IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4A1_3E-O3

Room A1

Date/Time: July 24 9:15-9:30

Buoyancy of plume-sourced ash clouds: implications for ash transport modelling.
Stephen RJ Sparks1 , Peter Baines2 , Rose Burden1 , Samantha Engwell1 , Andrew Hogg3 , Herbert E
Huppert1 , Chris Johnson3 , Jessica Kandlbauer1 , Jeremy C Phillips1 , Mark Woodhouse31

School of Earth Sciences, Bristol University, United Kingdom, 2 Department of Infrastructure
Engineering, University of Melbourne, Australia, 3 School of Mathematics, Bristol University, United
KingdomE-mail: Steve.Sparks@bristol.ac.uk

Volcanic plumes ascend high into the atmosphere where they spread out at a level of neutral buoyancy to form
intrusions. The structure of these intrusions depends on the relative strength of the intrusions, the ambient wind
and the local atmospheric stratification. In a strong wind, moderate to weak sized eruptions form bent over plumes
while in more powerful eruptions or in eruptions with a weak wind, they typically form umbrella clouds, which spreadin all directions. Irrespective of the wind the plumes reach dynamical equilibrium with the wind field further from thevolcanic source. The motion of such plume-fed intrusions is governed by buoyancy. The spreading of intrusions iscontrolled by the volumetric flux of the feeding plume at the height of neutral buoyancy and the density stratificationin the atmosphere, but not by the density of the intrusion itself. For the case of a symmetrically spreading umbrella
cloud the thickness decreases with distance. Although more complex in detail, intrusions affected by the wind also
thin quite rapidly with distance. Buoyancy thinning can explain why ash clouds are observed to become very thinquite close to source. Advection diffusion models are now widely used to forecast ash clouds dispersal and ash
deposition. Such models typically assume ash is dispersed vertically above the source and assume ash particles
act as heavy (sedimenting) tracers that are spread by atmospheric diffusion. Buoyancy effects are ignored. We
contend that such models are not a correct description of the physics of ash clouds in regions where buoyancy
effects are significant. For very powerful eruptions buoyancy effects are dominant to distances of hundreds ofkilometres or more; it seems unlikely that an advection-diffusion model could reproduce observed ash distributions
since such models cannot have upwind or very extensive cross-wind spreading. For weaker plumes that aremarkedly affected by wind, advection-diffusion models are useful mathematical descriptions that can be calibrated
to give good forecasts of ash transport. However, this does not mean they are good physical models of the process.
Models of buoyancy spreading suggest that it can be the main cause of lateral spreading of wind blown clouds tosignificant distances (perhaps tens or hundreds of kilometres). Operational models of ash dispersal are likely toremain structured as advection diffusion models, so they may need some empirical adjustments to take accountof buoyancy. For example it might be better to have source terms, which assume ash is concentrated in narrow
height intervals above the source (at one or more discrete levels rather than distributed vertically). It seems unwise
to continue using such models for very powerful eruptions.

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