine Research  Vol. 30, No. 2, 1998. P. 200-204. http://www.jstor.org/stable/1552135

Flin, F. and Brzoska, J-B. “The Temperature-Gradient Metamorphism of Snow: Vapor diffusion Model and Application to Tomographic Images.” Annals of Glaciology Vol. 49, 2008. P 17-21. (Printed copy; no link.)

Marbouty, D. “An Experimental Study of Temperature-Gradient Metamorphism.” Journal of Glaciology Vol. 26, No. 94, 1980. P. 303-311. http://www.igsoc.org/journal/26/94/igs_journal_vol26_issue094_pg303-312.pdf

Marchand, P.J. Life In The Cold. Hanover, NH: University Press of New England, 1996.

Sturm, M. and Benson, C. “Vapor Transport, Grain Growth and Depth-Hoar Development in the Subarctic Snow.” Journal of Glaciology Vol. 43, No. 143. 1997. P. 42-58. http://www.igsoc.org/journal/43/143/igs_journal_vol43_issue143_pg42-59.pdf