Review of "Why we need radar, lidar, and solar radiance observations to constrain ice cloud microphysics" by F. Ewald et al.

This is a very interesting paper focusing on achieving short-wave radiative closure using multi-instrument cloud observations. The

ability of such studies to constrain ice cloud particle habits is particularly noteworthy. The results are especially relevant to

upcoming active remote sensing satellite missions (e.g. the ESA/JAXA Earth Clouds and Radiation Explorer mission).

The paper is, on the whole, well written and structured. There are, however, some areas where the text must be improved.

Specific Issues

----------------

P-1, L-19: Bad sentence: I suggest: "In this case, collocated in-situ measurements indicate that the lack of closure may be linked to

unexpectedly high values of the ice crystal number density."

P-2: L-30: "...distribution contribute to..."

P-3: L-70 to 75: This paragraph is confusing (e.g. it is not clear to me at all when you are referring to lidar+radar, passive, or lidar+radar+IR

radiometer measurements).

I think the points you are trying to make here are:

1. Combined lidar-radar measurements can provided high-resolution (on the scale of 10s of meters) vertical profiles of cloud

properties. This capability can not be matched by passive sensor only based cloud retrievals.

However,

2. Lidar measurements are most sensitive to the particle extinction while radar reflectivity is mostly dependent on the squared-mass distribution.

3. Using lidar derived extinction together with radar reflectivity is not enough to constrain e.g. the effective size or IWC

unambiguously. The mapping between the lidar and radar measurements and e.g. IWC and Reff depends significantly on the assumed particle habit and size distribution.

4. Using IR emissivity measurements can help constrain the problem. However, even then ambiguity can remain as IR measurements can

saturate with optical depth quite quickly.

Please rewrite the first half of this paragraph (with appropriate references).

P-4: L 101: Concluding "..the paper concludes with the presentation of a case..."

P-77: L-177: "...online.." ? Do you mean to say that the lidar forward model you use is being run through a remote web interface ? I think

you want to say that the model is available for download. If the later is true, then please give a url, or just delete "online".

P-7: L-185: "..and the beam..."

P-8, First paragraph. Bad first two sentences. I suggest something like

The ice microphysical and scattering models employed in this study are of central importance. Both the lidar+radar (+radiometer) results as

well as the simulated SW radiances used in the closure assessment depend on the ice microphysical and scattering models assumed. In this

section, we describe the microphysical and scattering models employed in this study. We cover both the models/assumptions used in the

retrievals and in the simulation of of the short-wave radiances used to assess the closure."

P-8, L 193: "A commonly used framework which simplifies the... is the concept of an effective ice particle density..."

P-9: L-200: Delete the coma after "Analogous"

P-10, L-243: Very unclear sentence (e.g. what do you mean by "At first..?". I recommend deleting this sentence.

P-13: L-291: "..scientific objective.." is not the proper phrase here, I recommend simply "target".

P-13, L-295: Define WCB ?

P-14: L-328 "Overall, the lidar signal is extinguished much more rapidly ..."

P-17: L-348: "..for all the microphysical models considered..."

P-18: L-365: "Comparison of in-situ and remote-sensing observations"

P-18: Last line "..and its processing are..."

P-19: L-393: Define "ICNC"

P-19: End of page: A small bit of foreshadowing here would help guide the reader, e.g. end the section by stating "The implications of the occurrence of the regions of unexpectedly hight ICNCs are discussed in the next section"."

P-20: Last sentence of page: Awkawrd sencence. I suggest "The resulting oversmoothing accross this discontunity could lead to the undesired perturbation of ....."

P-21: L-445: "...water vapor absorption which insures that mainly light...."

The Authors discuss the use of passive measurements in the shortwave infrared to provide radiative closure to synergistic retrievals of cirrus cloud microphysical properties utilizing co-located airborne lidar and radar measurements. They discuss two test cases, one where passive measurements show evidence of a good radiative closure and one where they do not. For the latter case, they use co-located in situ observations to relate the lack of radiative closure to an exceptionally high ice crystal number concentration at an altitude of about 7.5 km, which poses challenges to both lidar and radar retrievals.

Overall, I found this paper very interesting and clearly written, and therefore I recommend publication with only minor technical corrections. Specific comments are below:

• Abstract, L19-20. The last sentence was not immediately clear to me when I first read the paper. You may want to replace “narrow” with “attribute” or “relate”.

• P2, L56. Do you actually mean “overlapping radar and lidar measurements”?

• P125, Section 2.1. At what altitudes did the aircrafts fly?

• P5, L127, “aircraft” -> “aircrafts”

• P6, L151, “spectroradiometer” -> “spectroradiometers”

• P7, P179, consider replacing “convergence with the actual measurements” with “convergence of the simulated measurements to the actual ones”

• P10, L243, “missing” -> “lack of”

• P10, L244, “can be reflected by all atmospheric consituents” -> “can also be reflected by liquid water clouds and aerosols”.

• P18, last line. Do you mean Fig. 9d instead of 9c?