Volcanic Ash Forecast Transport and Dispersion Modeling
Overview
The NOAA Air Resources Laboratory (ARL) developed
the time dependent 3-dimensional Volcanic Ash Forecast
Transport And Dispersion (VAFTAD) model to simulate the transport of
ash in the atmosphere. In 1992 VAFTAD was transferred
to the NOAA National Weather Service
(NWS)
to support the NOAA -
Federal Aviation Administration Memorandum of Understanding
(MOU) on Volcanic Hazards Alert.
VAFTAD output further supported the operations of the
Washington, DC,
and Anchorage, Alaska,
Volcanic Ash Advisory Centers
(VAACs),
which were established by the International Civil Aviation Organization
(ICAO) in the late 1990s.
The U.S. VAACs are operated by NOAA.
In 2005 the Hybrid Single-Particle Lagrangian Integrated Trajectories model
(HYSPLIT) replaced VAFTAD at the NWS.
The ICAO "International Standards and Recommended Practices,
Annex 3 to the Convention on International Civil Aviation,
Meteorological Service for International Air Navigation" states that
VAACs shall run a dispersion model to forecast the movement of volcanic ash
"clouds". NOAA began issuing dispersion model (then VAFTAD) output in the form
shown below. In the 16th edition (July 2007) of Annex 3, a new graphical
product, the Volcanic Ash Graphic (VAG), was instituted.
Recent operational VAG can usually be seen for the
Washington and/or
Anchorage VAAC.
Raw model output graphics by NOAA retained the format shown below. The model
output is disseminated to the public over the World Area Forecast System
(WAFS),
and is made available on the Internet (e.g.
ARL or
NWS).
HYSPLIT model output for hypothetical
volcanic eruptions are also available and the
model may be run on the Internet.
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Volcanic Ash Dispersion Model Brief Description
HYSPLIT is set to use the same volcanic ash
source term and produce look-alike output graphics as VAFTAD
(Heffter and Stunder, 1993). More information on
HYSPLIT is available here.
- 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) and an ash particle distribution
described below.
- Meteorological data input
HYSPLIT inputs gridded meteorological data in a format described
here.
Forecast meteorological fields in this format are routinely generated from
National Centers for Environmental Prediction
(NCEP) daily
runs of the
Global Forecast System (GFS) model and the North American Mesoscale (NAM) model.
Information on these HYSPLIT-compatible forecast datasets,
as well as others produced by ARL, are available
here.
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. 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
here.
- 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 volcanoes, 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 chart of the forecast visual ash
cloud is shown below. 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 as an aid for issuing significant
meteorological (SIGMET) advisories or 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).
VAFTAD Model Development and 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
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.
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