Modelled effects of temperature gradients and waves on the hydroxyl rotational distribution in ground-based airglow measurements

Franzen, Christoph; Espy, Patrick Joseph; Hibbins, Robert Edward

doi:https://doi.org/10.5194/acp-20-333-2020

Articles | Volume 20, issue 1

https://doi.org/10.5194/acp-20-333-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-20-333-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 20, issue 1

Research article

|

09 Jan 2020

Research article |

| 09 Jan 2020

Modelled effects of temperature gradients and waves on the hydroxyl rotational distribution in ground-based airglow measurements

Christoph Franzen, Patrick Joseph Espy, and Robert Edward Hibbins

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Final revised paper (published on 09 Jan 2020)
Preprint (discussion started on 03 Apr 2019)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of acp-2019-318', Anonymous Referee #1, 18 Apr 2019
RC2: 'Review of acp-2019-318', Anonymous Referee #2, 22 Apr 2019
AC1: 'Authors comment to Anonymous Referee #1', Christoph Franzen, 27 Aug 2019
AC2: 'Authors comment to Anonymous Referee #2', Christoph Franzen, 27 Aug 2019

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Christoph Franzen on behalf of the Authors (27 Aug 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (09 Sep 2019) by William Ward

RR by Anonymous Referee #2 (17 Sep 2019)

Suggestions for revision or reasons for rejection

General comments:

The revised manuscript "Modelled Effects of Temperature Gradients and Waves on
the Hydroxyl Rotational Distribution in Ground-Based Airglow Measurements" by
Christoph Franzen et al. shows significant improvements compared to the first
version. I appreciate these efforts. Nevertheless, several minor issues are
still present in the paper. In some cases, some information in the response
letter should also be added to the paper as this would also be helpful for the
general reader. In some cases, my comments might have not been sufficiently
clear and things can be overlooked. Before accepting the paper for
publication, the remaning issues should be handled.

Specific comments:

P.2,L.8: The term "radiative effects" is not easy to understand as it is very
general. For the reader, it would be helpful to add some words to explain the
specific impact on OH. For example, one could write "radiative effects (i.e.
the emission of airglow photons) on the roto-vibrational level population
distribution" or something similar.

P.3,L.2: "rotational level distribution" should be changed into "rotational
level population distribution". This is my fault as I forgot to add
"population" to the corresponding comment in the first review. In this
context, note that "rotational population distribution" (P.3,L.15) seems to be
another version with the same meaning.

P.3,L.2-6: Indeed, the sentence "Any instrument that integrates ..." will
still be true if non-LTE effects are considered. The problems are related to
the subsequent sentence "That is, the emission ...". Here, it is not clear
whether non-LTE effects are considered, although full thermalisation is needed
for the statement (as the OH non-LTE effects tend to increase with altitude
irrespective of the kinetic temperature gradient). The phrase "even if
thermalised at each altitude" (P.3,L.2) does not imply that the subsequent
sentences need this precondition. Moreover, it would still be prudent to
change "that occurs" to "that preferentially occurs" or something similar.
This sentence does not focus on extreme temperature gradients. In general, the
temperature difference between "cold" and "warm" regions should not be
sufficient to completely separate the rotational level population
distributions for both regions. Note that a similar statement in Sect. 2.3 was
corrected in a satisfying way.

P.4,L.1: The title "The OH model" is confusing as it suggests that the full
model including the rotational level population distribution is described.
However, the latter is somehow hidden in the section "Simulation of a
ground-based measurement" (Sect. 2.3). Sect. 2.1 is only focussing on the
height-dependent vibrational level population distribution, which is necessary
for the v'-specific VER profiles. It would be helpful if this was better
communicated in Sect. 2.1 by changing the title, giving a brief information on
the purpose of this section, and providing a reference to Sect. 2.3 if the
structure of Sect. 2 is not changed.

P.4,L.13: A brief note on the reasons for the neglection of N_2 as collision
partner would still be useful. This information should not only be provided to
the reviewers.

P.5,L.20: Similar to the previous comment, some information on the choice of
the Einstein coefficients in the response letter could also be added to the
paper. It could be said that the selection of Mies (1974) does not matter for
the purpose of this study.

P.6,L.8-9, P.7,L.9, P.8,L.3-4, P.8,L.12: In Sect. 2.4, it is not clear which
P-branch lines are considered. There is a note on the only use of lines
related to F = 1. However, Fig. 3 shows P_1- and P_2-branch lines, which are
called "the P-branch" in the figure caption. On the other hand, "P-branch" is
also stated in the caption of Fig. 4, but there it only refers to P_1-branch
lines. Hence, in Sect.2.4, it should be made clear again which lines are
discussed.

P.11,L.17-19: "above about 20 km" is ambiguous as it is not clear where the
upper altitude limit is. This is an issue as the statements in the sentence
become partly wrong at altitudes of about 30 km depending on the amplitude.

Technical corrections:

P.6,L.12 and Fig.4: The Boltzmann constant "k" in Eq. (7) is not introduced.
Moreover, the abscissa of Fig. 4 shows "k_B", which is inconsistent with Eqs.
(7), (8), (9), and (10).

P.7,L.14, P.8,L.1, P.9,L.1: The use of N_V' in Eqs. (8), (9), and (10) still
looks wrong. It is not introduced in the paper. However, the entire text
(including the sentences directly related to the equations) discusses N_v',
which is defined in Sect. 2.1 (P.4,L.19-20). Moreover, N_v' is also used in
Eq. (7), which is linked to the equations with N_V'.

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RR by Anonymous Referee #1 (19 Sep 2019)

Suggestions for revision or reasons for rejection

General comments:

Overall the authors did a good job replying to the comments in my earlier review. In my opinion the manuscript is close to being acceptable for publication. I have some more minor comments and ask the authors to consider them.

Specific comments:

Page 1, line 21: The Meinel papers are probably not the best reference for mesospheric remote sensing using the OH emissions.

Page 1, line 31: Add space in “).Low”

Page 2, line 6: “these two conditions”

It’s not clear or obvious what “these two conditions” refers to. I suggest mentioning this explicitly.

Page 2, line 12: Add space in “).Here”

Page 2, line 15: “..”

Page2, line 20: “..”

Page 3, line 15: suggest to replace “i.e. a single Boltzmann distribution” by “i.e. can be described by a single Boltzmann distribution“

Page 3, line 16: “..“

Page 4, line 20: “its lifetime .. are calculated“ -> “its lifetime .. is calculated“

Page 5, line 9: “the dry adiabatic lapse rate of -10 K/km”

The lapse rate is defined as -dT/dz, not dT/dz. I suggest sticking to this convention, otherwise your use of "exceed" above (line 1 of this page) is not correct. The figure doesn't have to be changed, but the correct sign should be used when using the term "lapse rate".

Page 6, equation (6) and line below: The Einstein coefficent in the line below eqn. (6) does not have the vibrational quantum numbers as indices. I’m also wondering, whether the equation is correct. If the Einstein coefficient also „includes“ the vibrational transition, then the 1/tau term should not be required.

Page 6, line 19: Shouldn’t the intensity I also include indices for the vibrational states?

Page 7, upper right panel: The spectrum is shown for a temperature of T = 120 K. But T is more like 140 K than 120 K at this altitude, looking at the left panel of the Figure.

Page 11, line 25: “can cause up to 30 % apparent excess populations“

This understanding of “apparent excess population” differs from your definition of this term. You defined apparent excess population as the ratio … (bottom of page 9), i.e. the above sentence should read “up to 130% excess population”, which – of course – sounds strange. I find your definition of “apparent excess population” not very intuitive, because an excess population of 1 or 100% does not actually correspond to an enhanced population (compared to LTE conditions). In this sense the use of “excess population” is misleading. You did define the term above, but I recommend considering changing the meaning of the term or the terminology. This affects many sentences of the manuscript. I leave it up to you to decide.

Page 12, line 12: “Fig. 6 and 7 show the results for the phase which created the strongest apparent excess population.”

This means, it’s a different phase for the each of the different combinations of wavelength and amplitude, right? Perhaps this can be mentioned explicitly.

Page 13, line 1 – 5: Perhaps you would like to mention the reason for the described behavior? The reason is that the internuclear distance is larger for higher vibrational levels, i.e. the molecule's moment of intertia is larger and therefore the value of the rotational constant is smaller, hence the spacing between the rotational levels.

Page 14, line 6: “can explain between 12 % and 30 %”

I suggest mentioning what cases/emissions these values refer to.

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ED: Publish subject to minor revisions (review by editor) (18 Oct 2019) by William Ward

AR by Christoph Franzen on behalf of the Authors (04 Nov 2019) Author's response Manuscript

ED: Publish as is (18 Nov 2019) by William Ward

AR by Christoph Franzen on behalf of the Authors (25 Nov 2019) Manuscript

Short summary

Ground-based observations of the hydroxyl (OH) airglow have indicated that the rotational energy levels may not be in thermal equilibrium with the surrounding gas. Here we use simulations of the OH airglow to show that temperature changes across the extended airglow layer, either climatological or those temporarily caused by atmospheric waves, can mimic this effect for thermalized OH. Thus, these must be considered in order to quantify the non-thermal nature of the OH airglow.