HR: 0800h
AN: V31A-0296    [Abstracts]
TI: Hydrometeor-Enhanced Tephra Sedimentation
AU: * Durant, A
EM: adam.durant@bris.ac.uk
AF: Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, United States
AU: * Durant, A
EM: adam.durant@bris.ac.uk
AF: Now at: Department of Earth Sciences/School of Geographical Sciences, University of Bristol, Queens Road, Bristol, BS8 1RJ, United Kingdom
AU: Rose, W
EM: raman@mtu.edu
AF: Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, United States
AB: New evidence presented here supports hydrometeor formation as a fundamental process in volcanic cloud sedimentation. Distal fallout from several recent eruptions was characterized using laser diffraction particle size analysis for diameters between 0.2-2000 μm. Based on simple modeling, sedimentation rates of most ash particles <100 μm are faster than single particle terminal velocities. In the case of the Mount St. Helens 18 May 1980 (MSH80) eruption, fallout at <300 km involved single particle fall in addition to multiple-particle aggregate fall. Fallout at >300 km was predominantly through the formation of aggregates composed of a particle subpopulation with a mode at 18 μm. Airborne measurements, satellite remote sensing, observations of mammatus clouds and simple model calculations indicate volcanic clouds contain abundant water and hydrometeors. Observational and modeling studies of mammatus clouds on thunderstorm anvils provide insight and constraints on volcanic analogues. In a conceptual model presented here, ash particles initiate hydrometeor formation and subsidence of the cloud deck occurs through mammatus generation. Rapid aggregation and fallout occur as the cloud passes through the melting level in a process analogous to snowflake growth. Cloud particles then settle en masse, forming the distal mass deposition maxima observed in many recent volcanic ash-fall deposits. Based on this new insight, VATDM should include the effects of hydrometeor formation to accurately model distal volcanic fallout.
DE: 0320 Cloud physics and chemistry
DE: 0370 Volcanic effects (8409)
DE: 8409 Atmospheric effects (0370)
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting