Modelling the volcanic ash plume from Eyjafjallajökull eruption ( May 2010 ) over Europe : evaluation of the benefit of source term improvements and of the assimilation of aerosol measurements

Equal space seems to given to each strand although it seems that the assimilation of ground based lidar at two locations has the most unexpectedly large impact on this case study. As with all research/developments related to volcanic ash, there are only a few case studies to consider. It is unclear whether the authors intend to apply the approaches presented here to eruptions in different geographical locations or to create “synthetic” eruptions to study.

A comparison of a simulation using parameterised ash source term with a simulation using a resolved source term by coupling MOCAGE with FPLUME. A comparison of simulations setup as in 1. but with additional assimilation of MODIS aerosol optical depth. MOCAGE simulations of the case study with continuous assimilation of ground based lidar profiles.
Equal space seems to given to each strand although it seems that the assimilation of ground based lidar at two locations has the most unexpectedly large impact on this case study. As with all research/developments related to volcanic ash, there are only a few case studies to consider. It is unclear whether the authors intend to apply the approaches presented here to eruptions in different geographical locations or to create "synthetic" eruptions to study.
It is generally well written, although in places sentences are very long and should be revised for clarity. Maybe the manuscript would benefit from being read by a native English speaker?
All figures should have panel labels as per the submission instructions. This makes referring to figures in the text and writing concise captions much easier.

Specific comments
Introduction: L1-24 More information could be added about what is issued by the VAACs (e.g. flight levels). What is the impact of issuing concentrations on aviation operations? What was the motivation of moving to a concentration approach? Do other VAACs issue quantitative information (I wasn't aware they did)?
L25 You mention short comings of the models used to produce the ash dispersion forecasts, however you do not include the impact of driving meteorology (and potential uncertainty) on the output. The driving meteorology also impacts on the plume model that is used to determine the resolved source term and source inversion.
Case study: Section 2.1 is rather brief and focusses mainly on listing the observations that are available rather than features of the case study. The observations then seem to be repeated in section 2.3.
L65 what ash properties are available from SEVIRI?
L86 What is the difference between 3D-VAR and 3D-FGAT? Which one is used in this study?

L96 I am unclear what an observation operator is. Can you define it?
L109 'rather good' is a subjective statement. Can you support this statement with some quantitative comparison?
I am not an expert in aerosol optical depth retrievals or data assimilation methods -When you assimilate AOD is this done at a specific height or does it somehow effect the whole column? L174 Is this value of 30% analogous to the distal fine ash fraction used by the London VAAC? This value seems quite high if this is the case.
L182 Is this the uniform distribution of ash in the parameterised case by design? Figure 2 What is meant by the yellow shading in the bottom panel below the green line? Is it ash at the vent?
L187 The authors state that the 'plume has a realistic shape which goes in the right direction' but they don't say what evidence they use to come to this conclusion. Is it compared to the VACOS information? Figure 3 Qualitatively, the ash distributions in both simulations are quite similar and are both a reasonable match to the VACOS retrieved information. What is the additional cost of running with FPLUME? Along the plume axis, the high ash column loadings that are simulated are not reflected in the VACOS retrievals. Is there an upper limit on the value that can be retrieved? At 0900 on 16 th May in the parameterised simulation there is a lot of ash to west of Ireland that isn't present in the FPLUME simulation. Is this ash at low levels? At 2000 on 17 th May there are large parts of the simulated ash plume that are not evident in the VACOS retrievals. Do you have a hypothesis for why this is? Over the Netherlands there is no ash simulated in either simulation. Could this be due to errors in the meteorology? L195 Remove this sentence L200 This 12% error seems quite small? Is it from literature? What is meant by hits for VACOS? It is just the number of grid cells with ash in the retrievals? Are all the hits along the plume axis and misses around the edges? It seems all the simulations perform similarly in the hit metric, although there are obvious phases in the period considered. The caption mentions grey shading but there is none in the figure. L224 Unsure what is meant by 'data spots' here L225 Can the author's say something more quantitative here than 'rather good'?
L235 What is meant for by a 'meaningful' metric?
L244 Is a radius used or is it a square of a specified number of grid boxes as per the original FSS methodology? Figure 6 Why did you choose these neighbourhood sizes? 500km is a very large area. The differences between each of the simulations is quite small especially one you get to the larger neighbourhood sizes. Do the authors need to show 200km and 500km? Highlight the FSS=0.5 level on the plots What happens on 19 May? Skill drops off very quickly at all scales. Is it possible to show how much information is being assimilated at each time? This might be interesting to know.
The metrics that are used in this study consider the presence of ash above a column loading threshold. This analysis could be extended to determine the performance of the simulations regarding the magnitude of ash column loadings.
Assimilation of ground-based lidar profiles L262 I don't think that the signature is 'obvious' if you are not used to looking at this type of data. Also, the maximum at Hamburg really isn't that clear. You only use a few snapshots of the insitu measurements. Is there more that can be used? (E.g. to create a plot that shows the impact of the assimulation more generallymaybe a scatter plot).

Conclusions
L336 change 'look more realistic' to 'compare more favourably to insitu measurements' L342 How general do you think that your findings are given that you have only run a small number of simulations for one case study? Do you intend to apply the approaches presented here to eruptions in different geographical locations or to create "synthetic" eruptions to study? Another approach to address some of issues used as motivation in this study is to use ensembles of simulations. Is this something your group has considered?

Technical corrections
L34 eruption not eruptions L34 I think that there is an -that is not required L47 purposes rather than purpose L47 can you expand out the acronym -I am not familiar with this project L48 replace ',' with and so that it becomes 'satellites and ground based….' L52 'terms' than 'term' L57 'are' rather than 'is' L59 This sentence sounds rather odd -maybe change to 'includes a discussion of possible future directions for this work'? L62 change 'ashes' to 'ash' L63 'and fly over continental Europe' doesn't make sense. How about 'and impacted aviation operations over continental Europe'? L63 This sentence is quite hard to follow too: 'During this period, lidar observations from EARLINET (Pappalardo et al., 2013) and airborne measurements (Schumann et al., 2011) of aerosols were acquired, that detected layers of significant ash concentrations.' Please revise.