Figure 4 - uploaded by Bruce Jamieson
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Relative frequency of the differences between the danger rating from the regional forecast (D RF ) and from the local nowcast (D LN ) for the Coast Mountains, Columbia Mountains and Rocky Mountains
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In the winters of 2004-05 and 2005-06, 235 local "nowcasts" of the avalanche danger at and below treeline in the Coast, Columbia and Rocky Mountains of western Canada were compared with the danger rating from the public avalanche bulletin for the region including the nowcast site. These bulletins are issued from three to seven times per week, and t...
Contexts in source publication
Context 1
... positive difference indicates that the regional danger rating was higher than the local nowcast, and negative difference indicates that the regional danger rating was lower than the local nowcast. The distributions of the differences are plotted in Figure 4 for each of the three mountain ranges. Cases in which the difference is zero, ΔD = 0, are called hits and the relative frequency of hits is the hit rate h (Wilks, 1995, p. 240). ...
Context 2
... (62) Columbia (106) Rockies (67) Regional danger -local danger, ΔD In some cases the field teams had read the regional bulletin before traveling to the site of the local observations. The relatively low hit rate, which is most evident for the Coast Mountains and Columbia Mountains (Figure 4), indicates that the local nowcasts were strongly influenced by their local observations. This potential bias is further assessed in Section 4.2 ...
Context 3
... are at least two reasons why there are more positive differences than negative differences in all ranges as shown in Figure 4. First, forecasters-especially when forecasting for large areas-may "err on the side of caution" or focus on the sub-regions with higher danger as a result of the uncertainty associated with forecasting over large areas and up to three days in advance. ...
Context 4
... distribution of ΔD by range for the reduced dataset (n = 192) is plotted in Figure 7. Coast (58) Columbia (106) Rockies (28) local danger lower local danger higher Figure 7. Relative frequency of the difference between the danger rating from the regional forecast and the local nowcast, after filtering the forecaster-study-plot bias. The relative frequencies for the Columbia Mountains are unchanged from Figure 4. ...
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Citations
... Bulletin regions in Europe are significantly smaller and bulletins are generally posted daily. For Canada, Jamieson et al. (2006a) showed that locally verified danger ratings agreed with the regional danger ratings posted in the bulletin in approximately 57 to 64 percent. This percentage was generally higher for smaller forecast areas and large-scale regional danger ratings tended to be more conservative. ...
An exceptionally high number of avalanche fatalities during the winter of 2003 forced the Canadian avalanche community to question the effectiveness of existing public ava-lanche safety programs in Canada. In response to the recommendations of several avalanche safety reviews, the Canadian Avalanche Association launched the ADFAR (Avalanche Decision Framework for Amateur Recreationists) Project for the development of a practical, science-based decision framework for amateur recreationists when planning for, or traveling in avalanche terrain. The goal of the project was to reduce recreational avalanche fatalities by improving risk commu-nication and risk awareness among the fast growing number of winter backcountry enthusiasts in Canada. The Avaluator is a new rule-based decision support tool for amateur recreationists, including backcountry skiers and snowboarders, snowmobile riders and out-of-bounds skiers and snow-boarders. A key part of the Avaluator is a pocket card that assists with planning backcountry trips and facilitates field decisions. The paper provides an overview of the ADFAR project, describes the usage of the Avaluator and discusses the underlying design principles.
... Hence, the value of stability tests would seem to be less in areas covered by a regional avalanche bulletin. However, in Canada many recreationists travel in areas not covered by regional forecasts (bulletins) or in areas for which the forecast regions are large and the bulletins issued three times per week (Jamieson, Campbell and Jones, 2006, subsequently referred to as JCJ). For a typical day of backcountry snowmobiling, snowboarding or ski touring, recreationists are exposed to avalanche paths within an area of roughly 10 km 2 . ...
In western Canada, various agencies issue public avalanche bulletins three to seven times per week for regions which range from less than 500 km 2 to almost 30,000 km 2 . Sometimes avalanche danger varies substantially within the larger regions. In this study, we assessed whether the results of local rutschblock tests (including whole block releases) and compression tests (including sudden fractures) could help recreationists assess the local avalanche danger. Since "weekend" recreationists cannot reliably select areas of below average stability for their snowpack tests, especially in wind affected areas, we restricted the test sites to sheltered areas at and below treeline where our observers were likely to get the same results as recreationists. Field studies in the Coast, Columbia and Rocky Mountains yielded stability test results and local danger ratings. After a small number of data were filtered to minimize an observation bias, the results of compression tests and rutschblock tests were assessed using ratings of the local avalanche danger. Without considering the danger rating from the regional bulletin, the results of stability tests correlated weakly but significantly with the local avalanche danger. The score from the rutschblock test, with its greater area, correlated better than any of the compression test variables with the local avalanche danger. Various combinations of the regional danger rating and stability test results were assessed in terms of their performance in recognizing when the local avalanche danger was higher than the regional rating. Again the rutschblock results were more predictive than the compression test results. Some simple results of stability tests such as the observation of sudden fractures in compression tests and the release of the entire block in rutschblock tests showed promising results.
The energy balance at the snow surface results in substantial diurnal temperature fluctuations within the top portion of the snowpack. These temperature fluctuations, which vary both spatially and over time, can have important effects on stability. During the winters of 2005 and 2006, field data for a near-surface warming study were collected on a treeline knoll located in the Columbia Mountains of British Columbia. Thermocouple arrays placed on the top and at different locations on the knoll side slopes recorded temperatures within the top 30 cm of the snowpack. During each test period, nearby measurements of incoming short and long wave radiation were collected, as were periodic observations of cloud cover, air temperature and the snow surface temperature at each array. The study aims to use field data to identify when, and to what extent, near-surface warming occurs on different aspects. Instrumentation challenges encountered in accurately measuring near-surface snow temperatures are also presented. Preliminary analysis of the data shows a significant correlation between aspect and the maximum daytime increase in near-surface snow temperatures. In one experiment, the magnitude of daytime warming measured at 10 cm depth was almost 7 ºC greater on the south aspect than the north aspect. The aspect dependent differences in daily temperature change decreased with increased cloud cover.
Since the landmark papers of Conway and Abrahamson many studies have tried to quantify spatial variability. Many different methods have been used and the studies covered a variety of scales. Accordingly, some results appear contradictory, suggesting that the degree of spatial variation varies widely. This is not surprising, and is partly due to the methodology used and of course, due to varying natural conditions. Spatial variability is doubtless an inherent property of the snowpack. One important result seems to be that the layering is less variable than, for example, the stability of small column tests. Whereas it is often perceived that the results of the studies were not conclusive, it seems clear that they completely changed our view of spatial variability. We realized the importance of scale issues. For example, the variation will strongly depend on the measurement scale – the so-called support – of the method (SnowMicroPen vs. compression test vs. rutschblock test). Geostatistical analysis has been intro-duced and used to derive appropriate input data for numerical models. Model results suggest that spatial variation of strength properties have a substantial knockdown effect on slope stability and that the effect increases with increasing spatial correlation. The focus on scale has also revealed that spatial variations can promote instability or inhibit it. With the awareness of scale we can now address the causes of spatial variability. Many processes such as radiation and wind act at several scales. The most challenging process is probably wind that might hinder prediction of variability at the slope scale. However, at the regional scale, already today, many avalanche forecasting services try to address differences in respect to slope aspect. We will review the present state of knowledge, discuss consequences for avalanche forecasting and snow stability evaluation, and recommend future research directions.
