A Volcanic Ash Advisory Center (VAAC) is a group of experts responsible for coordinating and disseminating information on atmospheric volcanic ash clouds that may endanger aviation. As of 2010, there are nine Volcanic Ash Advisory Centers located around the world, each one focusing on a particular geographical region. Their analyses are made public in the form of Volcanic Ash Advisories (VAA) and often incorporate the results of computer simulation models called Volcanic Ash Transport and Dispersion (VATD).[1][2]
The worldwide network of Volcanic Ash Advisory Centers was set up by the International Civil Aviation Organisation (ICAO), an agency of the United Nations, as part of the International Airways Volcano Watch (IAVW), an international monitoring system managed by the International Airways Volcano Watch Operations Group (IAVWOPSG)[3] which replaced the Volcanic Ash Warnings Study Group (VAWSG).[4] The individual VAACs are run as part of national weather forecasting organisations of the country where they are based, e.g. the US NOAA or the British Met Office.
The centers were set up in the 1990s to improve forecasts of the locations of ash clouds from volcanic eruptions following incidents where commercial aircraft had flown through volcanic ash resulting in the loss of engine power. British Airways Flight 9, a Boeing 747, lost power to all four engines in 1982 over Indonesia after an eruption of Mount Galunggung. KLM Flight 867, another Boeing 747, again lost power to all engines in 1989 over Alaska after Mount Redoubt erupted. It was recognised following these and other incidents that volcanic ash was a danger to commercial aviation and that the only way to ensure that there would be no loss of an aircraft was to alert pilots in a timely manner to divert their flight around the cloud.
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Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions,[5] less than 2 millimetres in diameter. The ash enters the atmosphere from the force of the eruption and convection currents from the heated air and is then carried away from the volcano by winds. The ash with the smallest size can remain in the atmosphere for a considerable period of time and be carried well away from the eruption point. The ash cloud can be dangerous to aviation if it reaches the heights and enters the paths used by aircraft.
It was recognised that there was an issue following the incident in 1982 with the British Airways Flight 9 and therefore the ICAO established the Volcanic Ash Warning Study Group in 1982. Due to the difficulty in forecasting accurate information out to 12 hours and beyond the VAAC were set up by the ICAO as part of IAVW.[6][7]
The purpose of the VAAC is to gather information from volcano observatories, satellite images, pilot reports on ash clouds released by volcanoes, these are then analysed and forecasts made on future movements. This alert and forecast is then issued to interested parties in the aviation industry including Meteorological Watch Offices and Area Control Centers for aircraft and adjacent VAAC where an ash cloud may move from one area to another.[8]
To be able to perform this function each VAAC develop models on the dispersion of volcanic ash. Using the reports available from satellites, volcanic observatories and pilot reports they identify ash clouds and then use the models to forecast where they likely to be a danger to aviation.[9]
There are nine VAAC locations each with a defined area to monitor. The centers coordinate with adjacent VAAC, flight control centers within and adjacent to their area as well as meteorological offices within and adjacent to their area of operation.
The areas covered by the VAAC are set either by coordinates or by Flight Information Regions (FIR) that are internationally agreed as part of the IAVW program of the ICAO and the areas that each has responsibility is set out in the IAVW handbook.[7]
| Name | Area of Responsibility | Part of National Organisation |
|---|---|---|
| Anchorage | FIR: Anchorage Oceanic; Anchorage continental; Anchorage Arctic
and west to 150 east north of 60 north |
NOAA |
| Buenos Aires | South of 10 south between 10 west and 90 west | |
| Darwin | Southward from 10 north and from 100 east to 160 east, and the Perth FIR between 100 east and 75 east, Colombo FIR, and those parts of the Kuala Lumpur, Bangkok, Chennai, Yangon and Calcutta FIRs lying within 10 north 100 east to 20 north 100 east to 20 north, 82 east to 10 north, 82 east to 6 north, 78 east to 2 south, 78 east to 6 south 75 east | Bureau of Meteorology |
| London | FIR: Bodø Oceanic, Reykjavik, Shanwick Oceanic, London, Scottish, Shannon | Met Office |
| Montreal | FIR: Gander Oceanic Canadian Continental FIRs (including the Arctic Ocean), Reykjavik Søndre Strømfjord | Meteorological Service of Canada |
| Tokyo | 60 north to 10 north– and from 90 east to Oakland Oceanic and Anchorage Oceanic and Continental FIR boundaries | Japan Meteorological Agency |
| Toulouse | Santa Maria Oceanic, AFI Region to 60 south, EUR Region west of 90 east (except for London, Scottish and Shannon FIRs) and MID Region: south of 71 north, west of 90 east | Météo-France |
| Washington | New York Oceanic, Oakland Oceanic and United States continental FIRs, North of 10 south 140 west | NOAA |
| Wellington | Southward from the Equator and from 160 east to 140 west | Meteorological Service of New Zealand Limited |
The areas of responsibility for VAAC cover most of the world, with the exception of part of northern Russia and the Arctic; part of the South Pacific and the Antarctic, the areas that are not covered are shown by the hatched areas on the map.
Satellite data is the main source of information on ash clouds; it can provide the timeliest information. The remote nature of many volcanoes, and that only a few have permanent monitoring from volcanic observatories mean that onsite reports will often not arrive quickly. Though they are remote in terms of population, aircraft may still fly over them regularly. For example the Aleutian Islands are on the great circle route between Asia and North America which has a number of volcanoes. Over 50,000 aircraft a year fly through the area which has 40 volcanoes that have been active in historic time.[9] Pilot reports if present will only be available for flights during daylight hours as they cannot observe ash clouds during the night.
Even satellites will have issues as cloud cover of the volcano can limit the detection capability. Volcanoes in high latitudes are not covered as well by weather satellites as those in lower latitudes due to the curvature of the Earth from the position of a geosynchronous orbit.[10] The details of the ash erupted (concentration and size of particles) are often not available to the forecasters, but with this data a better forecast can be made.[6]
When an ash cloud is detected the VAAC will gather all the available information on this and then make use their computer models to predict the path of the ash cloud at different flight levels used by aircraft. They will then issue an alert to aviation and meteorological offices as stated within the IAVW procedure manual.[7] This alert will be in the format of an alert stating that it is a Volcanic Ash Advisory.
Within the alert the following information will be provided: the name of the volcano, the country, location and crater elevation of the volcano, the source of the information, e.g. satellite or pilot observation, details of the eruption including time of day in UTC and date of the eruption, details of the ash cloud including the flight level and size, detail on the current movement of the ash cloud, forecast movement for 6, 12 and 18 hours ahead following the time of the advisory and finally the next update time.