|The authors have addressed my comments and suggestions but very little changes have been taken into account in the revised version, eluding the most important calculations. Also, I do not like the tone of the response, e.g. sentences like "We suggest the referee read more carefully the LP04 paper ..." or "... his/her strange way of thinking..." or "We invite the referee and his/her group to make their own research contributions, given the new knowledge" are completely out of place and should be omitted. That said, I have tried to be as objective as possible.|
I think the paper deserves to be published. I am making the less possible comments and are divided in: a) essentials (without which I would not recommend the publication) and b) important (highly recommendable). I also include a comment on the authors' comment at the end that do not affect the manuscript but, since this report will be public, I feel I should reply.
One of the major points comes from the interpretation of the sentence in LP04 of
“...We have investigated the SABER 4.3 µm radiances with the help of a non-LTE radiative transfer model for CO2 and found that the large radiances can be explained by a fast and efficient energy transfer rate from OH(v) to N2(1) to CO2(v3), whereby, on average, 2.8–3 N2(1) vibrational quanta are excited after quenching of one OH(v) molecule."
We may question if this mechanism is realistic or not but it is a mechanism and LP04 were able to reproduce the radiances for essentially all conditions (e.g. for 4 orbits of 4 days covering equinox and solstice (Figs. 12-14)) within +/-20%, that is, equally or better than with the new mechanism. True that so far there is no evidence of such a "direct" energy transfer process and this is why this manuscript is relevant, because it gives a plausible "indirect" way of such an energy transfer, even though the actual reaction rates have not been measured (only estimated) and the efficiency of OH(v) to O(1D) has not been measured either. Hence, I do appreciate this qualitative new mechanism. That is why I suggested to focus (title, abstract, etc.) on the new pathway more than on if it is able to reproduce better or not the measured SABER CO2 4.3 µm radiances. Then, my comments:
1) Title: (important). I would still recommend changing the part "New model calculations improve agreement with SABER observations".
2) Abstract (important): If with the "previous study" the authors refer to "Kumer et al." it is correct. However if to LP04, it is also correct but not complete (see above). My recommendation would be to give the whole story not only part of it.
3) Motivation of the work (clarification, not relevant for the manuscript). I tried to say that this work is important itself and does not need the additional justification of retrieving CO2 at night. Of course, in no way I meant to discourage the authors in pursuing such research.
4) (Essential). Since there have been already two papers dealing with this new mechanism (Sharma et al. (2015) and Kalogerakis et al. (2016)), I think this work should make the most accurate and consistent calculations as possible, making use of all available SABER data in a consistent and proper way. That it, in my opinion is not valid the argument that the purpose of this work is to make "estimates" and further work will be done later. Hence:
4a) They should use the retrieved CO2 from SABER (contrary to their reply, CO2 is publically available, (ftp://saber.gats-inc.com/Version2_0/Level2C/) and two of the co-authors are co-authors of the CO2 retrieval papers (Rezac et al., 2015a,b).
4b) My previous major comment on the OH SABER radiances (see below) has not been addressed adequately.
"As the new proposed mechanism affects also to the population of OH(v) and the emissions from these levels were measured by SABER in two different channels, I think it is essential that the authors demonstrate that the new OH(v) model explain very well the measured SABER OH radiances, as LP04 did. Thus, figures for different conditions with the SABER observations and modelled radiances for the two OH SABER channels should be presented in this work."
Thus, their replies of: "... but this is clearly out of the scope of this report."
or "The goal of our study was to estimate, ..."
I think it is crucial for this manuscript (not report), and I think it should be something more than an estimate, the title of the work reads "New model calculations improve agreement with SABER observations"
5) (Essential, in the same line as point 4). Atomic oxygen is key for the new excitation mechanism, therefore all models inputs and SABER radiances should be consistent. Thus, my previous point on this topic has not been adequately addressed. Taking from my previous report:
"About the O(3P) abundance and the OH(v) model, the authors state that they used the O(3P) retrieved from SABER measurements. The SABER O(3P) is derived from the SABER OH radiances but a photochemical OH(v) model is required for such inversion (Mlynczak et al., 2013)."
The authors should use the same photochemical OH(v) model or prove (with calculations and figures) that they are consistent.
Further on this topic, several works have shown that SABER atomic oxygen might be overestimated in a ~30%. It would very important to comment, how would this affect to the simulations of SABER CO2 4.3 µm nighttime radiances with this new mechanism?
6) Sec. 3.2 and Fig. 2 (Essential). I cannot see the reasoning of why using only the partial result of LP04. Do the authors want to validate their model? I see a high risk to misleading the reader as giving the impression that LP04 were not able to explain the SABER radiances when they did (see Figs. 10 and 11 in LP04). I strongly recommend to either show the two LP04 simulations or none.
7) (Essential) About the OH densities.
If the authors calculate OH(v) (does this include also v=0?) from SABER O3 and H, why they need OH (ground state? or total (i.e. OH(v) including v=0) from WACCM? If this is important, my previous argument whereby WACCM should sub-estimate OH still applies.
As mentioned above, the authors should use a consistent model and inputs for all quantities.
8) (important) Conclusions. As mentioned at the beginning I would focus more on the mechanism itself (find the way to transfer so much energy from OH(v) to N2(v) rather than on the "improve agreement" of the SABER radiances.
I do not fully agree with your statement that "(b) everything else that follows in our conclusions is rather measured ..." You need to make "estimates" of key parameters as how much energy is transferred from OH(v) to O(1D), both on the collisional rates and their temperature dependences.
I also do not fully agree either about the advantage of this work over previous work in the sense of making more viable the retrieval of nighttime CO2. True that the physics is known better but this is not superior to the energy efficiency factor derived by LP04 in reproducing the SABER 4.3 µm radiances. After all the two studies agree on that all energy comes from OH(v) and this is measured by SABER, so I cannot see much the advantage of using an "indirectly" derived atomic oxygen (derived from a model with potential uncertainties and that should be consistent with that used in the excitation of O(1D)=>N2(v)) or an empirical energy transfer efficiency.
Just one clarification to your statement:
• LP04 also showed that f=~3 (possible multi-quantum process, but mechanism is not explained) removes about 40% of differences for some selected scans studied. In our very extensive study we found that these differences can reach as high as 80%. Following the LP04 logic it would
require f=6 and higher to remove these differences, which is absolutely unrealistic. In other words, LP04 quantified the energy transfer efficiency that would be required for model calculations and observations to agree (for some limited set of scans), but no detailed mechanism was described, let alone validated by referring to theoretical or experimental investigations.
This is not fully true. LP04 showed, not only for some limited scans but for 4 orbits of 4 different days covering equinox and solstice conditions, that SABER measurement with f=2.8-3 could be reproduced within +/-20% (see Figs. 12-14). I would not be surprised to need larger f-values for the polar regions, where auroral excitation is prone and frequent. Also, if you analysed so many data, it would be very useful to the reader to show more than just only 2 days (Fig. 3).