Glide avalanche response to an extreme rain-on-snow event,Snoqualmie Pass, Washington, USA

摘要

Rain-on-snow events trigger immediate and delayed avalanches as liquid water penetratesthe snowpack. We present results from an extreme rain-on-snow event that triggered a glide avalanchenear Snoqualmie Pass, Washington, USA. Snoqualmie Pass recorded 463 cm of snowfall from13 December 2008 to 6 January 2009. This period of snowfall was followed by a strong southwesterlytropical flow that resulted in an extreme rain-on-snow event. Sensors at Snoqualmie Pass recorded285 mm of precipitation over a 52 hour period. Flooding, slush flows, landslides and avalanchesresulted from the influx of precipitation. Snow heights decreased rapidly over the period, withsettlement rates approaching 80 mm h~(-1).Liquid water infiltrated and flowed through the snowpackwithin a few hours of the arrival of rain, yet many of the major avalanches occurred 12-30 or morehours after the onset of rain and water outflow. A glide avalanche occurred -30 hours after the onsetof rain and the establishment of drainage through the snowpack. Increasing glide rates correlate withperiods of rapid snow settlement. Here glide rates approached 670 mm h~(-1).Although glide andsettlement rates increased during periods of intense precipitation, glide failure occurred 8 hours afterpeak precipitation and outflow.