Volcanic Ash Model Details
Figure 1. NOAA volcanic ash dispersion product.
Dispersion Simulations Using HYSPLIT
HYSPLIT is set to use the same volcanic ash
source term and produce look-alike output graphics as VAFTAD
(Heffter and Stunder, 1993). The
main differences between
running HYSPLIT for volcanic ash and
running HYSPLIT at the ARL HYSPLIT web page (unregistered or
registered) is in the source term (the initial conditions of the substance being
modeled) and the output graphics. For volcanic ash, a set of particles of a
given size distribution are uniformly distributed in a vertical layer above the
volcano. Additional options such as mass emission rate and wet deposition
are not available for volcanic ash runs. All HYSPLIT options are
available in the Windows-based version that may be downloaded
from the HYSPLIT main page.
Brief description of volcanic ash dispersion modeling:
Eruption input
Required inputs are:
- Volcano latitude and longitude
- Volcano summit height
- Eruption date and time
- Eruption duration
- Ash column height
- Ash reduction level (none-small-medium-large)
Preset input is a unit source (1 g) since the actual amount is not known in
real-time. The ash particle distribution is described below.
Meteorological data input
HYSPLIT inputs gridded meteorological data in a format described
in the meteorological archives web page.
Forecast meteorological fields in this format are routinely generated from
NWS National Centers for Environmental Prediction
(NCEP) runs of the
Global Forecast System (GFS) and the North American Mesoscale (NAM)
meteorological models
and are available on an
NCEP ftp server.
Information on these HYSPLIT-compatible forecast datasets,
as well as others produced by ARL, are available
in the meteorological forecast data web page.
In addition to these forecast files, GFS and NAM "archive"
data (analysis and short-term forecasts) for the previous two days are routinely created
at NCEP and are available on the
NCEP ftp server. This permits running volcanic ash forecasts for large eruptions that
occurred up to two days ago.
ARL also archives meteorological data in
the HYSPLIT-compatible format. Information on these files is available
in the meteorological archives web page.
Ash particle distribution
HYSPLITcalculates transport and dispersion of volcanic
ash from an ash column extending from the volcano summit
to the column top. The model uses spherical particles of density 2.5*10^6
g m-3 with diameters ranging from 0.3 to 30 microns.
A particle distribution was derived from aircraft
sampling of Mount St. Helens and Redoubt Volcano ash clouds.
Visual ash cloud
Calculated concentrations relative to the unit emission have been
correlated with satellite imagery (see verification discussion below)
for defining the visual ash cloud. The model computation
of the visual ash cloud includes the magnitude of the
volcanic eruption as determined by an algorithm based
on the ash column top height and column depth.
Reduced ash
For some volcanic eruptions, HYSPLIT forecasts a larger ash
cloud than what is observed, partly because of water
vapor in the eruption column. HYSPLIT is typically run
in a "reduced ash" mode when satellite imagery shows
a smaller ash cloud. Differences between the default
(no reduction) run and a reduced ash run, if any, reflect
some of the uncertainty in defining the eruption column.
Model output
An example 8-panel "VAFTAD-format" chart of the forecast visual ash
cloud is shown in Figure 1. The four panels in any column
are for a single valid time after eruption. Individual
panels are for layers applicable to aviation operations
and are identified at the side of a panel with upper
and lower flight levels (FL) in hundreds of ft. The
bottom panel is a composite layer, from the SURFACE
to FL550, and is useful for satellite
imagery comparisons. For each column, the forecast
valid time separates the upper three panels from the
composite panel. Volcano eruption information is at
the lower left. A description of the input meteorology
is at the lower right. The visual ash cloud symbol
is at the lower center. If a reduced ash level was
used, it is indicated below the visual ash cloud symbol.
HYSPLIT volcanic ash output issued by NOAA in response to a volcanic
eruption includes a message to "SEE CURRENT SIGMET
FOR WARNING AREA" at the lower right.
The example output shown, from one of the
hypothetical eruption simulations,
is valid for ERUPTION+6 hours (left column) and ERUPTION+12 hours
(right column).
Volcanic Ash Dispersion Model Verification
Verification has included three eruptions of Spurr in Alaska (June, August,
and September, 1992), multiple eruptions of Rinjani in
Indonesia (June & July 1994), a continuous 24-h eruption
of Klyuchevskoi in Kamchatka - eastern Russia (starting
September 1994), Soufriere Hills, Monserrat and Popocatepetl, Mexico (1996-1997).
List of Publications
Stunder, B.J.B., J.L. Heffter, R.R. Draxler (2007), Airborne Volcanic Ash Forecast Area Reliability, Weather and Forecasting, 22:1132-1139, DOI: 10.1175/WAF1042.1
Tupper, A., J. Davey, P. Stewart, B. Stunder, R. Servranckx, and F. Prata, 2006: Aircraft
encounters with volcanic clouds over Micronesia, Oceania, 2002/03. Australian Meteorological Magazine, 55, 289-299.
Heffter,J.L., 1996: Volcanic ash model verification using
a Klyuchevskoi eruption. Geophy. Res. Letters, 23-12, 1489-1492.
Heffter, J.L. and B.J.B. Stunder, 1993: Volcanic Ash
Forecast Transport And Dispersion (VAFTAD) Model. Wea Forecasting, 8, 534-541.
Heffter, J.L., B.J.B. Stunder, and G.D. Rolph, 1990: Long-range forecast trajectories
of volcanic ash from Redoubt volcano eruptions. Bull. Amer. Meteor. Soc. 71(12):1731-1738.