Modelling surface hoar formation and evolution on mountain slopes (original) (raw)
Related papers
Predicting the spatial distribution and persistence of buried surface hoar layers is important when evaluating avalanche hazard. This study used weather-based models to predict the formation of surface hoar and investigated how buried layers change over time. Seven years of study plot observations from the Columbia Mountains of British Columbia were used to calibrate models for surface hoar formation. The latent heat flux was modelled with weather station data and forecasted data from the Canadian numerical weather prediction model (GEM15). A linear relationship was found between vapour mass flux and observed surface hoar crystal size (r 2 of 0.84 with weather station data and 0.70 with GEM15 data), and was used to predict crystal size over seven winters. Crystal size predictions had root mean square errors of 2.4 and 4.1 mm with weather station and GEM15 data, respectively. The model was compared with other empirical weather-based models. Layers of buried surface hoar were tracked with shear frame tests, compression tests (CT) and propagation saw tests (PST). PSTs and fracture character in CTs indicated that the propensity for propagation in layers of surface hoar remained high for up to six weeks. Layers with large crystals were found to weakly indicate low stability. Results from this study could be used to improve the representation of surface hoar layers in snow cover models and make spatial predictions with NWP data.
2006
Buried layers of surface hoar often release skier-triggered avalanches in the Columbia Mountains of Canada and their shear strength can be used to assess the stability of a slab overlaying these layers. In 2001 Chalmers introduced an Interval Model to calculate the shear strength of layers of surface hoar based on manual snowprofile observations. We refined his model by adjusting the measured shear strength for the normal load and included only data points where the weak layer depth did not exceed 100 cm to better account for skier triggering. Further, we used average and daily loading rates as well as a regression analysis to determine the best estimate of the shear strength change. Our final Forecasting Model used a multivariate regression to calculate the shear strength on days with snowprofile observations and as well as average and daily loading rates to forecast the shear strength on days without manual snowprofile observations. The performance of the model (r 2 ) was 0.71 and 0.63 using average and daily loading rates, respectively. A companion paper, Part A, develops a forecasting model for weak layers of faceted crystals.
Meteorological, elevation, and slope effects on surface hoar formation
The Cryosphere
Failure in layers of buried surface hoar crystals (frost) can cause hazardous snow slab avalanches. Surface hoar crystals form on the snow surface and are sensitive to micro-meteorological conditions. In this study, the role of meteorological and terrain factors was investigated for three layers of surface hoar in the Columbia Mountains of Canada. The distribution of crystals over different elevations and aspects was observed on 20 days of field observations during a period of high pressure. The same layers were modelled over simplified terrain on a 2.5 km horizontal grid by forcing the snow cover model SNOWPACK with forecast weather data from a numerical weather prediction model. Modelled surface hoar growth was associated with warm air temperatures, high humidity, cold surface temperatures, and low wind speeds. Surface hoar was most developed in regions and elevation bands where these conditions existed, although strong winds at high elevations caused some model discrepancies. SNO...
Snowpack factors associated with strength changes of buried surface hoar layers
. Every winter in North America, failures in layers of buried surface hoar frost release many slab avalanches, some of which kill recreationists. Some surface hoar layers stabilize within a week of burial and others require a month or more. Little is known about whether snowpack factors such as crystal size, snowpack depth, slab thickness, load, temperature and temperature gradient are associated with strength changes of these layers. We tested buried surface hoar layers once or twice per week at study sites in the Columbia Mountains from 1994 to 1998, and measured over 300 changes in shear strength. We assess the factors associated with the rate of strength change using rank correlations. The factors are ranked to identify which are most relevant for forecasting changes in strength. Useful predictors include the total snowpack depth, the maximum grain size, and slab depth. The correlations between these predictors and the measured rate of strength change are discussed in terms of physical processes. We illustrate the predictive potential of combined factors by comparing measured values of the rate of strength change with fitted values from a regression tree. q 0165-232Xr99r$ -see front matter q 1999 Elsevier Science B.V. All rights reserved.
Micrometeorological and morphological observations of surface hoar dynamics on a mountain snow cover
Water Resources Research, 2010
1] The formation, growth, and destruction of surface hoar crystals is an important feature of mountain snow covers as buried surface hoar layers are a frequent weak layer leading to unstable snowpacks. The energy and mass exchange associated with surface hoar dynamics is further an important part of land-atmosphere interaction over snow. A quantitative prediction of surface hoar evolution based on local environmental conditions is, however, difficult. We carried out measurements of crystal hoar size and total surface mass changes in the period between January and March 2007 on the Weissfluhjoch study plot of the WSL Institute for Snow and Avalanche Research SLF, located above Davos, Switzerland, at 2540 m above sea level. For the first time, a direct comparison between eddy correlation measurements of latent heat flux and lysimeter-like measurements of surface mass change has been made. Results show that the growth of surface hoar crystals is very well correlated with deposition of water vapor during clear-sky nights as measured by two eddy correlation systems placed close to the ground. By analyzing local meteorological data, we confirm that low to moderate wind speed, humid air, and clear-sky nights are the necessary ingredients for the occurrence of significant vapor fluxes toward the surface and thus for the growth of surface hoar. We also confirm that surface hoar crystals tend to preserve during daytime, when strong sublimation occurs, although their size significantly reduces. Despite the complexities associated with mountain terrain and snow surfaces, such as nonequilibrium boundary layers and stratification effects, the hoar formation could be predicted by the snow cover model SNOWPACK, which uses a bulk Monin-Obukhov (MO) parameterization for the turbulent heat fluxes. On the basis of the comparison between direct observations and model predictions, we suggest that neutral stability conditions in the MO formulation provide the most stable and least flawed prediction for surface hoar formation. Citation: Stössel, F., M. Guala, C. Fierz, C. Manes, and M. Lehning (2010), Micrometeorological and morphological observations of surface hoar dynamics on a mountain snow cover, Water Resour. Res., 46, W04511,
2010 International Snow Science Workshop, 2010
Understanding the spatial distribution of weak layers is a significant challenge for avalanche forecasters. Thus, improving our understanding of the processes that dictate the formation and persistence of surface weak layers across large areas is critically important for improving backcountry avalanche forecasting accuracy. For this work, heli-skiing guides mapped unburied surface hoar and near-surface facets across the Chilkat and Takhinsha Mountains of southeast Alaska during two major formation events of the 2010 season. After burial, we monitored weak layer persistence and avalanche activity. Our study area encompasses 900 km 2 of rugged, glaciated, alpine terrain, at elevations ranging from 300 m to 2000 m. Guides collected information on crystal attributes and terrain characteristics at each location, and used handheld GPS units to reference locations. Incorporating the data into a Geographic Information System (GIS) proved to be invaluable for managing and visualizing observations. For example, the GIS allowed the creation of surface condition maps that we used operationally in guide meetings and in the helicopter for making run decisions. Data analyses quantified observed patterns. In particular, we often found that surface hoar crystal size lessened with decreasing elevation, possibly due to stronger katabatic winds in valley bottoms. Additionally, we observed areas of inhibited surface hoar formation and persistence which may have resulted from the influence of synoptic inflow and outflow drainage winds. By better understanding the distribution of surface hoar and near-surface facets, this work provides insights into improved backcountry forecasting of avalanche conditions over large areas.
Spatial Predictions of Surface Hoar and Crust Formation
International Snow Science Workshop Grenoble Chamonix Mont Blanc October 07 11 2013, 2013
Understanding the distribution of critical snowpack layers is important when assessing avalanche hazard. Two common critical layers, surface hoar and melt-freeze crusts, form under specific weather conditions. This study explores the possibility of modelling the formation of these layers with forecasted weather data. Surface hoar and sun crusts were tracked at study sites on two mountains in the Columbia Mountains of Canada. Weather data from automated stations near these sites were compared to forecast data from two numeric weather prediction (NWP) models (15 and 2.5 km grids). The latent heat flux and net shortwave radiation were modelled with the snow cover model SNOWPACK and related to observed surface hoar crystal size and sun crust thickness. Surface hoar formation was then predicted across western Canada with NWP data. Comparing these predictions with observations made by avalanche professionals at 112 study plots found that surface hoar occurrence was generally over-predicted. Spatial predictions with forecast data could help avalanche forecasting in data sparse areas.
Cold Regions Science and Technology, 2006
This paper presents an adjustment for the normal load for persistent weak layers in the Columbia Mountains of Canada and an improved empirical model to forecast the shear strength of layers of faceted crystals for indexing the stability of the snowpack. This model is based on manual snowprofile observations, as was a previous model; however, the measured shear strength is adjusted for the normal load by a constant factor of ϕ = 0.21. The study includes the analysis of more field data for the rate of change of shear strength, which is assessed for daily and average loading rates. The Forecasting Model predicts the shear strength of layers of faceted crystals with an accuracy of 77-79% depending on whether the shear strength change between snowprofile observations is based on daily or average loading rates. A companion paper, Part B, develops a forecasting model for layers of buried surface hoar.
Observations on buried surface hoar in British Columbia, Canada: section analyses of layer evolution
Observations on sections cut through buried surface hoar layers over time offer the opportunity to measure microstructural changes associated with strengthening. We present preliminary results of new analyses carried out with measurements on sections from snow sampled every week to ten days from midwinter to late winter over four years, 1995 Our measurements focused on changes in basic geometry of buried surface hoar layers, particularly where the buried surface hoar crystals were bonded to the underlying snow. As with previous results, over time the thickness of the layer containing surface hoar crystals decreased as strength increased. In samples from sloping snow, the effect of creep was evident in changing orientation of the cut profiles of buried surface hoar. We attempted to identify bond lines using Kry's criteria to assess bond area density, size and change. Results show that low shear strength of buried surface hoar stems from low area density of bonds at the base of layers of surface hoar crystals. The bond number density increases around the bases of some crystals over the course of several weeks. These measurements seem to support a recent conceptual model involving differential settlement of the buried surface hoar crystals, but evidence was statistically inconclusive pending more measurements.