Forecasting Tephra Dispersion Using TEPHRA2

The Challenge: Tephra sedimentation models are used to forecast the accumulation of tephra across a region due to volcanic eruptions, or to infer the characteristics of past eruptions from tephra fallout deposits. We need tephra fallout models that we can trust in order to make reliable forecasts crucial to hazard assessment!


painting

Volcan Cotopaxi, 1862, Painting by F. Church


Tephra Fallout Models must be physically realistic:
  • capture major elements of the eruption process (eg. plume diffusion)
  • account for major features of atmospheric transport (eg. wind advection, atmospheric diffusion)
  • consider particle size variation and settling velocity
Tephra Fallout Models must be tested (validated and verified):
  • given model assumptions, is the result correctly calculated?
  • does the model explain observed variation in tephra deposition?
TEPHRA2 forecasts tephra dispersion by:
  • relying on an analytical solution to tephra transport and sedimentation
  • making the code specific to a region by using:
  • Note: TEPHRA2 is an improved and optimized version of TEPHRA. Further details.


Steps and Assumptions in Our Implementation

1 A vertical eruption column is assumed to extend above the vent. This column is discretized. Particles fall from each of these heights.


2 The total grainsize distribution is estimated for the eruption, assuming a normal distribution in phi units.

particle distribution
height above vent

3 The total tephra mass is distributed vertically in the eruption column based on a probability density function for mass as a function of height. For example, for an umbrella cloud, mass might be equally distributed in the upper 20% of the total column height. Further details.

umbrella cloud

4 Calculate the total fall time of a particle from the point of release in the eruption column to the ground surface (accounting for the topography). The particle fall time depends on particle properties (density, diameter) and atmospheric density. Settling velocity is determined assuming spherical particles and accounting for the variation in particle Reynolds Number and atmospheric density.

single particle

5 Eruption column radius increases with height and this affects deposition. Particles are more spread out than they would be if they were released from a vertical line source. This effect is accounted for by increasing the diffusion time as a function of height in the column (see step 6). Further details.

eruption column

6 Diffusion of particles in the atmosphere is estimated using a bivariate Gaussian probability density function to approximate turbulence. The scale of diffusion (described by the diffusion law) depends on total particle fall time, which depends on particle size, release height, and elevation of the terrain the particle impacts. Further details.

gaussian diffusion

7 Particles fall through a stratified atmosphere. Wind speed and direction change between layers, based on REANALYSIS or locally collected wind data.

a stratified atmosphere

8 Total tephra accumulation is estimated as mass per unit area; most hazard results from excessive mass loading. The isomass is contoured over a region about the volcano. Mass per unit area can be easily converted into thickness knowing the deposit bulk density (i.e. thickness (m) = kgm-2/density).

tephra accumulation
advection-diffusion
tephra fallout

Volcan Reventador, 2002, Photo by G. Eguiguren.

HAZARD MODELS

Numerical models used for tephra hazard assessment (Hazard Models) typically result from the combination and integration of different theories and modeling approaches depending on the specific eruptive scenario and mitigation program required. They can be grouped within two main categories: particle-tracking models and advection-diffusion models. Particle-tracking models are Eulerian or Lagrangian models that can forecast volcanic-cloud position at specific times and space. They are mainly used for aviation-safety purposes. Advection-diffusion models are Eulerian models that describe the solution of the equations of particle diffusion, transport, and sedimentation and can forecast tephra accumulation on ground relative to a particle-release source. These models are mainly used for civil protection purposes, such as giving public warnings and planning mitigation measures. See the website of the IAVCEI Tephra Group for a detailed review of these models.

Examples of particle-tracking models:

  • CANERM (CANadian Emergency Response Model used by the Canadian Meteorological Centre; D'Amours 1998)
  • MEDIA (Model for DIspersion in the Atmosphere used by the Toulouse VAAC)
  • PUFF (particle tracking model used by the U.S. National Weather Service, Anchorage, Alaska; Searcy 1998)
  • VAFTAD (Volcanic Ash Forecast Transport And Dispersion model developed by the NOAA Air Resources Laboratory; Heffter and Stunder 1993)

Examples of advection-diffusion models based on the analytical solution by Suzuki et al. 1983:

  • ASHFALL (Hurst and Turner 1999)
  • FALL3D (Costa et al. 2006)
  • HAZMAP (Macedonio et al. 1988; Barberi et al. 1990)
  • TEPHRA and TEPHRA2 (Connor et al. 2001; Bonadonna et al. 2005)

the suspects

Co-perpetrators! (the brains and the brawn)

Costanza Bonadonna

Marc Byrne

Chuck Connor

Laura Connor

Mikel Diez

Sarah Kruse

Kristin Martin

Ivan Savov


Read the facts:

Suzuki's (1983) model computes the mass of tephra deposited at a location relative to the eruption source using an analytic solution to the diffusion - advection equation and a line source for tephra in the eruption column. A complete description of the original mathematical development is available in:

Suzuki, T., 1983. A theoretical model for dispersion of tephra, in: D. Shimozuru and I. Yokoyama (eds) Arc Volcanism: Physics and Tectonics, Terra Scientific Publishing, Tokyo, 95-116.

The model used here is slightly modified. See:

Connor, C.B., B.E. Hill, B. Winfrey, N.M. Franklin, and P.C. LaFemina, 2001, Estimation of volcanic hazards from tephra fallout, Natural Hazards Review, 2: 33-42. PDF 2.2 Mb

Bonadonna, C., C.B. Connor, B.F. Houghton, L. Connor, M. Byrne, A. Laing, and T. Hincks, 2005. Probabilistic modeling of tephra dispersion: hazard assessment of a multi-phase eruption at Tarawera, New Zealand, Journal of Geophysical Research, 110 (B03203). PDF 1.2 Mb

Further Reading:

Barberi, F., G. Macedonio, M.T. Pareschi, and R. Santacroce, 1990. Mapping the tephra fallout risk: an example from Vesuvius, Italy, Nature, 344, 142-144.

Costa, A., G. Macedonio and A. Folch, 2006. A three-dimensional Eulerian model for transport and deposition of volcanic ashes, Earth and Planetary Science Letters, 241 (3-4), 634-647.

D'Amours, R., 1998. Modeling the ETEX plume dispersion with the Canadian emergency response model, Atmospheric Environment, 32 (24), 4335-4341.

Heffter, J.L., and B.J.B. Stunder, 1993. Volcanic Ash Forecast Transport and Dispersion (Vaftad) Model, Weather and Forecasting, 8 (4), 533-541.

Hurst, A.W., and R. Turner, 1999. Performance of the program ASHFALL for forecasting ashfall during the 1995 and 1996 eruptions of Ruapehu volcano, New Zealand Journal of Geology and Geophysics, 42 (4), 615-622.

Macedonio, G., M.T. Pareschi, and R. Santacroce, 1998. A numerical simulation of the Plinian fall phase of 79 AD eruption of Vesuvius, Journal of Geophysical Research-Solid Earth and Planets, 93 (B12), 14817-14827.

Searcy, C., K. Dean, and W. Stringer, 1998. PUFF: A high-resolution volcanic ash tracking model, Journal of Volcanology and Geothermal Research, 80 (1-2), 1-16.


Download the source code:

A 1-processor version running under cygwin: tephra2.zip

A 1-processor version running under linux: tephra2.tgz

Questions? Check out the online FAQ..

Email: Laura Connor at lconnor@cas.usf.edu
Email: Chuck Connor at cconnor@cas.usf.edu
Email: Costanza Bonadonna at costanza@cas.usf.edu