Monitoring volcanic plumes from space
March 2012 - The Met Office is one of the International Civil Aviation Organisation Volcanic Ash Advisory Centres (VAACs) with responsibility for providing advisory information for volcanic eruptions in the north-east Atlantic area. Satellites are able to monitor volcanic ash plumes and gases in real time providing a valuable source of information for the VAACs.
The eruption of Eyjafjallajökull in Iceland during April and May 2010 provided further evidence of the importance of real time monitoring of volcanic ash to follow the trajectory of the plume, providing advice on the input to the atmospheric dispersion model, Met Office Dispersion Model, and for validating dispersion model forecasts.
For the London Volcanic Ash Advisory Centre (VAAC) the primary satellite used for monitoring volcanic ash plumes is Meteosat, which views Europe, the Atlantic Ocean and Africa every 15 minutes and so has good coverage of the VAAC's area of responsibility. The SEVIRI imaging radiometer on Meteosat has 12 channels (at visible and infrared wavelengths), and its data allow the retrieval of the height of the volcanic ash plume, the total column loading of ash and the effective radius of the ash particles in cloud-free areas or above cloud. The different radiative properties of the ash at different wavelengths make this possible and more details of the technique used can be found in Francis et al., 2012. An example from the Eyjafjallajökull eruption is shown in the plot at the top where the plume total column loading at 08 UTC on 13 May 2010 is shown to the north of Scotland exceeding 7 gm-2. A limitation of the satellite retrieval is that the depth of the plume cannot be inferred and so the concentration of the ash particles cannot be directly measured. A low concentration thick plume can appear the same as a high concentration thin plume when viewed from above. Plumes higher in the atmosphere are also easier to detect than at low levels. The height of the plume close to the source, also shown in the plot (when enlarged), is a useful tool in defining the input of the plume to the dispersion model, particularly for poorly monitored volcanoes. The 15 minute repeat cycle of Meteosat allows animations of the plume to be displayed and this can help to identify the ash in marginal situations.
VAACs currently have no formal mandate or requirement for providing advice on volcanic sulphur dioxide(SO2) concentrations, but various satellite-derived SO2 products are still monitored routinely for evidence of volcanic emissions. The potential impacts of SO2 on aircraft are currently not fully understood although preliminary research by the appropriate bodies is now being undertaken. SO2 can also be a potential problem if mixed with rain and converted to sulphuric acid or in high concentrations it can be a potential public health hazard. It is a gas which has distinct absorption bands in the infrared part of the electromagnetic spectrum which coincide with some of the SEVIRI radiometer channels allowing a retrieval of the total column amount of sulphur dioxide. An example of an eruption of Mt. Nabro in Ethiopia, which released a SO2 plume but only low concentrations of ash, is shown in the plot to the right. During some eruptions the ash plume and SO2 plume can be transported away from the volcano in different directions depending on what heights both constituents rise to. Also once inserted into the stratosphere the sulphur dioxide can circulate as a coherent plume for many days and satellites are useful to monitor this.
Simulated ash imagery
Ash forecasts from the atmospheric dispersion model can be compared with observations to assess the quality of the forecasts. A new technique has been developed to enable like-with-like comparison between satellite imagery of volcanic ash and simulated imagery using the forecast ash concentration data from the NAME model. These ash concentrations, together with temperature and humidity profiles from the Unified Model, are used as inputs to a fast radiative transfer model to compute radiances from which simulated satellite images are created. RGB (red-green-blue) observed and simulated images combining information from different SEVIRI infrared channels are mainly used. They are produced by assigning the difference in measured radiance between the 12.0 and 10.8 μm channels of SEVIRI to the red gun, the 10.8 - 8.7 μm radiance difference to the green gun and 10.8 μm radiance values to the blue gun. Volcanic ash tends to produce an orange or pink colour in these images. In addition to providing a useful tool for forecasters in the VAAC, the simulated images can be used to aid the physical understanding of how the ash affects the satellite imagery. An example for 12 UTC on 6 May 2010 is shown in the plot to the left where the left hand is the satellite observation and the right hand is the simulated RGB imagery to compare with the former. More details of how these are generated can be found in Millington et al., 2012.
Other satellite data are also used in the London VAAC which include conventional infrared and visible imagery from SEVIRI and imagery from polar orbiting satellites (e.g. AVHRR and MODIS) which are important for polar latitudes where the geostationary images have a very oblique view. There are also SO2 retrievals from IASI, a high spectral resolution infrared sounder, and aerosol and sulphur dioxide products from instruments measuring in the ultra violet and visible part of the spectrum such as OMI and GOME-2. Some of these products are accessed from the Support to Aviation Control Service site.
The use of satellite data for monitoring volcanic plumes is undergoing further developments to enable quantitative retrievals to be produced from the polar orbiting imagers which should enable a global capability for volcanic ash monitoring to be introduced. Also the possibility of directly assimilating the satellite products to improve the NAME model forecasts is under investigation.
Francis, P. N., M. C. Cooke, and R. W. Saunders (2012), Retrieval of physical properties of volcanic ash using Meteosat: A case study from the 2010 Eyjafjallajökull eruption, J. Geophys. Res., 117, D00U09, doi:10.1029/2011JD016788.
Millington, S.C., R.W. Saunders, P.N. Francis and H.N. Webster (2012)Simulated volcanic ash imagery: a method to compare NAME ash concentration forecasts with SEVIRI imagery for the Eyjafjallajökull eruption in 2010, J. Geophys. Res., 117, D00U17, doi:10.1029/2011JD016770.