Aircraft observations and model simulations of concentration and particle size distribution in the Eyjafjallaj?kull volcanic ash cloud

摘要

The Eyjafjallaj?kull volcano in Iceland emitted a cloudof ash into the atmosphere during April and May 2010. Over the UK the ashcloud was observed by the FAAM BAe-146 Atmospheric Research Aircraft whichwas equipped with in-situ probes measuring the concentration of volcanic ashcarried by particles of varying sizes. The UK Met Office NumericalAtmospheric-dispersion Modelling Environment (NAME) has been used to simulatethe evolution of the ash cloud emitted by the Eyjafjallaj?kull volcanoduring the period 4–18 May 2010. In the NAME simulations the processescontrolling the evolution of the concentration and particle size distributioninclude sedimentation and deposition of particles, horizontal dispersion andvertical wind shear. For travel times between 24 and 72 h, a 1/trelationship describes the evolution of the concentration at the centre ofthe ash cloud and the particle size distribution remains fairly constant.Although NAME does not represent the effects of microphysical processes, itcan capture the observed decrease in concentration with travel time in thisperiod. This suggests that, for this eruption, microphysical processes play asmall role in determining the evolution of the distal ash cloud. Quantitativecomparison with observations shows that NAME can simulate the observed column-integrated mass if around 4% of the total emitted mass is assumed to betransported as far as the UK by small particles (< 30 μmdiameter). NAME can also simulate the observed particle size distribution ifa distal particle size distribution that contains a large fraction of< 10 μm diameter particles is used, consistent with the ideathat phraetomagmatic volcanoes, such as Eyjafjallaj?kull, emit very fineparticles.