Observed trends in clouds and precipitation (1983–2009): implications for their cause(s)

Zhong, Xiang; Liu, Shaw Chen; Liu, Run; Wang, Xinlu; Mo, Jiajia; Li, Yanzi

doi:https://doi.org/10.5194/acp-21-4899-2021

Articles | Volume 21, issue 6

https://doi.org/10.5194/acp-21-4899-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-21-4899-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 21, issue 6

Research article

|

30 Mar 2021

Research article |

| 30 Mar 2021

Observed trends in clouds and precipitation (1983–2009): implications for their cause(s)

Xiang Zhong, Shaw Chen Liu, Run Liu, Xinlu Wang, Jiajia Mo, and Yanzi Li

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Final revised paper (published on 30 Mar 2021)
Supplement to the final revised paper
Preprint (discussion started on 21 Aug 2020)
Supplement to the preprint

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review of Zhong et al. 2020', Anonymous Referee #3, 27 Aug 2020
- AC2: 'Response to referee 3', Run Liu, 28 Nov 2020
RC2: 'Review of Zhong et al. (2020)', Anonymous Referee #2, 04 Sep 2020
- AC3: 'Response to referee 2', Run Liu, 28 Nov 2020
RC3: 'Review', Anonymous Referee #1, 06 Sep 2020
- AC1: 'Response to referee 1', Run Liu, 28 Nov 2020

Peer-review completion

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

AR by Run Liu on behalf of the Authors (28 Nov 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (12 Dec 2020) by Graham Feingold

RR by Anonymous Referee #2 (14 Dec 2020)

Suggestions for revision or reasons for rejection

I have read through the revised paper, and find that many of the major issues originally raised by the reviewers still largely remain in the manuscript.

1. Reviewer 3 stated that “the authors need to show that the relationships between global temperature and regional variations in cloud cover and precipitation are consistent when linear trends are removed.” The authors have done this analysis in Table S1, and found that this is actually not the case. In fact, they conclude that the “high correlation coefficients are nearly entirely contributed by the long-term linear trends.” As I stated in my original review, just because global temperatures are warming, it doesn’t mean that concurrent trends in clouds and precipitation are necessarily caused by global warming. The similarity in Figs. 1 and 4 is by construction, as the global temperature time series is dominated by an increasing trend (so any trend in clouds and precipitation will by definition be highly correlated with global temperature). While the authors have acknowledged this problem at various points in the manuscript (for example, see paragraph starting on line 325), the large amounts of variance attributed to global warming, AMO, and PDO are still discussed throughout as a key conclusion of the manuscript (line 11). A much more careful analysis (such as that discussed in Chen et al. 2019) needs to be performed to more precisely partition the recent trends into the global warming, PDO, and AMO components.

2. All three reviewers found the suggested linkage of the results with Hadley cell expansion to be inadequately supported and recommended further analyses. To address these comments, the authors have added two figures (Figs. 2-3) to describe a broadening of high cloud cover and precipitation over the deep convective region over the western Pacific Ocean centered on Indonesia. The authors conclusively show that this region has expanded in recent decades, which is an interesting new result. However, the authors attribute this very local feature to a widening of the global Hadley and Walker circulations (line 146), but do not show any evidence to support this claim. For example:
- Figures 2 and 3 combine changes in the zonal and meridional directions, making it impossible to tell which direction is most contributing to the changes.
- No analysis is provided for the eastern tropical Pacific Ocean. To show that the Walker circulation is widening, the authors need to show that the descending branch is not changing. Figure 3 only shows changes in the ascending branch.
- The Hadley cell is a zonal-mean quantity. Changes in the ascending region over the western Pacific Ocean do not necessarily imply changes in the zonal mean.
- To show that the Hadley cell is widening, the authors need to show that the descending branch is moving. Even if the deep tropical precipitation is broadening, it doesn’t necessarily imply that the descending branch is moving.

Because of these two major issues remaining in the manuscript, I cannot recommend publication at this time.

Line 11: “and negligible” --- awkward phrasing … I would remove referring to ENSO in the abstract if it’s negligible.

Line 166: rectangle

Lines 167, 210: I don’t see the close correspondence between Figs. 3e and 3f.

Lines 213-214: Incorrect figure (Fig. 2e) is cited here.

Line 242: AMO loses? … not sure what is meant by this, please rephrase

Line 247: Figure 6

Line 259: Fig. 7a

Line 266: Fig. 7b

Line 270: Fig. 8

Figure 2 caption: Need to clarify that these maps are showing cloud cover trends from Fig. 1.

Statistical significance in Tables 1 and 2: As per my previous comment, the authors shouldn’t be using a sample size of n to calculate the p-value of the correlation coefficient, but rather an effective sample size (n*) that accounts for the autocorrelation in the time series. See equation 31 of Bretherton et al. (1999):
https://doi.org/10.1175/1520-0442(1999)012<1990:TENOSD>2.0.CO;2

Table S2 is not discussed at all in the main text.

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RR by Anonymous Referee #1 (15 Dec 2020)

Suggestions for revision or reasons for rejection

Review of “Observed Trends of Clouds and Precipitation (1983–2009): Implications for Their Cause(s)” by Zhong et al.

I commend the authors for the significant work in revising their manuscript. Overall, it has been substantially improved. However, there still seems to be a bit of disconnect in some of their statements, particularly those related to what is meant by expansion of the Hadley cell.

To summarize, I feel as if the authors are not using correct terminology, as has been previously established in (many) prior publications. Which adds confusion and makes interpretation of the results more difficult than is necessary. I would recommend changing some of the basic terminology used throughout the paper, such that it is consistent with prior publications.

Comments

As mentioned in the first round of reviews, Hadley cell expansion refers to a poleward displacement of the outer edge of the circulation. There are numerous publications that have used this definition. In the context of precipitation, it has been defined as a poleward shift of the subtropical latitude where P-E = 0; the subtropical latitude where P is a minimum could also work. It has also been pointed out that looking at this separately in each hemisphere is important, because the NH and SH show different tropical expansion signals.

The authors choose to construct their own methodology to define “tropical expansion”, based on the rectangular boxes. That’s fine. But they do not really give a simple explanation for what this new methodology is calculating. It appears to me, based on panel a in Figure 3 from the response, that precipitation is not changing near the center of the region (inner rectangles), but it is increasing in the outer regions (outer rectangles). I assume precipitation is estimated in each rectangle independently, and not summed over the inner rectangles? In any case, this would seem to suggest an increase in tropical precipitation starting near layer 6 and extending outwards to layer 15. In other words, tropical precipitation in this region is moving outwards from the center rectangle? Or maybe a better description is that there is just an increase in tropical precipitation moving outwards from the local maximum (assuming the inner rectangle is a local maximum)? This is not “tropical expansion” as used in numerous other publications. To add confusion, the authors also describe this as not only Hadley cell expansion, but also expansion of the Walker circulation.

The center of the rectangles is “in the middle of Kalimantan, Indonesia which is located near the major ascending/wet zone of Hadley cell”. What is meant by “near” the major wet zone? Is the center rectangle chosen so that it represents a local maximum in precipitation?

The authors then go on to say (again, in the response): “In summary, the spatial distributions of the linear trends of total cloud cover and precipitation are characterized primarily by a widening of the center of precipitation (ascending/wet zone of Hadley cells) over the Maritime Continent in all directions”.

Yes, I agree with this description. But this is not what is meant by “tropical expansion”. This is more related to the thickness of the band of intense tropical rain, right? And the authors are showing an increase in this “thickness”? Perhaps this is not related to “tropical expansion”. But it does seem to be interesting, as others have shown that under continued GHG increases, the ITCZ is projected to narrow (or decrease its “thickness”). But the authors are showing the opposite. It would appear that the authors should remove “tropical expansion” type statements and verbiage, and instead replace this with something more similar to what they are quantifying—“thickness” or area, etc. of the intense precipitation over the Maritime continent.

I think the last reviewer summarized this concern well:

Line 69, 131-132, 138-140: See major comment #4. The expansion of the Hadley cells has nothing to do with enhancement of tropical precipitation. It is related to subtropical static stability (Chemke and Polvani 2019: https://doi.org/10.1175/JCLI-D-18-0330.1). If anything, an expansion of tropical precipitation would contradict the literature, which suggests a narrowing of the Intertropical Convergence Zone in a warming climate (Byrne and Schneider 2016: https://doi.org/10.1002/2016GL070396; Su et al. 2017: https://www.nature.com/articles/ncomms15771).

We understand this is a controversial point. Please see our response to your major comment #4.

But as can be seen, the authors have not really addressed this confusion. They continue to refer to their signal as “tropical expansion” (as well as expansion of the Walker circulation). Furthermore, their response again seems to be disconnected from the comment. The comment is pointing out that the authors are not using the term “tropical expansion” properly. And instead of changing it, they simply say that this is controversial? I don’t quite follow.

Regarding this response:

Direct effects of anthropogenic aerosols on clouds and precipitation tend to be regional and/or sub- yearly time scale, which are beyond the scope of discussion in this study

Obviously, this is a small point and not particularly important to the main analysis. But again, there seems to be a disconnect in the response. Why are possible aerosol effects on cloud/precipitation important on only sub-yearly time scales? I do not think this is true. There have been multi-decadal changes in anthropogenic aerosol emissions, which leads to multi-decadal changes in aerosol forcing. So it would stand to reason that such a multi-decadal forcing may in fact lead to long term changes in temperature, clouds, precipitation, etc.

It’s a bit odd (and frustrating) that I’ve pointed out past papers that have addressed causes of tropical expansion (e.g., aerosols, as well as the PDO), and the authors have chosen to disregard any acknowledgement of these prior papers. Why not add a simple sentence in the introduction that points out prior papers that have addressed the causes of tropical expansion? But then again, I do not think what the authors show is really “tropical expansion”, and therefore, the causes of their signal may very well have nothing to do with previously identified causes of tropical expansion. So in a round-about way, lack of addressing this point fine, I suppose.

The authors again use this terminology:

Further analysis of the widening of the Hadley and Walker circulations (Figures 3a-3h)
What is meant by widening of the Walker circulation?

The authors also use this description of their signal, which is more reasonable in my opinion: “widening of the center of precipitation over the Maritime Continent in all directions”. And maybe “broadening” in more appropriate than “widening”, as widening implies an increase in one spatial direction, whereas broadening is more general.

Another example of a disconnect between reviewer comment and author response pertains to significance:

Table 1: How are you evaluating significance? I have a difficult time believing that a correlation of 0.02 is still significant at the 95% confidence level. Are you taking into account autocorrelations among neighboring grid points, which would greatly reduce the number of degrees of freedom in your t-test? Table 2: Similarly, how is significance being evaluated here? A trend of 0% (see T60%) should not be statistically significant at all, especially at the 99% level.

We used the function imbedded in R named corr to do this significance test. The function corr we chose applies Pearson correlation formula…The p-value of the correlation is determined by calculating the t value as follow…then using t distribution table for the degrees of freedom: df = n-2 to get the p-value. We believe even when the correlation coefficient r is very small, due to the big value of n (the number of samples we used in calculation), the t value should remain a very big value, therefore brings a reliable significance.

Yes, the small correlations (e.g., 0.02) are deemed significant in this analysis likely due to the large n. But this comment is suggesting n is not as large as what the authors are using, due to spatial autocorrelation. But the authors do not address this point.

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ED: Reconsider after major revisions (24 Dec 2020) by Graham Feingold

AR by Run Liu on behalf of the Authors (01 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (16 Feb 2021) by Graham Feingold

AR by Run Liu on behalf of the Authors (18 Feb 2021) Author's response Author's tracked changes Manuscript

ED: Publish as is (22 Feb 2021) by Graham Feingold

AR by Run Liu on behalf of the Authors (23 Feb 2021) Manuscript

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Short summary

The distributions of linear trends in total cloud cover and precipitation in 1983–2009 are both characterized by a broadening of the major ascending zone of Hadley circulation around the Maritime Continent. The broadening is driven primarily by the moisture–convection–latent-heat feedback cycle under global warming conditions. Contribution by other climate oscillations is secondary. The reduction of total cloud cover in China in 1957–2005 is driven by the same mechanism.