Synoptic atmospheric circulation patterns associated with deep persistent slab avalanches in the western United States

Andrew R Schauer,Jordy Hendrikx,Karl W Birkeland,Cary J Mock

doi:10.5194/nhess-21-757-2021

Andrew R Schauer, Jordy Hendrikx + Show 2 more

Open Access

https://doi.org/10.5194/nhess-21-757-2021

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Abstract

Abstract. Deep persistent slab avalanches are capable of destroying infrastructure and are usually unsurvivable for those who are caught. Formation of a snowpack conducive to deep persistent slab avalanches is typically driven by meteorological conditions occurring in the beginning weeks to months of the winter season, and yet the avalanche event may not occur for several weeks to months later. While predicting the exact timing of the release of deep persistent slab avalanches is difficult, onset of avalanche activity is commonly preceded by rapid warming, heavy precipitation, or high winds. This work investigates the synoptic drivers of deep persistent slab avalanches at three sites in the western USA with long records: Bridger Bowl, Montana; Jackson, Wyoming; and Mammoth Mountain, California. We use self-organizing maps to generate 20 synoptic types that summarize 5899 daily 500 mbar geopotential height maps for the winters (November–March) of 1979/80–2017/18. For each of the three locations, we identify major and minor deep persistent slab avalanche seasons and analyze the number of days represented by each synoptic type during the beginning (November–January) of the major and minor seasons. We also examine the number of days assigned to each synoptic type during the 72 h preceding deep persistent slab avalanche activity for both dry and wet slab events. Each of the three sites exhibits a unique distribution of the number of days assigned to each synoptic type during November–January of major and minor seasons and for the 72 h period preceding deep persistent slab avalanche activity. This work identifies the synoptic-scale atmospheric circulation patterns contributing to deep persistent slab instabilities and the patterns that commonly precede deep persistent slab avalanche activity. By identifying these patterns, we provide an improved understanding of deep persistent slab avalanches and an additional tool to anticipate the timing of these difficult-to-predict events.

Highlights

Deep persistent slab avalanches are challenging to predict and generally more destructive than other snow avalanche types
We explored various self-organizing map (SOM) configurations using 9, 12, 15, 20, 25, 35, and 56 node arrays in order to determine an appropriate number of synoptic types to retain
The array shows a gradual transition from meridional flow in the top rows to zonal flow in the bottom rows

Summary

Introduction

Deep persistent slab avalanches are challenging to predict and generally more destructive than other snow avalanche types. They threaten infrastructure, transportation, and recreationists in snow-covered mountain regions around the world (McClung and Schaerer, 2006). Deep persistent slab avalanches occur due to two different mechanisms, depending on whether they are primarily dry or wet. Dry deep persistent slab avalanches occur when a weak layer in the snowpack fails due to applied stress from an external load such as new and wind-transported snow, cornice fall, explosives, or the weight of a human. In wet deep persistent slab avalanches, the weak layer typically fails due to the introduction of liquid water to the snowpack, which deteriorates the bonds between grains in the weak layer such that the weak layer can no longer support the weight of the overlying slab (Baggi and Schweizer, 2009; Marienthal et al, 2012; Pietzsch, 2009).

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Discussion

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