Impact of topography on black carbon transport to the southern Tibetan Plateau during the pre-monsoon season and its climatic implication

Zhang, Meixin; Zhao, Chun; Cong, Zhiyuan; Du, Qiuyan; Xu, Mingyue; Chen, Yu; Chen, Ming; Li, Rui; Fu, Yunfei; Zhong, Lei; Kang, Shichang; Zhao, Delong; Yang, Yan

doi:https://doi.org/10.5194/acp-20-5923-2020

Articles | Volume 20, issue 10

https://doi.org/10.5194/acp-20-5923-2020

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

Special issue:

Study of ozone, aerosols and radiation over the Tibetan Plateau...

https://doi.org/10.5194/acp-20-5923-2020

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

Articles | Volume 20, issue 10

Research article

|

19 May 2020

Research article |

| 19 May 2020

Impact of topography on black carbon transport to the southern Tibetan Plateau during the pre-monsoon season and its climatic implication

Meixin Zhang, Chun Zhao, Zhiyuan Cong, Qiuyan Du, Mingyue Xu, Yu Chen, Ming Chen, Rui Li, Yunfei Fu, Lei Zhong, Shichang Kang, Delong Zhao, and Yan Yang

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Final revised paper (published on 19 May 2020)
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Preprint (discussion started on 14 Oct 2019)
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AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Comments on "Impact of topography on black carbon transport to the southern Tibetan Plateau during pre-monsoon season and its climatic implication"', Anonymous Referee #1, 31 Oct 2019
RC2: 'Review comment', Anonymous Referee #2, 18 Nov 2019
RC3: 'Impact of topography on black carbon transport to the southern Tibetan Plateau during pre-monsoon season and its climatic implication', Anonymous Referee #3, 30 Nov 2019
AC1: 'Response to review comments of "Impact of topography on black carbon transport to the southern Tibetan Plateau during pre-monsoon season and its climatic implication"', Zhang Meixin, 20 Jan 2020

Peer-review completion

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

AR by Zhang Meixin on behalf of the Authors (21 Jan 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (27 Jan 2020) by Hang Su

RR by Anonymous Referee #1 (04 Feb 2020)

Suggestions for revision or reasons for rejection

1, METHODOLOGY:
i) The authors replaced the 20-km simulation by a 4-km simulation but with ‘smooth’ topography to be compared with. As I argued in the last round of review, the such topography (identically over 5 by 5 grid cells) cannot represent the smooth one of 20-km resolution. Taking the slope between neighboring grid cells as example, the values will be 0, 0, 0, 0, dz/4, which definitely defer from those of 20-km resolution (dz/20). Thus, we cannot say that the such topography is smoothened, but stepped. In fact, it is rather unrealistic.
The authors did not provide any reasonable explanation but argued that some other studies applied such a method. Following others but without a respect to the truth cannot be acceptable in the science.
Moreover, it is not appropriate that the 4-km topography refers to as ‘original’ topography. But this is a minor issue.

ii) The authors intended to investigate the impact of topography (particularly valley) -representation on the BC transport and further the local climate (precipitation) via radiative forcing. However, the precipitation can also influence the BC concentration via wet deposition, which is totally ignored in the MS.

2. RESULTS to CONCLUSIONS:
i), Their results obviously show that the BC transport difference between two simulations is due to the higher BC concentration simulated by the 4-km resolution rather than wind speed (fig 11, 14, s4, s5a,b, s6, s7), although I was confused by their bi-yaxis (do not know which, pressure or height, the plots relate to and whether they made interpolation. Do they mean that pressure level can be certainly converted to height?) of their cross-section plots and wind directions (do they take vertical wind into consideration?).
Again, weaker wind speeds are expected from finer-resolution simulation according to precious study considering that gravity-wave-drag and turbulence-orographic-form-drag to be explicitly resolved. The authors intended to emphasize the more valleys represented by finer-resolution, but they ignored the blocking effect of more and higher mountains. They discussed only one paper (Lin et al. 2018), but there are more studies addressing the weaker near-surface (and lower-model-level) including ones focusing on a broader domain.

ii) The author revised their conclusions as the enhanced BC transport in their finer resolution simulation is due to deeper-valleys plus PBL height and small-scale circulations. I did not find in any part of their results they have investigated the quantified difference in PBL height or circulations between the simulations. It can be agreed that the PBL height can somehow influence the BC transport but this study cannot lead to this conclusion. The case for resolved small-scale circulations is likewise. To diagnose the BC transport, the BC transport and wind are direct (and sufficient) factors. The resolved small-scale circulations may only mean the more realistically-simulated local winds but not stronger wind speeds.

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RR by Anonymous Referee #2 (09 Feb 2020)

ED: Reconsider after major revisions (18 Feb 2020) by Hang Su

AR by Zhang Meixin on behalf of the Authors (02 Mar 2020) Manuscript

ED: Referee Nomination & Report Request started (03 Mar 2020) by Hang Su

RR by Anonymous Referee #1 (03 Mar 2020)

Suggestions for revision or reasons for rejection

Here just come two points according to the revision and the responses:

1) 'Smooth' topography?
The authors argued that their method (applying single value to 5 by r grids) produces a smoothified topography by presenting an example. But I have to say that the example is misleading.

Considering a simple case, 10 grids at 4 km spacing have elevation values of (0, 1, 2, 3, 4), (5, 6, 7, 8, 9) along a horizontal dimension, meaning a constant slope of 1/4. After applying their method, so these grids turn to have values (2, 2, 2, 2, 2), (7, 7, 7, 7, 7). Now, the slopes are 0, 0, 0, 0, 5/4, 0, 0, 0, 0. As such, will you state that the topography is smoothified through their method? On the contrary, in this case a smooth topography becomes unsmooth (as being stepped) after applying their 'smoothing' method.

So, their methodology fails at all.

2) “which come first, chicken or egg?”.
The authors intended to conclude that the wind difference between simulations of different representation of topography leads to BC transport difference and further BC concentration difference. The reviewer found from their results that BC transport difference is rather contributed by the BC concentration difference simulated. Then the authors argued that this is a “which come first, chicken or egg?” question.

Well, the problem is not that the reviewer raised such a question but that the question comes from the non-linear nature. If the authors want to draw the such conclusion, they have to find a way circumventing it, as the magnitude of BC transport is determined by both wind and concentration (or mass). That is, the authors have to strictly prove that BC transport is the only factor affecting the BC concentration tendency so that to make their conclusion ('the wind difference between simulations of different representation of topography leads to BC transport difference and further BC concentration difference') valid.

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ED: Reconsider after major revisions (03 Mar 2020) by Hang Su

AR by Zhang Meixin on behalf of the Authors (04 Mar 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (08 Mar 2020) by Hang Su

RR by Anonymous Referee #4 (05 Apr 2020)

Suggestions for revision or reasons for rejection

The authors have properly addressed previous reviewers’ comments. I only have a few minor comments and suggestions:

1. Lines 64-67: “This study implies that global climate models generally with even coarser resolutions than 20 km and therefore relatively smoother topography may introduce significant negative biases in estimating light-absorbing aerosol radiative forcing over the TP.” I would hesitate to agree with this argument, since there may not be negative biases for global models even with coarser resolution considering different model physics/parameterizations used in global models. The negative bias caused by the topography issue may be canceled out by biases caused by other model processes. If the authors really want to make this argument, they need to be more specific and accurate in stating this to avoid any confusion.

2. One important aspect of the model description that is missing is related to the convective parameterization. Normally, one important advantage for using very high resolution (e.g., < ~4 km) is that WRF-Chem can be run at such resolution without using convective parameterization (commonly known as convection-permitting simulations) to explicitly resolve deep convection. How did the author set up their model simulations regarding this? How is this process coupled with aerosol wet deposition in the model? This is expected to significantly affect aerosol wet scavenging potentially.

3. The authors used WRF-Chem coupled with the SNICAR model to quantify the radiative forcing of absorbing aerosols in snow. Did the authors assume spherical snow grains externally mixed with aerosols in the model? A recent study (He et al., 2018, https://doi.org/10.5194/acp-18-11507-2018) has updated the SNICAR model to include the treatment of nonspherical snow grains and BC-snow internal mixing, which showed significant impacts on aerosol radiative effect in snow over the TP. Did the authors use this updated SNICAR model? I suggest including some clarifications and discussions on this aspect.

4. The model spin-up time is only 2 days, which seems not enough to me even for aerosols like BC with a typical lifetime of about one week. Any justification?

5. I agree that wind change due to topography is one key factor affecting BC transport in the region. However, another important factor that was not discussed/analyzed by the authors is how the cloud and precipitation fields change due to the topographic difference in their model simulations, which could affect aerosol wet deposition and further the amount of BC transported to the TP. This also seems to be a confounding factor for wind analysis. I suggest at least including some discussions on this aspect.

6. In the analysis of wind and transport, the authors mainly focused on the near-surface wind (within PBL) which is directly affected by topography. How about transport at a higher altitude? The topography changes near-surface BC concentrations and probably vertical transport to free troposphere over the source regions and on the way to the Himalayas. The changes in BC vertical profile will also alter BC transport in the free troposphere or even at a higher altitude, which may affect the total BC transport to the TP. Is this factor unimportant compared to near-surface transport? What is the relative importance of the high-altitude transport pathway and the near-surface valley-mountain transport pathway?

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ED: Publish subject to minor revisions (review by editor) (06 Apr 2020) by Hang Su

AR by Zhang Meixin on behalf of the Authors (10 Apr 2020) Author's response Manuscript

ED: Publish subject to technical corrections (19 Apr 2020) by Hang Su

AR by Zhang Meixin on behalf of the Authors (20 Apr 2020) Manuscript

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Short summary

Analysis of multiple numerical experiments over the Himalayas and Tibetan Plateau (TP) shows that the complex topography results in 50 % stronger overall cross-Himalayan transport during the pre-monsoon season primarily due to the strengthened efficiency of near-surface meridional transport towards the TP, enhanced wind speed in some valleys and deeper valley channels associated with larger transported BC mass volume, which leads to 30–50 % stronger BC radiative heating over the TP.