title: Hail formation triggers rapid ash aggregation in volcanic plumes.
abstract: During explosive eruptions, airborne particles collide and stick together, accelerating the fallout of volcanic ash and climate-forcing aerosols. This aggregation process remains a major source of uncertainty both in ash dispersal forecasting and interpretation of eruptions from the geological record. Here we illuminate the mechanisms and timescales of particle aggregation from a well-characterized 'wet' eruption. The 2009 eruption of Redoubt Volcano, Alaska, incorporated water from the surface (in this case, a glacier), which is a common occurrence during explosive volcanism worldwide. Observations from C-band weather radar, fall deposits and numerical modelling demonstrate that hail-forming processes in the eruption plume triggered aggregation of ∼95% of the fine ash and stripped much of the erupted mass out of the atmosphere within 30 min. Based on these findings, we propose a mechanism of hail-like ash aggregation that contributes to the anomalously rapid fallout of fine ash and occurrence of concentrically layered aggregates in volcanic deposits.
authors.affiliation: David A. Johnston Cascades Volcano Observatory, US Geological Survey, Vancouver, Washington 98683, USA.|School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA.
authors.forename: Alexa R
authors.initials: AR
authors.lastname: Van Eaton
authors.affiliation: David A. Johnston Cascades Volcano Observatory, US Geological Survey, Vancouver, Washington 98683, USA.
authors.forename: Larry G
authors.initials: LG
authors.lastname: Mastin
authors.affiliation: Department of Geography, University of Cambridge, Cambridge CB2 3EN, UK.
authors.forename: Michael
authors.initials: M
authors.lastname: Herzog
authors.affiliation: Alaska Volcano Observatory, US Geological Survey, Anchorage, Alaska 99508, USA.
authors.forename: Hans F
authors.initials: HF
authors.lastname: Schwaiger
authors.affiliation: Alaska Volcano Observatory, US Geological Survey, Anchorage, Alaska 99508, USA.
authors.forename: David J
authors.initials: DJ
authors.lastname: Schneider
authors.affiliation: Alaska Volcano Observatory, US Geological Survey, Anchorage, Alaska 99508, USA.
authors.forename: Kristi L
authors.initials: KL
authors.lastname: Wallace
authors.affiliation: School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA.
authors.forename: Amanda B
authors.initials: AB
authors.lastname: Clarke