Transcript Slide 1

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
 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
Leeds implementation: Methodology I.
One-way coupling: ambient atmosphere affects ash, but not vice-versa
“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=0
W≠0
Settling velocity is a function of height and
density – from Kasten et al. 1968
I.
II.
z
II(a).
z
y
U
x
Time = t
z
U
y
U
y
x
Time = t + Δt
x
U’
Time = t + Δt
U’ = (0,0,-w’)
Leeds implementation: Methodology II.
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 depend
upon surface type) [X?]
Wet Deposition: have included this but not tested it.
(Method: ash is removed when cloud water mixing ratio
is greater than Y g/Kg) [Y?]
N.B. no interaction with microphysics
at present
N.B. Grimsvotn 2011?
Leeds implementation: Methodology III.
One-way coupling:
Ambient atmosphere affects ash, but not vice-versa
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 ?
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 release
Order 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 release
Order of minutes forecast
Emission
rate
constant
for all ash
types
Plume height depends upon
thermal perturbation
Can be function of time?
(Not implemented)
Different formulations of the model II.
Near-vent: 15km resolution,
continental scale
Order of 60 hours forecast
Initialised via GFS / ECMWF
Emission
rate
constant
with
height
and for
all ash
types
Column source,
No thermal perturbation
Different formulations of the model II.
Near-vent: 15km resolution,
continental scale
Initialised via GFS / ECMWF
Plume height specified;
can be function of time
Order of 60 hours forecast
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
from NASA Earth
Observatory, 2010
6th May 12Z
from model: ash + cloud
6th May 12Z
Eyjafjallajökull,
May 2010
N.B. both images use the same domain.
Some Results. I. Long-range runs
total integrated column ash
(Different simulation times)
isosurface of ash
Some Results. II. Near-vent runs
The model is initialised with
a sounding from
Keflavikurflugvollur (24th
April 2010 )
Ash from above
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 collapsing
volcanic columns with particles of two sizes”
J. Geophy Res. B4, 8153-8174