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Scale analysis of avalanche activity on persistent snowpack weaknesses with respect to large-scale backcountry avalanche forecasting Haegeli, Pascal Thomas

Abstract

Information about existing snowpack weaknesses is pertinent for backcountry avalanche forecasting. The incorporation of detailed information about snowpack weaknesses, however, significantly increases the complexity of the forecasting process. The goal of this research was to examine the scale characteristics of persistent snowpack weaknesses and related avalanche activity with respect to large-scale backcountry avalanche forecasting (≥1000 km²). The study focuses on the snowpack of the mountain ranges in Western Canada, namely the maritime Southern Coast Mountains, the transitional Columbia Mountains and the continental Rocky Mountains. Scaling and scale issues are of fundamental importance in the avalanche forecasting process due to the multi-scale character of the avalanche phenomenon. While professionals have developed successful strategies to use information across scales, scaling has to be incorporated explicitly into formalized forecasting approaches. Hierarchy theory (Ahl and Allen, 1996) is used as a conceptual framework for discussing scale issues in avalanche forecasting. The two-dimensional reference system consists of a temporal hierarchy of seven levels representing the main groups of factors contributing to avalanches. Within each temporal level, there is an embedded spatial hierarchy of processes. The SNOWBASE database of Canadian Mountains Holidays (1996/97 - 2000/01) and the InfoEx dataset of the Canadian Avalanche Association (1991/92 - 2001/02) were used to examine the temporal and spatial characteristics of three main types of persistent snowpack weaknesses (weak layers of faceted grains, surface hoar layers and pure crust interfaces) and their related avalanche activity. While significant weaknesses of all types were frequently observed across large parts of the study area, avalanche activity related to the three different weakness types exhibited distinct patterns in space and time. 'Avalanche winter regimes' are suggested as a new classification scheme for describing local avalanche characteristics with respect to forecasting. While existing snow climate classifications (see, e.g., Mock and Birkeland, 2000) focus only on the average winter weather characteristics, it is the comprehensive character of a winter including the sequence of events that produce persistent weaknesses that is of crucial importance for backcountry avalanche forecasting. The analysis reveals three distinct avalanche winter regimes for Western Canada.

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