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WRF Volcano modelling studies, NCAS Leeds Ralph Burton, Stephen Mobbs, Alan Gadian, Barbara Brooks. Why use WRF?. WRF = Weather Research and Forecasting – NCAR, U.S. State-of-the-art numerical weather prediction model Can be run at a variety of scales, from O(100m) to many 10s of kms - PowerPoint PPT Presentation
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WRF Volcano modelling studies,
NCAS Leeds
Ralph Burton, Stephen Mobbs, Alan Gadian, Barbara Brooks
Why use WRF?
State-of-the-art numerical weather prediction model
Can be run at a variety of scales, from O(100m) to many 10s of kms
Full range of microphysics, boundary layer, radiation, convection, etc. etc. schemes
Open-source – used in over 140 countries
Code is modular
Initialisation fields easily obtained
Runs either on desktop machine or national supercomputer -
scales very well
WRF = Weather Research and Forecasting – NCAR, U.S.
“Ash” is a passive tracer, but is assigned a settling velocity to mimic the effect of mass.
Relative velocities between particle and gas phases: U = V = 0W ≠ 0
Settling velocity is a function of height and density – from Kasten et al. 1968
x
y
z
Time = t
U
I.
x
y
z
Time = t + Δt
U
II.
x
y
zU
II(a).
U’
Time = t + Δt
Leeds implementation: Methodology I.
One-way coupling: ambient atmosphere affects ash, but not vice-versa
U’ = (0,0,-w’)
Up to 7 tracers (or ash species) at the moment. Thus,7 different densities of ash (plus combined field).
Dry Deposition: have included this but not tested it.
(Method: X% of ash is removed at surface. X could dependupon surface type) [X?]
Wet Deposition: have included this but not tested it.(Method: ash is removed when cloud water mixing ratiois greater than Y g/Kg) [Y?]
N.B. no interaction with microphysicsat present
N.B. Grimsvotn 2011?
Leeds implementation: Methodology II.
Leeds implementation: Methodology III.
All ash “species” (i.e. bins) are emitted at same rate.Different emission rates for different densities?
Some key parameters:
Emission rates ? Emission rates for different types of ash ?
Plume height / thermal perturbation ?
Density of ash ?
One-way coupling:Ambient atmosphere affects ash, but not vice-versa
Different applications.
Near-vent: 100m resolution, 141 levels, 25km x 25km
Initialised via GFS / ECMWF or radiosonde profiles
Ash initialised with heat source and point releaseOrder of minutes forecast
Point source,Strong O(100K) thermal perturbation
Updraughts ~ 50m/s
Different formulations of the model I.
Near-vent: 100m resolution, 141 levels, 25km x 25km
Initialised via GFS / ECMWF or radiosonde profiles
Ash initialised with heat source and point releaseOrder of minutes forecast
Plume height depends uponthermal perturbationCan be function of time?(Not implemented)
Emission rateconstant for all ashtypes
Different formulations of the model II.
Column source,No thermal perturbation
Near-vent: 15km resolution, continental scale
Initialised via GFS / ECMWF
Order of 60 hours forecast
Emission rateconstant with heightand forall ashtypes
Different formulations of the model II.
Near-vent: 15km resolution, continental scale
Initialised via GFS / ECMWF
Order of 60 hours forecast
Plume height specified;can be function of time
Output
A) All standard variables, plus tracer concentration
B) netCDF – non-CF compliance
C) A variety of WRF-specific applications to extract,convert data, etc.
Very large files ~50Gb
Some Results. I. Long-range runs
Eyjafjallajökull, May 2010
N.B. both images use the same domain.
from NASA Earth Observatory, 20106th May 12Z
from model: ash + cloud6th May 12Z
Some Results. I. Long-range runs
total integrated column ash
isosurface of ash(Different simulation times)
Some Results. II. Near-vent runsAsh from above
The model is initialised with a sounding from Keflavikurflugvollur (24th April 2010 )
Further work: full multiphase WRF
N + 1 phases: 1 air (gas + liquid and solid water) phase, N particulate phases (size bins)
Fundamentally N + 1 momentum equations, one for each phase, with interaction forces (drag) between them
Integrate N particulate momentum equations plus the combined (summed) momentum equation
There is only one shared pressure field and so the combined momentum equation is simply the usual one in the model, taking account of the contribution of the particles to the density.
All interaction forces between phases are equal and opposite (Newton's 3rd law) so cancel in the combined momentum equation
Drag terms in each particulate momentum equation
Modified equation of state taking account of the compressible fraction (air).
Further Work.
Similar approach adopted by e.g. Neri and Macedonio, “Numerical simulation of collapsingvolcanic columns with particles of two sizes”J. Geophy Res. B4, 8153-8174