The London VAAC process

Introduction

The Met Office NAME dispersion model is initialised with both meteorological and volcanic eruption data. This includes winds, temperature, humidity, height of plume, size of ash particles and more.

NAME is the standard dispersion model used by London VAAC, but data from other dispersion model runs are also assessed. Forecasters will use information from available observational sources (satellite, radar, lidar and research aircraft etc.) in combination with the dispersion model output, in order to produce the standard VAAC products and supplementary products. Schemes are being developed to allow observations to be used automatically and intelligently as part of the dispersion model run.

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Observing Volcanic Ash

Dispersion Modelling

Forecasting and Advisories

Observing Volcanic Ash 

Satellite-based instruments

• Satellite imagery products are the first port of call after receiving news of an eruption.
• Data from different meteorological satellites are exchanged worldwide.
• Visible and infrared images are used to monitor the location of volcanic ash over large geographical areas.

• There are two different types of satellite operated by European and International Agencies (geostationary and polar orbiting):

  • Geostationary satellites orbit at around 36,000 km above the equator, and orbit at the same speed as the Earth's rotation so have the same field of view 24 hours a day. Information is relayed back to earth every 15 minutes over Europe and Africa.
  • Polar orbiting satellites orbit at 800 - 900 km above the Earth, circling the Earth around twice a day. Images are a higher resolution than geostationary satellites, and approximately 30 images a day are received that cover Iceland from all polar orbiters with infrared sensors.
• Multi-spectral sensors on both types of satellite platforms facilitate the production of derived imagery that can be used to identify ash-contaminated areas.
• Some multi-spectral data (e.g. over Europe and Africa) can be used to provide estimations of ash particle size, ash height and ash column mass loading (how much ash is in a vertical atmospheric column).

Ongoing developments and future plans

• Coverage of satellite-based ash imagery is being expanded by using data from other geostationary satellites (where suitable multi-spectral data are available).
• Simulated volcanic ash images are being developed from model ash forecasts and weather data. These images can be compared to the real satellite images to identify model errors.
• Volcanic ash and sulphur dioxide products are being developed from hyper-spectral sensors on polar orbiting satellites to improve the sensitivity to volcanic emissions and coverage in polar regions.

Radar 

• When an eruption takes place in Iceland, the near vent plume height is monitored by the Icelandic Meteorological Office (IMO) in near-real-time using their ground-based radars.
• Static long range C-band weather radar are now located at Keflavik airport and a site in eastern Iceland, resulting in almost full radar coverage of the country. In addition, two shorter range (but more sensitive) mobile dual-polarised X-band radars are available for the purpose of detecting volcanic ash plumes in close proximity to the eruption itself.
• The radar-recorded plume height is combined with expert interpretation of other observations by the IMO State Volcano Observatory to provide an estimation of plume height. This information is currently used as the primary initialisation data for the NAME dispersion model.

Lidar

• A lidar is an optical remote-sensing instrument, which can be located on the ground, mounted on an aircraft or satellite-based, and can be used to observe the location and vertical profile of aerosols such as volcanic ash.
• A lidar measures backscattered light from atmospheric targets e.g. aerosols including volcanic ash and Saharan dust, water droplets and ice crystals, using laser pulses transmitted at one or more given wavelengths (UV, visible or IR).
• Time-height plots of backscatter intensity produced from measurements can then be used to interpret the vertical profile of the atmosphere at each point where an instrument is located. However low cloud and fog will prevent detection of volcanic ash above the cloud layer.
• 'Lidarnet' is a UK network operated by the Met Office, of approximately 40 ground-based LCBRs - commonly known as ceilometers. These are a lower-powered less sensitive version of a lidar with more limited vertical range for aerosol detection: typically 6 km altitude or lower, in contrast to some lidars with a vertical range extending through the troposphere.
• LCBRs and lidars can offer near real-time height information to VAAC forecasters, but are not currently able to offer concentration (i.e. of ash in the volcanic cloud) measurements in near-real time.
• Lidars are used to validate satellite imagery and other observations; distinguish, by use of expert interpretation, between clouds and aerosols; determine geometrical properties (height/base) and movement of the aerosol/ash cloud, the latter by using observations from the whole network and ideally observations from other instruments.

Ongoing developments and future plans

• The Met Office is in the process of procuring 11 three-channel (dual polarised plus Raman channel) lidars co-located with sun-photometers for deployment around the UK. The dual polarised capability will allow discrimination between 'angular' particles, such as volcanic ash and ice crystals, and 'spherical' particles, such as boundary layer dust and water droplets. The Raman channel will allow assessment of the lidar ratio (with restricted altitude coverage during the daytime), enabling implementation of methods to provide further insight into the nature of detected aerosol in the future. Once operational these instruments will be integrated into the current Met Office 'Lidarnet' system.
• The use of automated algorithms to help distinguish between clouds and aerosols will be explored.

Sun Photometers

• A sun photometer is used to provide a measurement of the direct solar radiation at a point on the ground.
• Under cloud-free and broken-cloud conditions, sun photometers can also be used to detect the presence of atmospheric aerosols. This is because the aerosol layer, e.g. volcanic ash, causes a reduction in the direct solar radiation.
• A sudden and consistent increase in the observed AOD over the natural background level can indicate the arrival of an aerosol layer.
• When combined with LCBRs/lidars measuring the aerosol layer altitude and depth sun photometers can be used to estimate the mass concentration of the aerosol cloud.
• If the sky is cloud-free, some models of sun photometers co-located with lidars can also provide information on aerosol properties such as the size distribution or refractive index.

Ongoing developments and future plans

• There are plans to implement advanced retrieval methods for aerosol properties using co-located sun photometers and lidars, aimed at improving the confidence of volcanic ash observations in Lidarnet.

ATDNet (lightning detection)

• The 'Arrival Time Difference' (ATD) Network is the Met Office's long distance lightning detection network. This covers large areas of the globe.
• Lightning detection instruments use the time taken for the lightning to be detected at different locations to triangulate the signal and determine where it originated.
• During volcanic eruptions volcanic lightning is often generated by the erupting ash cloud.
• Detection of this lightning can be used to identify that an eruption is occurring and may be able to reveal information about the height of the eruption.

Research aircraft

• A number of research aircraft may carry out ad-hoc atmospheric measurements during volcanic ash events.
• One such aircraft is the FAAM, which is a shared aircraft between Met Office and NERC. It is the largest research aircraft in Europe - 18-20 people can be on board during flight.
• Most research aircraft are still restricted as to where they can fly in the vicinity of volcanic ash, just like commercial flights.

Dispersion Modelling

• The NAME model, developed by the Met Office, is the primary dispersion model used by the London VAAC and the Met Office's Hazard Centre.
• It is capable of modelling the dispersion of many atmospheric pollutants and their associated physical and chemical processes.
• NAME has been tested and proven during previous eruptions as well as incidents including Chernobyl (1986), the Buncefield oil depot explosion (2005), the Fukushima nuclear incident (2012), and the outbreak of Bluetongue disease (2008).
• For VAAC use, the model is initialised with both meteorological and eruption data. These include winds, temperature, humidity, and eruption source parameters including the height of the plume, mass eruption rate of ash and the size distribution of ash particles.
• Forecast meteorological data is taken from the latest run of the Met Office's global Numerical Weather Prediction model, which is amongst the most accurate weather forecasting models in the world.
• Ash particles are moved by the three dimensional forecast winds from the global weather prediction model with turbulence and diffusion applied by NAME parameterisations.
• Ash is retained in NAME until it is removed through the modelling of natural physical processes including sedimentation, deposition and wash-out.
• The model is run every 6 hours to make use of the latest eruption source parameter information on the volcanic eruption from the Icelandic Meteorological Office.
• Model forecasts are routinely validated and verified against all available observations (satellite, radar, lidar and research aircraft) and compared to model outputs from the other VAACs.

Ongoing developments and future plans

• An inversion modelling system is currently being developed to improve upon the eruption source term used in NAME. The method uses a 'best fit' approach to intelligently compare a set of modelled plume concentrations to observation data (satellite retrievals of ash column loadings). This technique results in a new source term profile and a new modelled plume which more closely resembles the satellite retrievals. In a future phase, it is planned that the inversion system will be able to also use other observation data e.g. lidar, aircraft, etc.
• There is an increasing interest in the impact of large gas-rich eruptions in Iceland from health and environmental perspectives. The Met Office's Atmospheric Dispersion and Air Quality team are leading an Effusive Eruption Modelling Project to assess the potential hazard to the UK from such an eruption. This involves modelling the sulphur dioxide gas and sulphate aerosol produced in these eruption clouds.
• An intercomparison of buoyant eruption column models is being conducted, including the Met Office model developed by Devenish et. al. This will improve our understanding of how these models could be used to inform the eruption source parameters for dispersion models.

For more information about how we are using the NAME model and how to interpret the results, download our technical summary 'The current volcanic ash modelling set-up at the London VAAC'. 

Forecasting and Advisories

• The London VAAC is hosted and run by the Met Office and is one of 9 VAACs worldwide. It provides forecast guidance concerning the location of volcanic ash.
• The London VAAC has responsibility for issuing advisories from volcanic eruptions originating in the north-eastern corner of the North Atlantic which includes Iceland and Jan Mayen (a volcanic island belonging to Norway).
• The forecasts are provided to the standard and tolerances set by the regulator. The forecast model can be configured to provide forecasts to any tolerance of ash as required by the aviation regulatory authorities.
• The Met Office are active participants in a number of cross-disciplinary and multi-organisational volcanic ash related working groups, projects and initiatives, and exercises. See VAAC: related activities, organisations and links for further information.

Advisories and Products

• Standard advisories issued by the Met Office London VAAC are a VAA text file and VAG for three flight levels: FL000-200; FL200-350 and FL350-550. Note that a flight level (FL) is a measure of altitude in hundreds of feet, so FL350 denotes 35,000 feet.
• Supplementary guidance products are provided by the Met Office under designation from the UK CAA in the form of concentration charts (i.e. of ash in the volcanic cloud) and annotated satellite images in support of the European Volcanic Ash Contingency Plan.
• Advisories and charts are issued on a 6-hourly basis during a volcanic eruption, providing guidance up to 18 hours ahead for the VAA/VAG and up to 5 days ahead for the CAA. Annotated satellite images are issued every three hours.
• VAAC Forecasters use an 'intervention tool' which allows them to make adjustments, where necessary, to the raw dispersion model output e.g. to adjust the polygon lines produced by the model prior to issuance of the graphical advisory, based on evidence from observations.