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.

1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.