David J. Weinbrenner

National Centers for Environmental Prediction

W/NP12, World Weather Building,

Washington, DC 20233, USA


The Volcanic Ash Forecast Transport and Dispersion (VAFTAD) model was fully transferred to the NWS/NCEP Cray computer system on 22 April 1999 at 1200 UTC. For approximately the last 7 years the VAFTAD model has run operationally on a NOAA Air Resources Laboratory (ARL) computer which was not in a suitable environment for use in an operational 7/24 program. Also accomplished during the transfer was the introduction of a new "reduced ash" version (Figs. 1 and 2) in addition to the standard ash version (Figs. 3 and 4) ) of the VAFTAD model. The standard ash version was reviewed several years ago by Nick Heffter (ARL). In 1988 NOAA and the FAA signed a MOU for Volcanic Hazards Alert program. This resulted in a trajectory model which was run on the then NMC 9000 computers to support jet aviation interests. Information was transferred to the airlines via phone fax. The trajectory model was replaced by the development of the VAFTAD model on the ARL computer.


The VAFTAD model is a time dependent 3-dimensional volcanic ash transport and dispersion model. It was developed for global forecasting of volcanic ash in response to volcanic eruptions around the world. The "visual ash cloud" is depicted on the model output charts since ash density and particle size hazardous for aviation operations have not been determined. The focus of the VAFTAD model is for the in-flight hazard to aviation operations in support of the NOAA Volcanic Ash Hazards Alert Program.

VAFTAD model input

- 48hr analysis (pre-forecast period)

- 48hr forecast

VAFTAD model source

- density 2.5 x 106 g/m3

- diameter 0.3 to 30 microns

VAFTAD model ash particle dispersion

Visual ash cloud depiction

VAFTAD products

The VAFTAD model charts consist of a series of maps showing the instantaneous ash cloud in time and space starting 12 hours after eruption time and continuing in 12 hour intervals through hour 36 or 48. The 4 panels in any column are for a single valid time after the eruption time. Individual panels are for layers applicable to aviation operations and are identified at the side panel with upper and lower flight levels (FL) in hundreds of feet. The bottom panel is a composite from the SURFACE to FL550. 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 and run description are at the lower center. The volcano site is depicted on the maps by a circle containing cross-hairs.

Ash cloud reduction

Included in this implementation is the relatively new (Dec. 1998) ash reduction alternative to the standard VAFTAD ash output, which was modeled on larger type eruptions . Most (90%) of the volcanic eruptions handled by the Washington and Alaskan Volcanic Ash Advisory Centers (VAACs) are of the short duration (visible on satellite imagery less than 18 hours) and low mass variety, or "puffs". The standard VAFTAD model simulation often results in cloud excessive ash concentrations (CEAC), which makes it difficult for the model to handle these "puffs" with accuracy, especially when there are large, but unknown, concentrations of steam in the eruption column. Small (1), medium (2) or large (3) ash reductions can be manually entered in the model inputs to more closely simulate the actual ash cloud observed in satellite imagery. Differences among the standard and the various ash reduction runs reflect some of the uncertainty in defining the eruption column. The result of using the reduction mode is usually a more accurate VAFTAD forecast.

Model horizontal resolution Northern and Southern hemispheres

- AVN - 102 km

- AVN - 190.5 km at 60 deg

Vertical resolution


ARL web page -

Heffter, J.L. and B.J.B. Stunder, 1993: Volcanic Ash Forecast Transport And Dispersion (VAFTAD) Model. Weather Forecasting, 8 534-541.


The ARL is responsible for continued verification, research and development for modeling volcanic ash clouds and making model changes. ARL also assists implementing VAFTAD upgrades at NCEP.

NCEP is responsible for maintaining the VAFTAD model in a 7/24 operational environment on its computer system. NCEP also runs the AVN global model four times a day in 6 hour cycles for input to the VAFTAD model while also providing other meteorological models for further VAFTAD model development. AVN analysis fields for the last 48 hours are also run and stored every 6 hours for VAFTAD use. The AVN and VAFTAD models reside in normal production suites where both production and communications are fully supported by a staff of programmers and technicians.

VAFTAD suite

The NCEP Senior Duty Meteorologist (SDM) or the forecaster at the Anchorage Aviation Weather Unit (AAWU) can set up a file containing the inputs for the VAFTAD model. The SDM then runs the model by requesting the computer operator to release the VAFTAD. Upon model completion a postscript file is created and the output is reviewed for consistency with the meteorology fields, satellite imagery, and any observed data in the vicinity of the ash cloud. The postscript file is also automatically sent to AAWU and the NESDIS Satellite Analysis Branch. A standard VAFTAD or an ash reduction VAFTAD may be run before the SDM has the operator release the VAFTAD fax job which communicates the product to the customers. The model may be rerun later if additional more accurate information is received. If the model is not verifying properly or if the visible ash cloud no longer exists, an Alert Ended VAFTAD is also run and issued. The SDM makes recommendations to ARL regarding model improvements.


It could be said that the Washington VAAC and Anchorage VAAC are the primary users of the VAFTAD model, and having the VAFTAD model run on the NCEP operational computer system adds to the convenience and stability of VAAC operations. In 1997 the International Civil Aviation Organization (ICAO) mandated graphical support for volcanic ash guidance, forecasts and warnings for aviation from many of the national meteorological services across the world. The Washington and Alaskan VAACs were officially established by the end of that year. The Washington VAAC is responsible for providing volcanic ash guidance for all the US flight information regions (FIRs), excluding the Alaskan FIR, southward to 10 degrees south of the equator. It also tries to support the Buenos Aires VAAC, which is still not operational, over South America. For the two years prior to 1998, the Washington VAAC was tasked with covering the world and averaged almost 350 VAFTADs per year. With the inauguration of the other VAACs around the world, and "WATCH" VAFTAD runs being made available on the Internet instead of being issued by the VAAC, the number of VAFTADs issued has dropped in 1998 to just over 150. The NESDIS Satellite Analysis Branch (SAB), part of the Washington VAAC, issued over 1000 Volcanic Ash Advisory Statements (VAASs) during 1998 in text format. The VAFTAD helps NESDIS/SAB determine where ash exists (analysis), while the NCEP/SDM determines where ash is going (forecast).

Other VAAC VAFTAD customers


Volcanic ash presents a significant hazard to jet aircraft operations. Modern day eruptions have nearly downed two Boeing 747s in incidents near Alaska and Indonesia. About two years ago during a satellite eclipse an Air Canada DC-10 had a problematic encounter from a new eruption on Montserrat in the Caribbean. Volcanic ash clogs jet engine fuel injectors and Pitot tubes, while scouring windscreens until they lose there translucency. Often present sulphur dioxide gas is corrosive to aircraft materials. The aviation community is desperate for any ash guidance they can receive in a timely fashion. At risk are the lives of passengers and crew and the loss of millions of dollars in maintenance costs to just a single aircraft. The Washington VAAC issued over 150 VAFTADs and over 1000 VAAS text messages during 1998 in support of both US and foreign met watch offices, flight services and airlines.

Implementation accomplishments


The VAFTAD is officially released when the SDM runs the VAFTAD fax job out of the VAFTAD job suite. The VAFTAD graphics are communicated as a 6 bit fax graphic file to National Weather Service Telecommunications Gateway for official dissemination as is other NCEP fax production. Gateway converts the fax file for WAFS and the NWS web server.

Communications to

(36/48 hr header PHBI10 KWBC - official)

(36/48 hr header PHBI10 KWBC - official)

Note that the FAA has no graphics system to which the VAFTAD can be sent directly other than phone fax.