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This paper presents a new version of the EMEP MSC-W model called eEMEPdeveloped for transportation and dispersion of volcanic emissions, both gasesand ash. EMEP MSC-W is usually applied to study problems with air pollutionand aerosol transport and requires some adaptation to treat volcanic eruptionsources and effluent dispersion. The operational set-up of model simulationsin case of a volcanic eruption is described. Important choices have to bemade to achieve CPU efficiency so that emergency situations can be tackled intime, answering relevant questions of ash advisory authorities. An efficientmodel needs to balance the complexity of the model and resolution. We haveinvestigated here a meteorological uncertainty component of the volcaniccloud forecast by using a consistent ensemble meteorological dataset (GLAMEPSforecast) at three resolutions for the case of SO<sub>2</sub> emissions from the2014 Barðarbunga eruption. The low resolution (40 × 40 km)ensemble members show larger agreement in plume position and intensity,suggesting that the ensemble here does not give much added value. To comparethe dispersion at different resolutions, we compute the area where the columnload of the volcanic tracer, here SO<sub>2</sub>, is above a certain threshold,varied for testing purposes between 0.25 and 50Dobson units. The increased numerical diffusion causes a larger area(+34 %) to be covered by the volcanic tracer in the low resolutionsimulations than in the high resolution ones. The higher resolution(10 × 10 km) ensemble members show higher column loads farther awayfrom the volcanic eruption site in narrower clouds. Cloud positions are morevaried between the high resolution members, and the cloud forms resemble theobserved clouds more than the low resolution ones. For a volcanic emergencycase this means that to obtain quickly results of the transport of volcanicemissions, an individual simulation with our low resolution is sufficient;however, to forecast peak concentrations with more certainty for forecast orscientific analysis purposes, a finer resolution is needed. The model isfurther developed to simulate ash from highly explosive eruptions. Apossibility of increasing the number of vertical layers, achieving finervertical resolution, as well as a higher model top, is included in the eEMEPversion. Ash size distributions may be altered for different volcaniceruptions and assumptions. Since ash particles are larger than typicalparticles in the standard model, gravitational settling across all verticallayers is included. We attempt finally a specific validation of thesimulation of ash and its vertical distribution. Model simulations with andwithout gravitational settling for the 2010 Eyjafjallajökull eruption arecompared to lidar observations over central Europe. The results show thatwith gravitation the centre of the ash mass can be 1 km lower over centralEurope than without gravitation. However, the height variations in the ashlayer caused by real weather situations are not captured perfectly well byeither of the two simulations, playing down the role of gravitationparameterization imperfections. Both model simulations have on average an ashcentre of mass below the observed values. Correlations between the observedand corresponding model centres of mass are higher for the model simulationwith gravitational settling for four of the six stations studied here. Theinclusion of gravitational settling is suggested to be required for avolcanic ash model

The operational eEMEP model version 10.4 for volcanic SO&lt;sub&gt;2&lt;/sub&gt; and ash forecasting

The operational eEMEP model version 10.4 for volcanic SO<sub>2</sub> and ash forecasting