|The paper “On the impact of the temporal variability of the collisional quenching process on the mesospheric OH layer” by Kowalewski et al uses the SD-WACCM4 chemistry-climate model to investigate processes driving the seasonal and diurnal variability of the mesospheric OH layer peak shifts. Observations of two vibrational transitions of OH from the SABER instrument are used to show that the modelled seasonal / daily variability is qualitatively consistent with observations; so it is valid to use this model to investigate the underlying processes even though they do not agree quantitatively. They find that in contrast to previous investigations based on observations alone, quenching by O2 also contributes to the seasonal variability of the vertical shifts of the OH layer, while the diurnal variability seems to be more driven by changes in the production of OH*, e.g., changes in the source gases H and/or O3 during the night. The paper is well written and the main findings are stated clearly; considering the reviewers comments to the first version of the paper, the paper must have improved considerably. As ground-based observations of different vibrational transitions of the OH layer are used for trend studies of mesospheric temperatures at a certain altitude, any process affecting the peak altitude and peak width of the vibrational transitions of OH is of great interest. In my opinion however, the conclusions can probably be strengthened and established more clearly with a bit more work as discussed in more detail below. The methods used for the analysis, concerning both the model data and observations, are valid in the sense that they present probably the best possible, and in my opinion this justifies using this model and data. However, both model and SABER data have limitations which should be discussed in more detail, some suggestions for improvements of the description of the methods can also be found below. I recommend publication after the points listed below have been addressed, which will probably amount to mostly minor changes.|
Line 19-20: What is meant by “changes of the entire OH emission layer width”? Does that mean that the total emissivity calculated from the product of H and O3 changes, or that the emission layer width of all vibrational transitions change in the same way?
Line 152-153: Whatever Hoffman et al said, nudging does not turn a chemistry climate model into a chemical transport model, “essential” or not. Even in the nudging region the model still calculates its own temperatures and dynamics, the prescribed temperatures than forcing one term of the tendencies. Above the nudging region, the model is essentially free-running with lower boundary conditions determined by the nudging; however, unfortunately that does not necessarily mean that the model represents reality. It is justified to use this kind of model because it is the nearest to reality one can get above the altitude where meteorological analyses are available. However, this misconception has already been mentioned by reviewer #1, and must be corrected here.
Line 167: “we may assume” suggests that the assumption is justified, which you can’t really know; you make this assumption because it is the only possible option apart from redoing the model run with a better temporal sampling. However, you should formulate this more carefully, e.g., “we make the assumption”. However, this point should be discussed as a potential error source when discussing comparison between model results and observations.
Line 188-191: I am not an expert in the IR region, but is the emissivity of a vibrational transition not dependent on temperature there? E.g., OH concentrations should be proportional to the VER profiles only for a fixed temperature? Please discuss, possibly also add an error estimate.
Line 192-195: You state here that the Saber atomic oxygen is derived from the OH radiance, as was discussed by reviewer #2. However, you do not discuss how this might affect the applicability of the SABER O for investigating the dependency of properties of the OH layer on atomic oxygen. As atomic oxygen is derived from OH*, it might be correlated to a property of the OH layer – as the vertical shift – just because of this, and not because this property of the OH layer depends on O in a real physical sense. Using O in this way might lead to a circular argument as pointed out by reviewer #2, and you should discuss this point, and how it might affect an analysis of the SABER data.
Line 214-216: the sentence “Despite the less pronounced vertical …” I’ve read this sentence several times, and still don’t understand it. What it means seems to be “The vertical shifts at the profile peak altitudes are less pronounced, and the temporal changes in the OH profile width should also be less pronounced in this altitudes, but should play a larger role because of part (1).” Why? Maybe you could formulate this as follows: “The temporal changes in the OH profile width are less pronounced in the profile peak altitudes, where vertical shifts are also less pronounced.” Is that what you meant?
Line 219: “… significantly higher precision ….” Significantly higher compared to what?
Line 268-269: you could provide a formula for the weighting.
Lines 342-348: When I looked at panels c) and d) of Figure 4, I wondered whether they would add up to the total peak shift as shown in panel a). I tried to do this by hand and found immediately that they do not add up as might be expected if quenching with O and O2 were the only relevant processes. Can you discuss what is missing? However, I found that probably the sum of panel c) and d) shows a seasonal variability highly correlated to the total peak height as shown in panel a). The correlation appears to be non-linear as the amplitude of the seasonal variation is large by a factor ~4 in panel a) than in panels c) + d), but the structure seems to be represented much better than in the single panels, suggesting that the driving factor of the seasonal variability is indeed a combination of quenching by O and O2. This would strengthen your conclusions quite considerably, I think, and I suggest that you show such a figure and invest some more work in investigating the combined effects of O and O2 quenching.
Figure 5: The seasonal / semi-annual variability in Figure 5 is hard to discern because four years in a row are shown; my first impression was that the agreement with the model results is quantitatively not very good, only by marking every May was I able to see that the agreement is actually qualitatively quite good. I would suggest that your horizontal scale should only cover one year from May to May as for the model results; you can think about than either showing a mean +- standard deviation, or all three years superimposed. I think that would help comparability with the model results.
Line 12: no comma after “question”, actually “whether” would be better than “if”
Line 12 to 14: The sentence starting “Hence, it remains ….” is not necessary.
Line 13: “Accordingly” is also not necessary
Line 14: no comma after “study”
Line 43: (McDade, 1991) McDade (1991)
Line 53: if whether
Line 259: associated “to” the vertical motions …