Forcing mechanisms of the terdiurnal tide

Lilienthal, Friederike; Jacobi, Christoph; Geißler, Christoph

doi:https://doi.org/10.5194/acp-18-15725-2018

Articles | Volume 18, issue 21

https://doi.org/10.5194/acp-18-15725-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-18-15725-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 21

Research article

|

02 Nov 2018

Research article |

| 02 Nov 2018

Forcing mechanisms of the terdiurnal tide

Friederike Lilienthal, Christoph Jacobi, and Christoph Geißler

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Final revised paper (published on 02 Nov 2018)
Preprint (discussion started on 02 May 2018)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'reviewer comment', Anonymous Referee #1, 16 May 2018
- AC1: 'Answer to RC#1', Friederike Lilienthal, 30 May 2018
RC2: 'Review of the Lilienthal et al paper. ACPD 2018-154', Anonymous Referee #2, 25 Jul 2018
- AC2: 'Answer to RC #2', Friederike Lilienthal, 25 Jul 2018

Peer-review completion

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

AR by Friederike Lilienthal on behalf of the Authors (21 Aug 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (23 Aug 2018) by Franz-Josef Lübken

RR by Anonymous Referee #2 (01 Oct 2018)

Suggestions for revision or reasons for rejection

The authors have improved their paper significantly. The explanation of their method and analysis is much clearer and in much of the paper the writing is better.

I still have some comments and suggestions which I think the authors should address before the paper is accepted in ACP.

I have looked carefully at the Du and Ward (2010) paper and at the end of their Section 4.2 and in their conclusions they state that based on their correlation analysis that "non-linear interactions are unlikely to be the source of the terdiurnal tide in their model" and suggest further study (a Hough mode analysis is suggested) is needed to investigate this possibility further. They also note that correlations exist for longer time scales. I don't think that their analysis precludes non-linear interactions being present but instead introduces another facet of terdiurnal tides which needs to be understood.

In the paper under review, the authors examine the forcing for steady state conditions (i.e. without the variability inherent in a full GCM - this situation is similar to the longer time-scale situation discussed by Du and Ward (2010)), demonstrate that there are regions where non-linear forcing terms are non-zero and show using suitably selected runs that the tidal amplitudes vary in a way that is consistent with non-linear interactions playing a role in the terdiurnal tidal amplitudes. They thereby forward our understanding of aspects of this tide. However, the authors do not directly show that the forcing terms generate propagating terdiurnal tides as opposed to simply resulting in a local forcing nor do they distinguish whether the modes which might be forced non-linearly are the same as those which are thermally forced. It seems that the Hough mode analysis suggested by Du and Ward could contribute to clarifying what is going on with MUAM also (I am not suggesting that this be attempted as a revision to this paper). I do suggest that authors, provide more nuanced conclusions (page 13) to their paper, point to their contribution to studying this question and note that the issue of non-linear interactions for the terdiurnal tide is still an open question which warrants further study.

The new discussion on the criteria (page 3) associated with the generation of terdiurnal tides is not complete. Since this tide is a global mode with specific parameters associated with each mode, an additional criteria is the extent to which the forcing conditions match the parameters associated with the tidal mode. If there is a significant mis-match then there will be local forcing but no propagating modes will be excited. This aspect of the forcing should be included as part of this discussion.

I am still curious as to how the gravity wave scheme in the model is implemented (page 3) and suggest a little more explanation is still needed. The authors state that the two schemes are separated in terms of the wavenumber range that each is associated with and suggest that this results in each scheme affecting different height ranges. However, won't the wind filtering in the stratosphere and mesosphere affect phase speeds outside the +/- 35 m/s boundary between the two schemes so that both are active throughout the full altitude range? Please comment on this

The exp(-z/2H) weighting used in this paper is not a "density weighting" but a term associated with conservation of wave energy which normalizes the wave growth with height due to the decrease in density. I think this is an appropriate way to compare different forcings but suggest you use a different term to describe it.

Pg 5, Line 13: I apologize for the typo in my last review. The species which recombine are O and O2 to produce O3 as the authors show in the reactions that they list in their response. O3 is the resulting species not the molecule which recombines. The text should be changed to "recombination of O and O2".

Pg 8, line 1: "subharmonics" should be changed to "harmonics". Subharmonics are modes with frequencies lower than the parent waves. Harmonics are modes with frequencies higher than the parent waves.

Pg 8, line 4-5: I don't understand why the scaled amplitudes are omitted from the figures since they are of a higher order of magnitude. If they are of a higher order of magnitude, doesn't that mean that the interaction is more significant? Please comment.

I have a number of suggestions for improvements to the wording which I will forward separately as comments on the revised manuscript.

Referee Report: PDF

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ED: Publish subject to minor revisions (review by editor) (10 Oct 2018) by Franz-Josef Lübken

AR by Friederike Lilienthal on behalf of the Authors (22 Oct 2018) Author's response Manuscript

ED: Publish as is (22 Oct 2018) by Franz-Josef Lübken

AR by Friederike Lilienthal on behalf of the Authors (23 Oct 2018) Manuscript

Short summary

The terdiurnal solar tide is an atmospheric wave, owing to the daily variation of solar heating with a period of 8 h. Here, we present model simulations of this tide and investigate the relative importance of possible forcing mechanisms because they are still under debate. These are, besides direct solar heating, nonlinear interactions between other tides and gravity wave–tide interactions. As a result, solar heating is most important and nonlinear effects partly counteract this forcing.