28 February 2011
In April and May 2010, the Eyjafjallajökull volcano in Iceland erupted explosively emitting a complex cocktail of ash and gases into the atmosphere. The resulting plume was blown into UK and European airspace causing disruption to aviation, leaving tens of thousands of passengers stranded and causing considerable costs to the airline industry and the global economy. Here, Jim Haywood, Met Office Aerosol Research Manager, provides an insight into the observations and science of the volcanic ash plume.
The Met Office London Volcanic Ash Advisory Centre (VAAC) is responsible for monitoring and forecasting the movement and dispersion of volcanic ash originating from volcanoes in the north-eastern Atlantic including Iceland. Volcanic eruptions are extremely complex to model and are influenced by many geophysical factors. Eyjafjallajökull was particularly complex because of the interaction of the molten magma with the overlying glacial ice which led to the production of small ash particles.
The Met Office's Numerical Atmospheric-dispersion Modelling Environment (NAME) is used to model a range of atmospheric dispersion events, including volcanic eruptions. When modelling such events, it is important to know both the total mass of ash emitted and the ash particle size distributions because large particles are removed from the atmosphere more quickly than small particles via gravitational settling. Gravitational settling rates also depend on the particle shapes and densities. Additional observations are vital for forecasters and researchers to be able to validate and add value to the model forecasts. Observations were provided from the Met Office, European National Meteorological Services and the university research communities. They came from a variety of sources including laser cloud-based recorders, research lidars, satellite imagery, balloon-borne ash sensing instruments, and aircraft instruments.
Among the most equipped aircraft of the European research aircraft fleet is the FAAM BAe 146-301 which is run jointly by the Met Office and the Natural Environment Research Council (NERC). It performed a series of 12 flights between 20 April and 18 May. In compliance with Civil Aviation Authority regulations, the FAAM aircraft was not permitted to fly in areas where volcanic ash concentrations were greater than 2 milligrams per cubic metre, so the aircraft targeted areas where concentrations forecast by the Met Office NAME model were in the range 0.2 to 2 milligrams per cubic metre.
The FAAM aircraft was equipped with remote sensing equipment including a compact backscatter lidar which emits laser light and measures the intensity of the light backscattered from particles in the atmosphere. This enabled mapping of the geographic distribution of the volcanic ash plume without the aircraft having to penetrate the plume. The lidar was also able to provide information on the altitude of the volcanic ash layers and on the size and shape of the ash particles which enabled them to be distinguished from sulphuric acid particles, another component of the plume.
When forecast levels were below 2 milligrams per cubic metre, the FAAM aircraft was able to fly in areas of volcanic ash to measure the mass concentration. However, this is not a straightforward operation. Aerosol particle sizes are typically measured by Optical Particle Counters (OPCs) that bounce laser light off each particle and measure the scattered light which is proportional to the particle size. Many thousands of particles may be measured per second. OPCs are calibrated using spherical pure glass beads. This presents a complex problem; volcanic ash particles are not spherical or composed of pure glass so corrections have to be made that typically double the derived mass of the particles.
Other instrumentation used to estimate the volcanic ash mass included a nephelometer which measures the scattering of light not from individual particles, but from bulk samples of volcanic ash laden air and if the particle size distribution is known from the OPCs, then the mass may be determined.
While the measurements were able to provide quantitative information as to the validity of the NAME model forecasts, much work is still ongoing to improve the accuracy and the speed that mass concentrations derived from the lidar, OPCs and other measurements so that accurate near-real-time measurements can be provided for model validation should future eruptions occur. Given the frequency of explosive volcanic eruptions in Iceland is around once every five years, we may not have long to wait.
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