Role of vertical and horizontal mixing in the tape recorder signal near the tropical tropopause

Glanville, Anne A.; Birner, Thomas

doi:https://doi.org/10.5194/acp-17-4337-2017

Articles | Volume 17, issue 6

https://doi.org/10.5194/acp-17-4337-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-17-4337-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 17, issue 6

Research article

|

30 Mar 2017

Research article |

| 30 Mar 2017

Role of vertical and horizontal mixing in the tape recorder signal near the tropical tropopause

Anne A. Glanville and Thomas Birner

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Final revised paper (published on 30 Mar 2017)
Supplement to the final revised paper
Preprint (discussion started on 09 May 2016)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'anonymous review', Anonymous Referee #1, 31 May 2016
- AC1: 'response to criticism raised by reviewer 1', Thomas Birner, 08 Jun 2016
  - RC3: 'response to author comments', Anonymous Referee #1, 21 Jun 2016
    - RC4: 'comment to authors and reviewer 1', Anonymous Referee #2, 01 Jul 2016
    - AC2: 'response to reviewer 1, 2nd round', Thomas Birner, 03 Aug 2016
RC2: 'Review of Glanville and Birner, "Role of vertical and horizontal mixing..."', Anonymous Referee #2, 09 Jun 2016
- AC3: 'response to reviewer 2', Thomas Birner, 03 Aug 2016

Peer-review completion

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

AR by Thomas Birner on behalf of the Authors (02 Aug 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Aug 2016) by Rolf Müller

RR by Anonymous Referee #1 (26 Aug 2016)

Suggestions for revision or reasons for rejection

Some comments regarding my reasons for rejecting this paper:
My previous review and comments on this paper have focused on the lack of including dehydration in the idealized 1D model calculations, which may lead to spurious derived vertical diffusion near the cold point tropopause. The authors have addressed this criticism by including calculations with idealized dehydration during winter, but the strongest dehydration is specified at 100 hpa, not near the cold point (the minimum in observed water vapor during boreal winter is near 83 hPa in both MLS and HALOE retrievals). In the end the authors discount the importance of explicit dehydration from these tests because it “…does not improve the overall simulation throughout the year…and increases the dry bias during the summer”. However, the summer results (at 80 hPa) seem unphysical to begin with (in my opinion – see below), so this is a poor argument for not including dehydration (which is the fundamental cause of the tape recorder to begin with).

While intuitively I don’t like the neglect of an important term in the idealized model, it is the detailed output of the model that is problematic in my opinion. Fig. 7 shows vertical mixing maximizing with identical timing to vertical advection, with extrema during Nov-Dec and Aug-Oct (which they term ‘summer’). This timing seems unphysical to me; why should vertical mixing be tied to mean vertical advection? And what is the physical mechanism for these seasonal maxima in derived mixing? Also, the diagnosed horizontal advection is relatively small and peaks during boreal spring (~May), very different from large boreal summer transport expected from the Asian summer monsoon (e.g. Ploeger et al 2012 explicitly calculate a 400 K maximum for water vapor horizontal mixing during August-October). Because of the fundamental problems in these important details, I am not convinced of the key result of large diagnosed vertical mixing from this analysis. As a note, it would have been helpful to have some statistical uncertainty estimates included with the results, although I did not recommend this in my previous review.

The authors argue that the lack of strong vertical mixing derived using isentropic coordinates is physically consistent with the pressure coordinate calculations. However, I am not familiar with the quoted statement that “vertical mixing due to breaking gravity waves may be assumed to take place quasi-adiabatically”, and a reference is not provided. The derived horizontal mixing for the isentropic case (Fig. 12) still peaks in the wrong season compared to previous work. Overall, the isentropic-coordinate calculations do not convince me that the pressure-coordinate calculations are correct (the p-coordinate calculations are more straightforward, and I believe they are not correct based on my comments above).

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RR by Anonymous Referee #2 (07 Sep 2016)

RR by Timothy J. Dunkerton (25 Oct 2016)

Suggestions for revision or reasons for rejection

This paper extends earlier analysis of the tape recorder signal in water vapor by including the seasonal cycle and focusing primarily on the lowermost part of the signal near 80 hPa. If I were interested in further research on this topic I would certainly want to consult this paper to build on the authors’ results. I regard their findings as a partial improvement, perhaps half-way to where we need to go to understand fully the implications of water vapor behavior for mean vertical motion, vertical diffusion, and lateral in-mixing. The MLS data are inferior to HALOE for this purpose, the model is, well, just a model, and ERA-I is excessively diffusive, perhaps worthless of this purpose. I would probably rely on extending the HALOE analysis (Dunkerton, 2001 JAS) with the precision (for lowermost levels) illustrated by the authors.

Two issues are overlooked that otherwise limit my appreciation for this work. First, it is now well established that the moist part of the tape recorder is driven by vertical, then lateral, transport of water vapor injected by overshooting convection in the Asian summer monsoon (Gettelman et al.,, 2004 JGR, et seq.). It is surprising if, in fact, no one has repeated Mote et al. with time-varying side boundary conditions to mimic this effect. Second, although the authors seem to casually ascribe vertical diffusion to breaking gravity waves, it is well-known that such waves (if undergoing local convective instability in their phase of overturning) are not effective in mixing heat and constituents vertically (Coy et al., 1988 JAS). Inertia-gravity waves may undergo shear instability at large amplitude, altering this result possibly in a significant way (Dunkerton, 1985 JAS, et seq.).

Meanwhile, what is the role of overshooting convection in penetrating local theta surfaces? The main conclusion emerging from our work and subsequent studies of (de)hyrdration by overshooting convection is that ambient relative humidity determines the outcome: if high, leading to freeze-drying, if low, leading to hydration. While their latest additions to the text address dehydration, nothing is said to address hydration. In summary, while there are many reasons to publish this work, it will be a stepping stone, not close to any final answer, on a most intriguing problem. Publication is recommended.

Minor suggestion: be sure to emphasize that only the lowermost part of the signal is analyzed. Whether your analysis benefits higher levels, with respect to annual & QBO influences on parameters, remains to be determined. I did some forward modeling prior to Mote et al. to ensure that vertical diffusivity must decrease rapidly with height above 80 hPa. Indeed, it must, otherwise the signal is too wide and decays much too fast. Frankly, if you have HALOE results not shown, I would add them prior to publication.

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ED: Reconsider after major revisions (28 Oct 2016) by Rolf Müller

AR by Thomas Birner on behalf of the Authors (22 Dec 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (05 Jan 2017) by Rolf Müller

RR by Anonymous Referee #1 (12 Jan 2017)

RR by Anonymous Referee #2 (12 Jan 2017)

ED: Reconsider after minor revisions (Editor review) (28 Feb 2017) by Rolf Müller

AR by Thomas Birner on behalf of the Authors (05 Mar 2017) Author's response Manuscript

ED: Publish as is (06 Mar 2017) by Rolf Müller

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Short summary

Nearly all air enters the stratosphere through the tropical tropopause, which exerts a control on stratospheric chemistry and climate. By combining satellite observations with a simple 1-D transport model, we study the roles of vertical and horizontal mixing in transport near the tropical tropopause. We find that vertical mixing may play a more significant role than previously assumed, which is potentially as important as slow vertical transport by the residual mass circulation.