To define 'Discernible Ash'

The London Volcanic Ash Advisory Centre (VAAC) is one of nine VAACs worldwide with International Civil Aviation Organisation (ICAO) mandated responsibilities for providing flight safety advisory information for international air navigation. The rules of engagement for the VAACs are maintained by the ICAO International Airways Volcanic Watch Operations Group (IAVWOPSG). For volcanic ash originating from eruptions within its designated area of responsibility, a VAAC is responsible for:

(i) Real-time monitoring of volcanic ash, primarily using satellite imagery;

(ii) Production of 18-hour Volcanic Ash Advisories (VAA), based on forecaster interpretation of dispersion model output and observations;

(iii) Coordination with neighbouring VAACs including the establishment of back-up procedures and agreements on the hand-over of volcanic ash as it crosses into a neighbouring VAAC's area of responsibility.

There are currently no globally agreed ICAO requirements that talk to safety limits for volcanic ash contamination (how much ash) or any requirements for the monitoring and forecasting of any other volcanically related contaminants, the most common types being SO2, CO2, fluorine and chlorine.

Many of the issues raised during the eruption of the Icelandic volcano, Eyjafjallajökull (Eyja), in April/May 2010 were dealt with in ‘crisis mode' during the event. The duration of the event coupled with the prevailing north-westerly winds exposed gaps in European volcanic ash observational capabilities and most significantly, the lack of quantitative flight safety ash contamination thresholds. The Original Engine Manufacturers (OEMs) ultimately provided a ‘safe' ash concentration threshold of 2mgm-3 with which it was possible to redraft the ICAO EUR/NAT (European/North Atlantic) volcanic ash contingency plan1 and the subsequent introduction of the Met Office ‘raw model' ash concentration forecast charts as the primary flight planning tool.

1 An additional ‘higher' contamination threshold of 4000 µgm-3 was added in May 2010.

Post Eyja, ICAO established the cross-discipline International Volcanic Ash Task Force (IVATF) to fast track global harmonisation of volcanic ash contingency planning and processes to include agreed definitions of hazardous, ‘visible,' and ‘discernible' volcanic ash. IVATF3 (February 2012) recommended that the use of ash concentration charts as the primary user decision making tool be discontinued although without agreed definitions of what constitutes ‘hazardous,' ‘visible,' and ‘discernible' ash it has not yet proved possible to redraft the ICAO EUR/NAT volcanic ash contingency plan. The IVATF concluded its activities in June 2012 with agreement that the following volcanic ash definition proposals should go for further discussion to the next IAVWOPSG meeting in Bangkok in March 2013:

‘Visible ash' – volcanic ash that can be seen by the human eye.

‘Discernible ash' - volcanic ash that can be detected by: defined impacts on aircraft; or agreed in-situ and/or remote sensing techniques.

Given that it is the ‘discernible ash' definition on which the VAACs, as part of their agreed best practice processes, base their advisory forecasts and that the current OEM minimum jet engine safety threshold is currently set at 2mgm-3, this presentation looks to focus on achieving a more detailed definition of ‘...agreed in-situ and/or remote sensing techniques' with a view to further informing discussions at the forthcoming ICAO IAVWOPSG. In doing so the following assumptions will be explored:

• Minimum satellite ash concentration detection threshold is 0.2mgm-3.

• Qualitative satellite data can be defined as ‘visible', ‘true colour', ‘IR' and 2-channel temperature brightness difference;

• Post-processed SEVIRI and polar-orbiter satellite imagery can give ‘quantitative' estimates of ash mass column loadings (gm-2) but require supplementary information relating to the vertical distribution/structures of the ash layers (e.g. utilising lidar, ceilometer or CALYPSO profile data) to derive satellite-based ash concentration (gm-3) estimates.

• Ceilometer and lidar ash concentration detection thresholds are currently thought to be < 0.2mgm-3. In daytime, clear sky conditions ‘high grade' lidar coupled with sun photometers can give ash concentration estimates accurate to within half an order of magnitude.

• Suitably instrumented research aircraft are able to measure ash concentration values, accurate to within a factor of 2 or 3.

• Research is continuing into the development of aerosol measuring equipment on board commercial aircraft, Unmanned Airborne Systems and sondes.

• Global availability of quantitative volcanic ash observation capabilities within reasonable error margins (order of magnitude) will remain limited pending the introduction of enhanced geostationary satellite platforms over the next 5 to 10 years.