In their manuscript "SynRad v1.0: A radar forward operator to generate synthetic radar return signals from volcanic ash clouds" the authors present a new valuable tool to directly convert the output ash concentrations of 3D numerical eruption cloud models into the signals that would be measured by weather radars, ground-based or from satellites. The described version is specifically bound to the eruption cloud model ATHAM but the authors emphasize that the model is stand-alone and written in python. Therefore it can be used on any other model output as long as its output is in netCDF file format and includes ash concentrations. The idea is not completely new as a similar model has been published before (without the code though, see: Scharff, L.; Ziemen, F.; Hort, M.; Gerst, A. & Johnson, J. B.; A detailed view into the eruption clouds of Santiaguito volcano, Guatemala, using Doppler radar; 2012; doi: 10.1029/2011JB008542). But the model presented here extends the old one by performing ray tracing and thus accounting for signal attenuation due to the presence of scatterers within the radar beam.

The manuscript is scientifically sound and contains only a minor amount of technical corrections (see below). 

Specific Comments

• The title could be specified by using the expression "weather radar". This manuscript exclusively addresses weather radar measurements, but volcanic clouds are also measured using other non-weather radar systems. The latter systems not necessarily produce the radar observables listed here. This distinction between weather (scanning) radars and other non-weather (profiling) radars should be clear also throughout the introduction. A good review on the observation of volcanic ash clouds using (any) radar system is given by Hort and Scharff (Hort, M. & Scharff, L.; Detection of Airborne Volcanic Ash Using Radar; In Book: Volcanic Ash; 2016; doi: 10.1016/b978-0-08-100405-0.00013-6)
• L12: higher frequency microwave radars (K-band and higher) that observe volcanic activity do already exist. However, they may not operate in scanning mode. Please specify again: weather radar
• L22-25: again this describes scanning radars. Profiling radars will have a much better temporal and even spatial resolution, but only in one single direction.
• L38: Another observation of volcanic activity using dual-polarization weather radar was done in New Zealand (Crouch, J. F.; Pardo, N. & Miller, C. A.; Dual polarisation C-band weather radar imagery of the 6 August 2012 Te Maari Eruption, Mount Tongariro, New Zealand; 2014; doi: 10.1016/j.jvolgeores.2014.05.003)
• L42: volcanic particles can have any size near the vent. Their size will decrease with the distance from the vent. The formulation used in the manuscript is correct ("particle sizes are smaller than raindrops especially far from the vent"), but it must also be clear that this assumption becomes invalid near the vent.
• L62-63: the listing could be extended by the paper by Scharff et al (2012) mentioned above. With this reference you would acknowledge the old model without explicitly mentioning it.
• L115-119: neglecting attenuation due to water vapor in the atmosphere might be feasible, but water vapor is highly abundant in the eruption cloud. Is it considered there? Effects of volcanic SO2 can be neglected at common radar-relevant frequencies but may become more important in higher frequency radars. Please mention up to what frequency this assumption is valid.
• L125: When polarization is included in the future, do you also reevaluate assumption 1 on the sphericity on particles?
• L168-179: This paragraph does not make clear whether a one-way or two-way attenuation is calculated. From the formulation it appears as if only the attenuation from the radar to each cell is calculated.
• L214: what is "m"? This parameter appears before its explanation in line 219.
• L337-339: Please elaborate more on the way the ATHAM VMIs are calculated. To me the difference in calculation to SynRad did not become clear enough. This leads to follow-up questions in paragraphs that include comparisons between the two VMIs (e.g. in L427). What do you expect to see from ATHAM VMIs in comparison to SynRad VMIs. Is it simply the inclusion/absence of attenuation?
• L407-408: I do not understand this sentence. Could you please rewrite it. I suggest at least exchanging "at odds" with the more common "in contradiction to".
• L410: Isn't it a combination of range, particle size and concentration that lead to a return signal lower than the MDS?
• L430-431: This application is not shown here. Please rephrase.

Technical Corrections

• L3: ... module that traces ...
• L8: ... between a stronger return signal and the increased path attenuation ...
• L101, L108, etc: formatting of abbreviation "i.e." differs
• L103: maybe use range bin length instead of width as it describes a beam-longitudinal rather than transversal size
• L150: can you please list the parameters of eq. (6) that are sub-summarized in parameter Cr?
• L166: a closing parenthesis is missing
• L172: ... segment that acts ...
• L214: ... functions depending on ...
• L263: ... permittivity calculation ...
• section 3.4: the usage of parameters epsilon' and epsilon'' for water and ice without any further subscript is confusing.
• L291: please give the distance between radar and vent (and refer to figure 5 for the positioning)
• L310: ... can result in overestimating the volume density ...
• L328: ... threshold are guaranteed ...
• L337: remove the word 'colorblue'
• L365-366: This is not a complete sentence.
• L376-377: The lines are pink rather than red.
• L380: the color bar does not include white (no attenuation due to absence of cloud). see also Figure 8
• Figure 8: please indicate the vertical exaggeration in the caption.
• L401-403: Something appears wrong with this sentence. You may split this into two sentences for easier reading.
• L404: the comment 'move to appendix' does not make sense here: there is no appendix
• L412: this chapter is rather a summary than a conclusion. I suggest to rename it accordingly.
• L432: ... retrieval algorithms for ...

Dear editor,

in this contribution, Nair et al. describes a new radar forward operator, SynRad, which simulates the radar measurement process and generates synthetic radar signals in the three-dimensional output: equivalent radar reflectivity, vertical maximum intensities, and echotops. This system allows to simulate the radar measurement related to volcanic clouds taking into account the attenuation due to the volcanic ash and the hydrometeors. This last point is derived using the ray tracing, under certain simplifications. The 2019 eruption of the Raikoke volcano is used as the case study to showcase the capabilities of SynRad. The measurement process of a C-band radar is simulated and the equivalent reflectivities, VMIs and echotops are calculated and compared with those generated from ATHAM, showing a good agreement.

In general, the text is well-written, and each section of the work is clear and essential, however it requires more bibliographic references as reported in the attached PDF. The state of the art and the addressed problem are introduced in the first section in a clear manner, and this problem is well addressed in the manuscript (although several references are need too). The methodology is clear and responses to the introduced problem. Discussion and conclusions are effectively based on results, and presentation of results is accompanied by descriptive figures that effectively show the results highlighted in the manuscript.

Certainly the effective validation of SynRad can be performed directly with the radar measures of an eruptive event (such as Etna, Grimsvotn, etc.) and including an analysis of sensitivity, but this will probably be your next work.

All in all, I recommend publication of this manuscript in EGUSphere after minor but essential revisions. In the attached pdf, I include a set of detailed and editorial comments and suggestions.