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Vertical distribution of ash at source Time-height plots of mass concentration at Chilbolton. The height above the summit into which ash particles are emitted at the source is (a) 3-3.5km, (b) 4-4.5km, (c) 5-5.5km and (d) 6-6.5km. The height at which ash is emitted into atmosphere influences the plume evolution as wind speed and direction vary with height. The operational model assumes a uniform distribution of ash from the summit to the plume height to account for a fluctuating plume. Emitting ash in layers at decreasing heights above volcano decreases the plume depth and width and delays the arrival time over Chilbolton. Particle size distribution at source Mass concentration from 0-12km at 00UTC on16/04/10 for (a) 0-30 μ m (b) 30-10 μm (c) 100-300 μ m diameter particles Time-height plots of mass concentration for particle size distribution (a) operational, (b) more fine particles. Particle size distribution influences plume evolution as coarse particles have greater fall speeds than fine particles. Particles with diameter > 100μm fall out within 1000km of volcano. Operational model uses a particle size distribution with 20% of mass in 3-10μm range and 70% of mass in 10-30μm range (distribution A). Emitting a larger proportion of fine particles increases the peak mass concentration and the altitude of the centre of mass over Chilbolton. How sensitive are NAME ash plume forecasts to input source characteristics? Helen Dacre, Robin Hogan, Stephen Belcher, University of Reading, UK (h.f.dacre@reading.ac.uk) Introduction The Eyjafjöll volcano erupted on 14/04/10 causing widespread disruption. The Met Office dispersion model, NAME, is used to predict the evolution of the ash plume. The accuracy of these predictions depends heavily on the input of accurate source characteristics. The aim of this work is to analyse the sensitivity of the NAME model results to uncertainty in the input source characteristics. Case Study: 16 th April 2010 A high-pressure system was located over the UK and the north Atlantic and a low-pressure system was located over northern Europe. The ash plume was observed by a ground-based lidars at Chilbolton. Overview Observed Mass Concentration Time-height plot of mass concentration determined from a combination of lidar retrievals and sun photometer measurements at Chilbolton from 10-18UTC on 16/04/10. The lidar observes the ash plume with a depth of 3km, width of 500m and arrival time at 12UTC on 16/04/10. Peak concentrations of 800μg m -3 are observed at a height of 1.8km Conclusions Vertical and horizontal structure of ash plume predicted by NAME model is very sensitive to height at which ash is emitted above volcano and is also to sensitive to particle size distribution. Magnitude of mass concentration is determined by the %of mass in sub 100μm range. Comparison with observations suggests between 1 and 10% of total emitted mass is contained in this range. Synoptic Analysis 00UTC 16/04/10 Emission rate of source Time-height plots of mass concentration at Chilbolton for varying mass flux in the sub 100 μ m range, (a)10 6 kg/s (operational), (b) 10 5 kg/s and (c)10 4 kg/s. The mass flux attributed to the sub 100μm range determines the magnitude of the mass concentration contours. An empirical relationship is used to determine the emission rate H = 0:365M 0:225, where H (km) is the maximum plume height above the summit an M is the total emission rate (kg/s). For an initial plume height of 8.5km above the summit, this implies a total emission rate of ~ 10 6 kg/s. The mass concentration over Chilbolton scales linearly with the percentage of the total mass flux in the sub 100μm range. Initially the ash plume is advected SE from Iceland towards Europe. After 24 hours the ash plume diverges; one branch is advected around the high-pressure system whilst another branch is advected around the low-pressure system. During the16/04/10 the anti-cyclonic branch of the ash plume was advected over the UK. The vertically slanted structure observed by the lidar is captured by the NAME model and is a result of vertical wind shear. Mass concentration at 00UTC on 16/04/10. (Left) average concentration from 0-12km. (Right) vertical cross- section taken from 45-55 o N and at 2 o W. Note the contours are factors of 10 concentrations in arbitrary units. Lidar backscatter 10-18UTC 16/04/10 Chilbolton

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