Meteorological conditions associated with rain-related periglacial debris flows on Mount Hood, Oregon and Mount Rainier, Washington

Abstract

In November of 2006 an intense rainstorm of tropical origin, known colloquially as the "Pineapple Express," inundated the Pacific Northwest region of the United States, initiating numerous periglacial debris flows on several of the stratovolcanoes in the Cascade Range of Oregon and Washington. These debris flows rapidly aggrade channels, deposit thick sediments in their path, and severely damage infrastructure. Consequently, this work seeks to understand the potential meteorological triggering mechanisms of these flow events. Here we focus on Mount Hood, Oregon and Mount Rainier, Washington in the investigation of the meteorological conditions associated with rain-related periglacial debris flow events and the variability of these conditions over time. The objectives of this research are to assess the correlation between "Pineapple Express" and "Atmospheric River" events and rain-related debris flows, and to explore the meteorological conditions associated with debris flow events based on 5 parameters: storm track based on geostrophic flow patterns, temperature, precipitation and orographic enhancement, integrated atmospheric moisture transport, and antecedent snow water equivalent (SWE). Dates for the debris flow events for each mountain were linked with corresponding Pineapple Express circulation and Atmospheric River events. Analysis from this work suggests that there is not a strong correlation between the occurrence of debris flows and the occurrence of Pineapple Express or Atmospheric River events as they are presently defined in the literature. NCEP/NCAR reanalysis data were used to determine geostrophic flow from 500h-Pa heights. Radiosonde data from Salem, Oregon and Quillayute, Washington were used to examine freezing altitudes. Precipitation data from Government Camp and Paradise meteorological stations were used to determine total rainfall amounts for rain events, and these data were compared with precipitation data from coupled lower elevation sites (Three Lynx and Longmire, respectively) to determine orographic enhancement values for each event. Reanalysis data were again used to determine the strength and direction of atmospheric moisture transport. Snowpack Telemetry (SNOTEL) data were used to examine the antecedent snowpack conditions for each debris flow event. Debris flows on both Mount Hood and Mount Rainier were found to be associated with both meridional and zonal flow regimes, variable precipitation, and unimpressive orographic enhancement values. However, the debris flow events virtually all experienced significantly high freezing altitudes and little or negligible antecedent SWE. Further, nearly all debris flow events were coupled with plumes of atmospheric moisture transport with high values relative to the surrounding region, implying Atmospheric River-like conditions. This finding evokes a potential need to re-examine the metrics used to classify or characterize Atmospheric Rivers, particularly through the lens of their relationship to natural hazards. This research suggests that given the complexity of debris flow mechanics, the dynamic nature of the atmospheric system, and the small sample of data presented here, definitive conclusions cannot yet be made concerning the correlation between specific meteorological parameters and the occurrence of periglacial debris flows in the Cascades.