Effects of Aspect on Faceting in the Rocky Mountain Snowpack

1. Effects of Aspect on Faceting in the Rocky Mountain Snowpack by Vivian Underhill Winter Ecology, Spring 2011 Mountain Research Station University of Colorado, Boulder

2. Brief Overview: Faceted Snow Large temperature gradient 10 degrees/m (Marchand 1996) Depends on temperature and snow depth Vapor particles sublimate up Create hexagonal pyramid crystals TG snow requires large temperature gradient. Some discrepancies in exact amount; D. Marbouty (1980) says .25 degrees/cm , or 25 degrees/m while our book (Marchand 1996) says 10 degrees/ m. This could be because Marbouty was writing somewhat earlier than Marchand; new data could have changed the number. Also, Marbouty was working with a 2-D model and lab experiments while Marchand’s figure seems to apply more to natural experiments; in the outdoors, other factors may also take effect. Assuming that pore spaces within the snowpack exist at 100% saturation (Birkeland et al 1998), this temperature gradient results in a vapor pressure gradient, because cold air can hold less moisture than warm air can. Water vapor molecules will then follow this gradient, moving from spaces of higher vapor pressure (further down in the snowpack) to spaces of lower pressure (higher in the pack). (Birkeland et al 1998, Marbouty 1980). They only travel small distances, just to the crystal just above; they refreeze onto the bottom of that crystal because in the slightly colder air, dewpoint is lower. This makes for crystals that are rounded on the top (because vapor molecules sublimated off) but six-sided pyramids on the bottom. (Sturm and Benson 1997; Marchand 1996). TG metamorphism has been studied since the 1950s, and is often seen as the most interesting type of snowpack decomposition (Flin, Brzoska 2008). Currently, much interest seems to lie in making computer models of faceting processes (Flin, Brzoska 2008). However, much of the work on faceting has been done with field work similar to mine. Most used more-sophisticated data collection techniques than I did, with permanent thermocouple systems, for example (Birkeland et al 1998). All the studies I found state that faceting most often occurs during cold, clear nights when the temperature gradient is the strongest (Birkeland et al 1998; Flin, Brzoska 2008). Most studies also emphasize the relation between temperature and vapor pressure gradient, since it's the latter that actually drives faceting. To this end, I plan to use an equation relating vapor pressure to temperature found in Birkeland et al 1998. TG snow requires large temperature gradient. Some discrepancies in exact amount; D. Marbouty (1980) says .25 degrees/cm , or 25 degrees/m while our book (Marchand 1996) says 10 degrees/ m. This could be because Marbouty was writing somewhat earlier than Marchand; new data could have changed the number. Also, Marbouty was working with a 2-D model and lab experiments while Marchand’s figure seems to apply more to natural experiments; in the outdoors, other factors may also take effect. Assuming that pore spaces within the snowpack exist at 100% saturation (Birkeland et al 1998), this temperature gradient results in a vapor pressure gradient, because cold air can hold less moisture than warm air can. Water vapor molecules will then follow this gradient, moving from spaces of higher vapor pressure (further down in the snowpack) to spaces of lower pressure (higher in the pack). (Birkeland et al 1998, Marbouty 1980). They only travel small distances, just to the crystal just above; they refreeze onto the bottom of that crystal because in the slightly colder air, dewpoint is lower. This makes for crystals that are rounded on the top (because vapor molecules sublimated off) but six-sided pyramids on the bottom. (Sturm and Benson 1997; Marchand 1996). TG metamorphism has been studied since the 1950s, and is often seen as the most interesting type of snowpack decomposition (Flin, Brzoska 2008). Currently, much interest seems to lie in making computer models of faceting processes (Flin, Brzoska 2008). However, much of the work on faceting has been done with field work similar to mine. Most used more-sophisticated data collection techniques than I did, with permanent thermocouple systems, for example (Birkeland et al 1998). All the studies I found state that faceting most often occurs during cold, clear nights when the temperature gradient is the strongest (Birkeland et al 1998; Flin, Brzoska 2008). Most studies also emphasize the relation between temperature and vapor pressure gradient, since it's the latter that actually drives faceting. To this end, I plan to use an equation relating vapor pressure to temperature found in Birkeland et al 1998.

3. 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 Near-surface faceting: occurs with large diurnal swings in temperature. This can cause a negative gradient as well, with air being much warmer than the bottom of the pack, and create upside-down faceting. Then, as night comes, this gradient switches and faceting begins going upward again. This can create especially large, bidirectional facets (Birkeland et al 1998). Near-surface faceting: occurs with large diurnal swings in temperature. This can cause a negative gradient as well, with air being much warmer than the bottom of the pack, and create upside-down faceting. Then, as night comes, this gradient switches and faceting begins going upward again. This can create especially large, bidirectional facets (Birkeland et al 1998).

4. Why Do We Care? Subnivean Organisms Tunneling in depth hoar Insulative capacity Avalanches Facets form weak layers “ball bearings” Low-density subnivean space. The upward transport of mass in these layers without accompanying change in volume makes a layer of low-density depth hoar. This is important for mice and other winter mammals who burrow through the snow in winter (Marchand 1996). Insulative capacity. The insulative capacity of snow is a function of its density. Much of the winter ecosystem relies on this insulation: perennial plants, microbes, mammals, even some birds rely on it (Marchand 1996). More knowledge about changes in density with metamorphism could add to knowledge about preference of hibernation or torpor locations and the success of overwintering species. Hydrologic cycle: Increasing world temperatures probably mean earlier and faster melting of snow, although at some high elevations, a warmer temperature could mean a deeper snowpack. There are probably many other changes too that we can’t predict, but more knowledge about the interactions between solar radiation and snowpack can help us to understand better. Most importantly, skiers: These facets can form a weak layer on which higher layers can slide. They’re shaped just like ball bearing, in a way, and are strongly un-cohesive. Most of the study of snow metamorphism is by skiers or in the interest of skiers. Low-density subnivean space. The upward transport of mass in these layers without accompanying change in volume makes a layer of low-density depth hoar. This is important for mice and other winter mammals who burrow through the snow in winter (Marchand 1996). Insulative capacity. The insulative capacity of snow is a function of its density. Much of the winter ecosystem relies on this insulation: perennial plants, microbes, mammals, even some birds rely on it (Marchand 1996). More knowledge about changes in density with metamorphism could add to knowledge about preference of hibernation or torpor locations and the success of overwintering species. Hydrologic cycle: Increasing world temperatures probably mean earlier and faster melting of snow, although at some high elevations, a warmer temperature could mean a deeper snowpack. There are probably many other changes too that we can’t predict, but more knowledge about the interactions between solar radiation and snowpack can help us to understand better. Most importantly, skiers: These facets can form a weak layer on which higher layers can slide. They’re shaped just like ball bearing, in a way, and are strongly un-cohesive. Most of the study of snow metamorphism is by skiers or in the interest of skiers.

5. Research Question How does aspect (and therefore a difference in solar radiation) affect temperature-gradient metamorphism in the snowpack? We know that faceting occurs from a high temperature gradient. What are the factors that affect this gradient? A temperature gradient can be created either by very low air temperatures (making a larger temp. difference) or by a shallow snowpack. Solar radiation can affect both sides of this; while snow itself doesn’t easily absorb short-wave radiation, small dust or litter particles on the surface can. Also, solar radiation warms the air, which warms the snow. This would reduce the temperature gradient. However, solar radiation can also melt snow, beginning a melt-freeze cycle or decreasing the total depth of the snow. This leads to a greater temperature gradient. It’s really a double-edged sword, and it’s this dichotomy of effects that I’m interested in. We know that faceting occurs from a high temperature gradient. What are the factors that affect this gradient? A temperature gradient can be created either by very low air temperatures (making a larger temp. difference) or by a shallow snowpack. Solar radiation can affect both sides of this; while snow itself doesn’t easily absorb short-wave radiation, small dust or litter particles on the surface can. Also, solar radiation warms the air, which warms the snow. This would reduce the temperature gradient. However, solar radiation can also melt snow, beginning a melt-freeze cycle or decreasing the total depth of the snow. This leads to a greater temperature gradient. It’s really a double-edged sword, and it’s this dichotomy of effects that I’m interested in.

6. 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?- On the one hand, south-facing slopes receive more radiation during the day. Warmer air temperature means a smaller temperature gradient; on particularly warm days, surface temperatures become warmer than the ground and a negative gradient can even be established (Birkeland et al 1998, Fig. 2; Marchand 1996). On the other hand, this increased solar radiation could also increase melt rates on south aspects, decreasing depth of snowpack and increasing the gradient. Another factor to consider is that most faceting occurs during cold, clear nights (Flin and Brzoska 2008), when there is no insulating cloud cover and surface snow radiates energy back to space (Marchand). At night, there should be no difference between north and south aspects. Even if south-facing snow did absorb more energy, much of it would have gone into the latent heat required for phase changes and shouldn't still be warming the snowpack. Lastly, these diurnal temperature swings could make for more near-surface faceting, because of the large positive and negative gradients (Birkeland et al 1998). On the one hand, south-facing slopes receive more radiation during the day. Warmer air temperature means a smaller temperature gradient; on particularly warm days, surface temperatures become warmer than the ground and a negative gradient can even be established (Birkeland et al 1998, Fig. 2; Marchand 1996). On the other hand, this increased solar radiation could also increase melt rates on south aspects, decreasing depth of snowpack and increasing the gradient. Another factor to consider is that most faceting occurs during cold, clear nights (Flin and Brzoska 2008), when there is no insulating cloud cover and surface snow radiates energy back to space (Marchand). At night, there should be no difference between north and south aspects. Even if south-facing snow did absorb more energy, much of it would have gone into the latent heat required for phase changes and shouldn't still be warming the snowpack. Lastly, these diurnal temperature swings could make for more near-surface faceting, because of the large positive and negative gradients (Birkeland et al 1998).

7. 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) Berthoud Pass: west to east: since weather comes directly from the west, there should be a fairly even amount of snowfall on both aspects. Also, this means that there is a clear distinction between north and south aspects. Snow pit site selection: This was to protect from the effects of wind and tree cover. (Wind can compact snow as well as transport it; trees make a shallower snowpack until they shed that snow from their branches, making a sudden increase in depth.) They were both at about the same elevation and slope angle (about 20 degrees), and looked untouched. They were also not where skiers might want to go, so hopefully they hadn’t previously been affected by tramping by skiers. Dug both “into” the hill. It was a very cold day, and temperatures actually dropped from -9.5 to -10.5 during the course of the experiment. Light to heavy snow, and complete cloud cover. Mild to moderate winds. Snowpits grew through the experiment b/c snowing so hard. Effects from trees radiating infrared should be about the same for the two pits, because I dug them in similar-sized clearings. Profiles were done using the basic protocol that we used in class. Later, I changed the temperature data into vapor pressure using an integration of the Clausius-Clapeyron equation commonly used by other researchers in the literature (Birkeland et al, 1998). Berthoud Pass: west to east: since weather comes directly from the west, there should be a fairly even amount of snowfall on both aspects. Also, this means that there is a clear distinction between north and south aspects. Snow pit site selection: This was to protect from the effects of wind and tree cover. (Wind can compact snow as well as transport it; trees make a shallower snowpack until they shed that snow from their branches, making a sudden increase in depth.) They were both at about the same elevation and slope angle (about 20 degrees), and looked untouched. They were also not where skiers might want to go, so hopefully they hadn’t previously been affected by tramping by skiers. Dug both “into” the hill. It was a very cold day, and temperatures actually dropped from -9.5 to -10.5 during the course of the experiment. Light to heavy snow, and complete cloud cover. Mild to moderate winds. Snowpits grew through the experiment b/c snowing so hard. Effects from trees radiating infrared should be about the same for the two pits, because I dug them in similar-sized clearings. Profiles were done using the basic protocol that we used in class. Later, I changed the temperature data into vapor pressure using an integration of the Clausius-Clapeyron equation commonly used by other researchers in the literature (Birkeland et al, 1998).

8. 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 These pits were done during the day, and most TG happens at night. Really need a set of permanent thermocouples, installed in the spring, to measure temperature year-round (as in Birkeland et al 1998). Also, the possibility of previous disturbance is definitely plausible, given the popularity of the area. Would like to be able to do more samples, to make sure our results weren’t just anomalies. The whole process is inexact. These pits were done during the day, and most TG happens at night. Really need a set of permanent thermocouples, installed in the spring, to measure temperature year-round (as in Birkeland et al 1998). Also, the possibility of previous disturbance is definitely plausible, given the popularity of the area. Would like to be able to do more samples, to make sure our results weren’t just anomalies. The whole process is inexact.

9. Snowpack Profiles North Aspect South Aspect South aspect had more faceting further up the snowpack than the north aspect did. Also had a significant ice crust, which shows that melting is much more common on the south aspect. North aspect’s large ice layer at the bottom: probably from an anomaly in the fate of early snowfall. Possibly fell, then followed by a long warm period. South aspect completely melted away, while north aspect melted and refroze (because of colder overall temperatures). North also has faceting, just further down. Probably because when it was a shallower snowpack, it was more susceptible to faceting. Now that there’s much more snow on top, the entire pack is insulated. These layers correspond nicely to the density profile; in general, faceted layers tend to bring density down, while rounded or neutral layers increase density. South aspect had more faceting further up the snowpack than the north aspect did. Also had a significant ice crust, which shows that melting is much more common on the south aspect. North aspect’s large ice layer at the bottom: probably from an anomaly in the fate of early snowfall. Possibly fell, then followed by a long warm period. South aspect completely melted away, while north aspect melted and refroze (because of colder overall temperatures). North also has faceting, just further down. Probably because when it was a shallower snowpack, it was more susceptible to faceting. Now that there’s much more snow on top, the entire pack is insulated. These layers correspond nicely to the density profile; in general, faceted layers tend to bring density down, while rounded or neutral layers increase density.

10. Pressure and Temperature Gradients, Compared Temperature Gradients Vapor Pressure Gradients Interestingly, the temperature and vapor pressure gradients didn’t match at all with the layers I found. Ignore the inversions in the middle of the pack; these probably come from thermometer malfunction. As for the inversions at the top: The top 10 cm or so of the snowpack can be heavily affected by diurnal variations in temperature or radiation, so these are probably attributable to daytime warming. The gradients written on the graphs were calculated from the bottom to just before the inversion on the north aspect, and just before the equitemperature layer on the south aspect. This is probably more informative about the long-term history of the pack than the top few layers, since they’re affected by current conditions more. Overall, neither aspect had a gradient large enough to promote faceting right now, though from 20-45 cm on the south aspect the gradient is exactly 10 degrees C/m. They’re also very similar; this makes sense. It was cloudy when we took these profiles, so solar radiation shouldn’t have been affecting the south differently. The temperature profiles are really not very informative, because they were taken in the daytime. The real question lies in what it looks like on a cold night, when faceting would be going on. Interestingly, the temperature and vapor pressure gradients didn’t match at all with the layers I found. Ignore the inversions in the middle of the pack; these probably come from thermometer malfunction. As for the inversions at the top: The top 10 cm or so of the snowpack can be heavily affected by diurnal variations in temperature or radiation, so these are probably attributable to daytime warming. The gradients written on the graphs were calculated from the bottom to just before the inversion on the north aspect, and just before the equitemperature layer on the south aspect. This is probably more informative about the long-term history of the pack than the top few layers, since they’re affected by current conditions more. Overall, neither aspect had a gradient large enough to promote faceting right now, though from 20-45 cm on the south aspect the gradient is exactly 10 degrees C/m. They’re also very similar; this makes sense. It was cloudy when we took these profiles, so solar radiation shouldn’t have been affecting the south differently. The temperature profiles are really not very informative, because they were taken in the daytime. The real question lies in what it looks like on a cold night, when faceting would be going on.

11. 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. Early snowfall was probably followed by a period of warm temperatures; this completely melted it away from the south side, but only melted and then refroze the north side. This is why the south doesn’t exhibit the same ice layer as the north does. Early snowfall was probably followed by a period of warm temperatures; this completely melted it away from the south side, but only melted and then refroze the north side. This is why the south doesn’t exhibit the same ice layer as the north does.

12. 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 Subsequent layers (up to about 50 cm) probably both experienced similar faceting. They were both fairly shallow, so any difference in temperature would make a large temperature gradient and start that faceting process. Though the south was still experiencing more solar radiation, so more diurnal temperature shifts, which definitely exceeded the 10 degree/m threshold. The north side also exceeded that threshold, even though its conditions were more constant. Depth hoar became very similar between the two packs. Subsequent layers (up to about 50 cm) probably both experienced similar faceting. They were both fairly shallow, so any difference in temperature would make a large temperature gradient and start that faceting process. Though the south was still experiencing more solar radiation, so more diurnal temperature shifts, which definitely exceeded the 10 degree/m threshold. The north side also exceeded that threshold, even though its conditions were more constant. Depth hoar became very similar between the two packs.

13. 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 As more snow fell, the bottom layers became insulated. On the north side, grains generally consolidated and rounded with the more constant, cooler conditions. The increased depth of the snow probably discouraged faceting processes. The south aspect, though, probably experienced much more near-surface faceting. It got much warmer in the day but had the same cold temperatures at night, meaning pronounced positive and negative gradients and bidirectional faceting. The solar radiation also probably melted some snow, meaning a more shallow pack which would exacerbate existing differences in temperature. This would explain the ice layer I found near the surface of the south pit. As more snow fell, the bottom layers became insulated. On the north side, grains generally consolidated and rounded with the more constant, cooler conditions. The increased depth of the snow probably discouraged faceting processes. The south aspect, though, probably experienced much more near-surface faceting. It got much warmer in the day but had the same cold temperatures at night, meaning pronounced positive and negative gradients and bidirectional faceting. The solar radiation also probably melted some snow, meaning a more shallow pack which would exacerbate existing differences in temperature. This would explain the ice layer I found near the surface of the south pit.

14. 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. Subnivean mammals: the bottom layers are almost exactly the same, and over the entire depth, density is about equal. Ease of burrowing and the snow’s insulative capacity are probably about equal. Subnivean mammals: the bottom layers are almost exactly the same, and over the entire depth, density is about equal. Ease of burrowing and the snow’s insulative capacity are probably about equal.

15. 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 With only two samples, it’s hard to know whether these patterns were anomalies or not. An important future project would be to install thermocouple arrays on north and south aspects during the summer, so that through the winter they could monitor temperature gradients constantly. It would also be interesting to compare the layers found with data from the Berthoud Pass weather station. We could compare precipitation history, to try to match large layers with large snow events, as well as general weather data. If we monitored the changes throughout the season, we might be able to match particularly cold or warm temperatures with faceted layers or ice crusts. Similarly, a long period of cloudy, moderate temperatures might be able to be linked with a period of rounding in the snowpack. With only two samples, it’s hard to know whether these patterns were anomalies or not. An important future project would be to install thermocouple arrays on north and south aspects during the summer, so that through the winter they could monitor temperature gradients constantly. It would also be interesting to compare the layers found with data from the Berthoud Pass weather station. We could compare precipitation history, to try to match large layers with large snow events, as well as general weather data. If we monitored the changes throughout the season, we might be able to match particularly cold or warm temperatures with faceted layers or ice crusts. Similarly, a long period of cloudy, moderate temperatures might be able to be linked with a period of rounding in the snowpack.

16. 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. At lowest layers, probably insulated from any solar radiation effects. Only changes higher layers. Supports the second part of my hypothesis, that greater solar radiation will lead to greater faceting because of the greater positive and negative gradients. This goes very nicely with the results of Birkeland et al (1998), who studied near-surface faceting over a variety of pits. They pointed out large positive and negative temperature gradients as the most important factor in near-surface faceting. My project agrees with their conclusions because the south aspect had much more swings in temperature.At lowest layers, probably insulated from any solar radiation effects. Only changes higher layers. Supports the second part of my hypothesis, that greater solar radiation will lead to greater faceting because of the greater positive and negative gradients. This goes very nicely with the results of Birkeland et al (1998), who studied near-surface faceting over a variety of pits. They pointed out large positive and negative temperature gradients as the most important factor in near-surface faceting. My project agrees with their conclusions because the south aspect had much more swings in temperature.

17. 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