The operational eEMEP model for volcanic SO2 and ash forecasting
Authors: Steensen Birthe Marie , Schulz Michael , Wind Peter , Valdebenito Álvaro , Fagerli Hilde 
Published: January 2017

Abstract. This paper presents a new version of the EMEP MSC-W model called eEMEP developed for transportation and dispersion of volcanic emissions, both gases and ash. EMEP MSC-W is usually applied to study problems with air pollution and aerosol transport and requires some adaptation to treat volcanic eruption sources and effluent dispersion. The operational setup of model simulations in case of a volcanic eruption is described. Important choices have to be made to achieve CPU efficiency so that emergency situations can be tackled in time, answering relevant questions of ash advisory authorities. An efficient model needs to balance complexity of the model and resolution. We have investigated here a meteorological uncertainty component of the volcanic cloud forecast by using a consistent ensemble meteorological dataset (GLAMEPS forecast) in three resolutions for the case of SO2 effusion from the 2014 Barðarbunga eruption. The low resolution (40 × 40 km) ensemble members show larger agreement in plume position and intensity, suggesting that the ensemble here don't give much added value. For comparing the dispersion in different resolutions we compute the area where the column load of the volcanic tracer, here SO2, is above a certain threshold, varied for testing purposed between 0.25–50 DU Dobson units. The increased numerical diffusion causes a larger area (+34 %) to be covered by the volcanic tracer in the low resolution simulations than in the high resolution ones. The higher resolution (10 × 10 km) ensemble members show higher concentrations farther away from the volcanic eruption site in more narrow plumes. Plume positions are more varied between the high resolution members, while the plume form resemble the observed plumes more than the low resolution ones. For a volcanic emergency case this means: To obtain quickly results of the transport of volcanic emissions an individual simulation with our low resolution is sufficient, however, to forecast peak concentrations with more certainty for forecast or scientific analysis purposes a finer resolution is needed. The model is further developed to simulate ash from highly explosive eruptions. A possibility to increase the number of vertical layers, achieving finer vertical resolution, as well as a higher model top is included in the eEMEP version. Ash size distributions may be altered for different volcanic eruptions and assumptions. Since ash particles are larger than typical particles in the standard model, gravitational settling across all vertical layers is included. We attempt finally a specific validation of the simulation of ash and its vertical distribution. Model simulations with and without gravitational settling for the 2010 Eyjafjallajökull eruption are compared to lidar observations over Central Europe. The results show that with gravitation the centre of ash mass can be 1km lower over central Europe than without gravitation. However the height variations in the ash layer caused by real weather situations are not captured perfectly well by either of the two simulations, playing down the role of gravitation parameterization imperfections. Both model simulations have on average ash centre of mass below the observed values. Correlation between the observed and corresponding model centre of mass are higher for the model simulation with gravitational settling for four of six stations studied here. The inclusion of gravitational settling is suggested to be required for a volcanic ash model.