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(Buenos Aires, Argentina, 5 - 15 October 1999)

Agenda Item 7: Aeronautical meteorological offices and their functions, and meteorological observation networks.


(Presented by the United States)


This paper presents a summary of the operations at the Washington Volcanic Ash Advisory Center (VAAC) in the CAR/SAM Regions. These operations are conducted in accordance with the standards and recommended practices of ICAO Annex 3, Meteorological Services for International Air Navigation.


1.1 In November 1997, the Washington Volcanic Ash Advisory Center (VAAC) joined a global network formed by the International Civil Aviation Organization (ICAO) to provide worldwide coverage of volcanic ash events. The governing documents for this system are ICAO's Annex 3, Meteorological Service for International Air Navigation, and the International Airways Volcano Watch's Operational Procedures. The network consists of nine VAACs: Anchorage; Buenos Aires; Darwin; London; Montreal; Tokyo; Toulouse; Washington; and Wellington. Each VAAC has responsibility for monitoring volcanic eruptions and ash within a specified area of responsibility (Figure 1). Most, but not all, of the world's airspace is within the total area of responsibility of the VAACs.

1.2 In addition to the VAACs, this global system also includes Meteorological Watch Offices (MWOs), geological/volcanological observatories and Area Control Centers (ACCs). Each of these facilities has a role in exchanging and/or communicating information about a volcanic ash event to each other and/or to the aviation community. Whenever a volcanic eruption is reported or ash is discovered in satellite imagery for a VAAC's area of responsibility, the role of the VAAC is to: 1) utilize satellite data and other in situ information obtained from the global network to detect and track volcanic eruptions and ash clouds; and to 2) utilize volcanic ash numerical trajectory/dispersion models to track ash clouds.

1.3 This information is made available to the VAAC's primary users, the MWOs and ACCs, in the form of text messages known as Volcanic Ash Advisory Statements (VAAS). Some VAACs, including the Washington VAAC, distribute the output of a volcanic ash trajectory and dispersion model. The VAAS and model output provide guidance to the MWOs and ACCs about when to issue ash warnings for aviation (SIGMETs) and their content. At the same time, a Notice to Airmen (NOTAMs / ASHTAM) is issued by an Air Traffic Service facility on the known eruption or ash cloud based upon information provided by a VAAC. Within the CAR/SAM regions, the Washington VAAC supports over 20 MWOs and numerous ACCs.


2.1 VAAS text messages concerning volcanic events within the area covered by the Washington VAAC contain general information about: 1) the volcano involved (volcano name, number, location, and summit height); 2) the sources of information that were used in producing the VAAS; and 3) any available information on the time and type of eruption, Figure 2.

2.2 The heart of the VAAS is the instantaneous description of the ash. Ideally, the VAAS indicates not only the location but also the relative density of the ash (thin versus heavy), the estimated ash height, and the current movement(s) of the ash (noting when different areas of the ash move in different directions). This information is primarily derived from satellite imagery. Currently the VAAS statement incorporates the Volcanic Ash Advisory Information in Graphical Format (VAG). In the United States the graphic format is referred to as the Volcanic Ash Forecast, Transport and Dispersion (VAFTAD). In addition to a text message, the Washington VAAC also issues a graphic depiction of ash, e.g., the outline of the visible ash cloud, if ash is detectable from satellite data, Figure 3.

2.2 A VAAS is issued after the Washington VAAC becomes aware of a new ash event within its area of responsibility. Updates are issued no more than 6 hours apart or more frequently if there is a major change in the situation such as another eruption, shift in direction of the ash cloud, etc. If for any reason the Washington VAAC is unable to disseminate a VAAS statement or VAFTAD output, the VAAC will request that another VAAC issue the products on their behalf. This situation will be clearly indicated in the VAAS.

2.3 In addition, when ash moves from one VAAC's area of responsibility to another's, the responsible VAAC coordinates a "hand off" of responsibility to the other VAAC. The two coordinate to determine when the responsibility for the event will be transferred to the receiving VAAC. Also involved in the coordination are the relevant MWO(s). During hand off situations, VAAS messages will clearly indicate which VAAC has responsibility for the ash event.

2.4 If reliable information is received from a volcanological or geological observatory that volcanic activity has increased and there is a threat of an eruption, the Washington VAAC will issue a daily VAAS to provide updated information about these situations. This VAAS is discontinued once the threat has passed or an eruption occurs. For an eruption, a standard VAAS is issued.


3.1 A VAAS depicts current ash location and movement but provides no information about its future location. Therefore the Washington VAAC has developed the VAFTAD model to provide information about the forecast position of the ash 12 hours or more beyond the eruption time, up to 48 hours after the eruption. The model uses ash particle densities based on empirical observations of Mount St. Helens and Redoubt ash events, and/or several additional meteorological forecast models, depending on the area under observation. Current model resolution is 32 - 100 kilometers.

3.2 Initialization of the VAFTAD model is done with the latest meteorological observations relative to the time of the eruption. Several specific inputs are included: volcano name or latitude/longitude of the volcano; summit height; ash column height; date and time of the eruption; and duration of the eruption. These parameters are obtained from satellite data, volcanological observatories, pilots or MWOs. If unknown, the following default values are assumed: an ash column height of 50,000 feet and duration of one hour. Another option for the analyst is to set an eruption intensity parameter to stimulate a short "puff" or a longer and higher plume event. Not every eruption will require a VAFTAD, especially if it is a small or short lived event.

3.3 VAFTAD output is a series of graphics depicting forecasted ash location at low (below 20,000 feet), middle (20,000-35,000 feet) and high (35,000-55,000 feet) levels of the atmosphere as well as a composite forecast (surface to 55,000 feet). Graphical forecasts are depicted at 12 hour intervals after an eruption, out to 48 hours after the eruption, Figure 4. Model output is often compared to what is depicted in satellite imagery or described in pilot reports and can be rerun if necessary. For more information about VAFTAD, see Heffter and Stunder's article in Weather and Forecasting 8(4), pp. 533-541, 1993.


4.1 Often the most important source of information used in creating a VAAS is satellite imagery. Imagery is also used to verify or provide quality control for VAFTAD model output. Specifically, it is used to determine the instantaneous boundary of ash, speed, direction(s) of movement, height, and relative thickness. Loops of visible, infrared and multispectral satellite imagery are continuously updated to provide the latest available data centered over recently active or potentially active volcanoes.

4.2 The Washington VAAC uses imagery from the National Oceanic and Atmospheric Administration's (NOAA) geostationary satellites, GOES-East and GOES-West. Reliance on geostationary satellite data is due to the frequent refresh rate of the satellite, every 15 minutes, for a large portion of the Washington VAAC area of responsibility. Occasionally, the VAAC utilizes single channels and multispectral channels found on the NOAA-12 and -14 polar orbiting satellites (POES), the Total Ozone Mapping Spectrometer (TOMS) data from National Aeronautical and Space Administration's Earth Observing System, and single channels and multispectral channels on Japan's Geostationary Meteorological Satellite (GMS-5). In the case of TOMS data, the Washington VAAC receives it twice a day from NASA. But this infrequency of data availability and its coarse resolution limits its operational usefulness especially for small or low level ash events. TOMS detects sulfur dioxide which is emitted by volcanoes and sometimes drifts with the ash.

4.3 Obtaining reliable ash information from satellite imagery is a special challenge in two situations: in the presence of meteorological clouds/moisture and during nighttime hours. Due to the spectral characteristics of all the meteorological satellites used, clouds and ash can "appear" the same in satellite imagery. However, the ability to distinguish the two through comparison of visible imagery with the results of infrared channels is not available at night. The use of all available satellite channels, especially in combination for multispectral comparison, can minimize this problem.

4.4 In addition to visible imagery, longwave infrared and three types of multispectral imagery are used to monitor and track ash. Shortwave infrared channel imagery is used to look for "hot spots" i.e., heat radiating from volcanoes, possibly indicating an eruption or build up activity which could lead to an eruption. One of the multispectral comparison techniques used is "split window" channel differencing.

4.5 Of critical importance to providing the best quality information in VAAS' is verification of information. MWOs, volcano observatories, pilot observations and surface observations (METARS) can verify information, particularly when clouds or high moisture conditions obscure the satellite signal. Due to the time critical nature of a volcanic eruption, the Washington VAAC prefers to receive information about eruptions from phone calls from the above sources. The VAAC has instituted an automatic alert notification program based on a keyword search engine as an additional source of information. The search engine scans incoming Global Telecommunications System (GTS) Pireps, METARS and SIGMETs for "words" such as VOLC, ASH, VA, VOLCANO, VLCN, CENIZA, and for bulletin headers that begin with FV (indicating a VAAS message) or WV (indicating a volcanic ash SIGMET). When the engine has a "hit", an audible alarm sounds on a personal computer within the Washington VAAC work area. The identified report or bulletin is then displayed and reviewed for potentially useful information.


5.1 VAAS' are transmitted to customers via several communication systems, including FAA circuits (AFTN), GTS, Family of Services (FOS), and World Area Forecast Satellite Broadcast System (WAFS). The message headers are:


5.2 Many users (including several airlines) obtain VAAS' through commercial systems such as Alden or WSI. These systems in turn receive their information from FOS or one of the other United States' communication systems. In addition, VAAS', graphics, available satellite imagery, and other satellite-derived products centered over volcanoes in the Washington VAAC area of responsibility are posted on the internet at the following address: .

5.3 This internet site contains the most recent VAAS for each active volcano as well as up to 10 previous VAAS' for each volcano. When received, the Washington VAAC also posts VAAS' from other VAAC' at this site. In the future, this site will also contain VAFTADs if issued for a volcanic eruption. Currently it contains a link to an Internet site where ash dispersion and trajectory model output for hypothetical volcanic eruptions are posted. The site also allows a user to interactively run an ash dispersion model for a particular volcano.

5.4 VAFTADs are disseminated on WAFS, facsimile circuits and on internet web sites as follows:

5.4.1 VAFTAD headers for WAFS:

PHBE10 KWBC for the 12 and 24 hour valid times
PHBI10 KWBC for 36 and 48 hour valid times

5.4.2 Facsimile chart numbers:

00Z model run 12Z model runs

DIFAX D0291/D0292 D0293/D0294
HNLFAX H0186/H0187 H0188/H0189

5.4.3 The current internet web site is:

5.5 VAFTADs will soon be posted to the operational Washington VAAC web site ( In the near future, facsimile versions of the VAFTAD will be discontinued.


6.1 Recent volcanic activity at several MWOs has indicated that two-way communications and operational procedures could be improved. Improvement may provide a safe environment for not only high altitude aircraft, but commuter, cargo and local aviation interests. To this end, the Washington VAAC is pursing closer cooperative relationships with the airlines, MWOs and volcanological observatories involved. To help facilitate improved relations and communications, site visits and/or attendance at conferences are under consideration in cooperation with ICAO and WMO.

6.2 In addition, there are ongoing technological improvements and advancements planned with regard to satellite remote sensing techniques and operational procedures. One of the multispectral satellite techniques, the split window capability, will be removed from the NOAA GOES satellite around 2002 and replaced again around 2012. The split window capability will remain on the NOAA POES series. Research is being conducted at a number of facilities, including NOAA's, to develop new remote sensing techniques to mitigate the loss of the GOES split window and to improve upon current techniques.

6.3 The VAAC is developing agreements with other VAACs and aviation facilities for backup situations, and is constantly implementing efficiency and quality improvements to its operations. For example, at the request of the airlines, the Washington VAAC is planning to change the units of measure used in its VAAS messages to degrees and minutes instead of degrees and tenths. This change will be coordinated with the MWOs and ACCs. Also under development are options for providing interpreted information for the "outlook" section of the VAAS, in order to better support the 12 hour outlook of the SIGMET. Current efforts are focusing on utilizing the VAFTAD and other ash trajectory and disperson model output and analyzing/validating case studies of eruption events. Additional planned improvements include improving VAFTAD model resolution, testing and evaluating additional ash trajectory models and creating model ash output that is valid 6 hours or less after eruption time.

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