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.
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
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
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.
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'.