ATMOSPHERIC IMPACTS ON PLUME STABILITY

Explosive volcanic eruptions almost always occur in an active wind field. This poses a challenge to hazard mitigation because wind is known to influence plume stability. Specifically, there is evidence that different ratios of mass eruption rate to ambient wind may cause the plume to become more or less stable. The transition between stable and unstable plumes depends on eruption column characteristics controlled by conditions at the volcanic vent and in the surrounding atmosphere. Since column stability determines the types of hazard associated with a volcanic eruption, there is a need to quantify the transition between stable and unstable eruptions. We investigate atmospheric impacts on plume stability thresholds using ATHAM (Active Tracer High-resolution Atmospheric Model) to simulate volcanic eruptions in 3-dimensions in no-wind and with-wind environments under different atmospheric profiles. The mass eruption rate (MER) is a key parameter for constraining the stability threshold as it is a first-order control on column height. Simulation output is used to find thresholds of plume instability for MER-wind ratios with tropical and high-latitude atmospheric profiles. There are significant differences in plume rise heights between eruptions in high and low latitudes, with eruptions in tropical latitudes reaching plume heights 1.3-1.5x higher than their high-latitude counterparts due to the height of the tropopause. Wind can stabilize plumes that were unstable in the no-wind case by increasing the rate of ambient air entrainment and, therefore, increasing local buoyancy. Conversely, plumes may become unstable with too much wind; in that case, they are blown over to produce large pyroclastic flows with low phoenix clouds. Wind-induced instability is a new concept in volcanic plume modeling and has significant implications for future hazard analyses.