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.
For example, the freezing altitudes of nearly all the storms that caused debris flows are at least one standard deviation warmer than other significant rainfall events occurring in the same season (Figure 1).
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.
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.
The work on freezing level determination demonstrated that using WSR-88D reflectivity returns and the radar brightband height phenomenon provides frequent estimates freezing level during storms. This method is most effective in the strongest land falling storms providing potential to create a series of measurements during select storms to study how freezing levels change between soundings. Directional measurement is also possible, but
depends heavily on storm structure and continuity. This method is best used for case studies or providing additional data during exceptional storms when debris flows are possible.
Analyses of atmospheric temperature profiles from radiosonde data at Salem, OR and Quiillayute, WA indicate that isothermal or inverted atmospheric conditions were typical during debris flow storm events but not characteristic of those storms that did not cause debris flows. Temperature inversions or isothermal conditions were present with greatest frequency at the Quillayute sounding station on days when a debris flow occurred at
Mount Rainier. They are also present in some cases at the Salem Sounding station corresponding to Mt Hood debris flows, however less frequently. We hypothesize that the very warm subtropical-originating Atmospheric River storm events are overriding cooler ambient air causing this characteristic temperature profile phenomenon. Ongoing study will seek to determine the relation between the temperature maximum of the inversion and debris flow triggering elevation as well as the reason for the difference in pattern between Salem and Quillayute.
