Naturally triggered persistent deep slab avalanches in western Canada Part II: weather trends from model forecasts

Abstract

ABSTRACT. For 175 difficult-to-forecast persistent deep slab avalanches, weather data were obtained from Global Environmental Multiscale (GEM) models produced by Environment Canada. The focus was to determine critical parameters and thresholds for avalanche forecasting from GEM and compare them with weather station data analyzed in Part I (Conlan and Jamieson, this issue). The high-resolution GEM-limited-area model (2.5 km resolution) forecasted higher median precipitation amounts than both the lower-resolution GEM15 (15 km resolution) and weather stations within a small dataset. Air temperatures were lower for both weather models compared with the weather station data, likely because of elevation differences. A multivariate classification tree created with GEM15 data correctly classified 29 of 36 avalanches by their primary cause-of-release, using a primary split of modelled solar warming of 5.9°C, 10 cm into the snowpack. For all 175 avalanches, GEM15 forecasted significantly less precipitation than observed at the weather stations, particularly with multi-day cumulative amounts. The majority of GEM15 surface wind speeds were between 0 and 10 km h , producing negligible wind loading amounts. The parameter values may be helpful for predicting future persistent deep slab avalanches. However, GEM output is not always representative of field conditions and should be used in conjunction with other sources.

A Theoretical Model of Drumlin Formation Based on Observations at Múlajökull, Iceland

“…A final observation is that drumlins beneath the outermost~500 m of the glacier, where the ice is less thañ 100 m thick (Lamsters et al, 2016;Minchew et al, 2016), are coincident with overlying crevasse swarms (Johnson et al, 2010). This observation applies to the glacier as photographed in 1995 after the 1992 surge (Johnson et al, 2010), as well as to the margin more recently Johnson et al, 2010).…”

“…Erosive spatial gradients in till flux in a deforming bed could depend on effective stress, but during quiescent flow-particularly near the margin where drumlins have formed-the bed is unlikely to deform. Consideration of the Coulomb strength (e.g., Clarke, 2005) of the Múlajökull till, which has a high peak friction angle of 35°and a cohesion of 18 kPa (McCracken, 2015), indicates that for the basal shear stress calculated within 1 km of Múlajökull's margin (~50 kPa, Minchew et al, 2016), effective stress on the bed would need to be less than~54 kPa to enable till yielding in shear. This value indicates, for example, that for ice 100 m thick near the margin, the pore water pressure equivalent to 85 m of head (very close to that required for complete flotation) would be necessary for till yielding.…”

“…Patterns of basal till movement and effective stress on the bed within and between drumlins are provided by, respectively, 2,200 measurements of till anisotropy of magnetic susceptibility (AMS) calibrated to laboratory ring shear experiments and a similar number of till-density measurements calibrated to laboratory consolidation tests . Additional guidance and constraints are provided by stratigraphic observations Johnson et al, 2010), observations and analyses of drumlin shape Jónsson et al, 2014) and forefield elevation , ground-penetrating radar data that bear on drumlin structure, and baseline glaciological data from Múlajökull (Björnsson et al, 2003;Minchew et al, 2016).…”

Naturally triggered persistent deep slab avalanches in western Canada Part I: avalanche characteristics and weather trends from weather stations

“…This study focuses on weather data obtained from weather stations. Since weather data from a nearby weather station are sometimes unavailable to a forecasting operation, Part II of the study examines similar weather parameters from numerical weather prediction models and compares the data sources (Conlan and Jamieson, 2016).…”