Ascent dynamics of large phreatomagmatic eruption clouds: The role of microphysics

摘要

We examine the impact of abundant surface water interaction on the development of volcanic clouds from large-scale (>10~8 kg s~(-1) magma) phreatomagmatic eruptions,presenting the first 2-D large-eddy simulations of "wet" volcanic plumes that incorporate the effects of microphysics.The Active Tracer High-Resolution Atmospheric Model was forced with field-derived inputs from an exceptionally large phreatomagmatic eruption:the 27 ka Oruanui supereruption (Taupo volcano.New Zealand).Surface water contents were varied from 0 to 40 wt% for eruptions with equivalent magma eruption rates of ～4.3 x 10~8 and 1.1 x 10~9 kg s~(-1) Our findings confirm that increased surface water has a pronounced impact on column stability,leading to transitional column behavior and hybrid clouds resulting from simultaneous ascent of material from stable columns and pyroclastic density currents (PDCs).Contrary to the suggestion of previous workers, however,abundant surface water docs not systematically lower the spreading level or maximum height or volcanic clouds,owing to vigorous microphysics-assisted lofting of PDCs.The simulated behavior and ash cloud dimensions provide a close match to field evidence from the Oruanui case study.Cloud heights from the collapsing eruption columns also show a notable sensitivity to changes in the altitude of the tropopausc,while ambient humidity primarily impacts the abundance of airborne hydrometeors (particularly ice) associated with the volcanic clouds.General relationships between eruption style,meteorological conditions,and the resulting vertical profiles of volcanic emissions outlined here could also be adapted for use in operational volcanic ash forecasting and deposit reconstruction techniques already in existence.