The role of heat-flux–temperature covariance  in the evolution of weather systems

Marcheggiani, Andrea; Ambaum, Maarten H. P.

doi:https://doi.org/10.5194/wcd-1-701-2020

Articles | Volume 1, issue 2

https://doi.org/10.5194/wcd-1-701-2020

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

https://doi.org/10.5194/wcd-1-701-2020

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

Articles | Volume 1, issue 2

Research article

|

10 Nov 2020

Research article |

| 10 Nov 2020

The role of heat-flux–temperature covariance in the evolution of weather systems

Andrea Marcheggiani and Maarten H. P. Ambaum

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Final revised paper (published on 10 Nov 2020)
Preprint (discussion started on 27 May 2020)

Interactive discussion

Status: closed

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

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RC1: 'Referee Report', Anonymous Referee #1, 29 Jun 2020
RC2: 'Referee Comment', Anonymous Referee #2, 14 Jul 2020
AC1: 'Authors response to reviewers', Andrea Marcheggiani, 15 Aug 2020

Peer-review completion

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

AR by Andrea Marcheggiani on behalf of the Authors (13 Sep 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (25 Sep 2020) by Sebastian Schemm

RR by Anonymous Referee #2 (12 Oct 2020)

For final publication, the manuscript should be

accepted as is

accepted subject to technical corrections

accepted subject to minor revisions

reconsidered after major revisions

I would be willing to review the revised paper, if the editor considers it necessary.

I am not willing to review the revised paper.

rejected

Suggestions for revision or reasons for rejection

I appreciate the authors' effort in substantially revising this manuscript. In my opinion, the discussions have improved. Nevertheless, there are still some issues that the authors should address before the paper can be accepted.

1) My main remaining issue is the authors' contention that the air-sea fluxes act to decrease baroclinicity (lines 189-200). I pointed out in my previous review that the fact that baroclinicity decreases while the FT increases does not imply the flux causes decrease in baroclinicity, as the synoptic eddy is growing when FT increases, and the synoptic eddy growth could be the cause of the decrease in baroclinicity. In the response and in the revised manuscript, the authors acknowledged that they can't separate the effect of the heat flux from the effect of the eddy growth in reducing baroclinicity. Nevertheless, they still contended that since the air-sea flux damps the temperature variance, this provide strong evidence for the eddy flux damping baroclinicity. I don't really follow this point how the air-sea flux damping eddy amplitude would imply it would erode the MEAN temperature gradient. This needs to be better explained. This also relates to the discussions in the paragraph on lines 210-213. I can see how the air-sea flux is important in the budget of eddy APE, but I can't really see any "strong" evidence (apart from Fig. 5, which I have argued that this does not show causality) that these fluxes deplete the mean baroclinicity.

Other comments:
2) Lines 32-33: Note that as I pointed out in my previous review, Chang et al (2002) showed that total diabatic generation of transient EPE is largely negative except over parts of the Pacific, with the damping effect of sensible heating largely cancelling the positive generating due to latent heating. In fact, Chang and Zurita-Gotor (2007, JAS 64, 2309, see their table 1) have shown that over the northern hemisphere mid latitudes, and especially in the Atlantic, diabatic heating damps transient eddies. These results do not contradict those cited in this study (Li et al, 2007; Marques et al., 2009), because in Lorenz's definition of EAPE, eddies are defined as deviations from the zonal mean, and includes stationary eddies. In winter, net diabatic heating is positive over the warm oceans, and negative over the cold continents (e.g. Held et al., 2002, J. Climate, 15, 2125, Fig. 8), leading to generation of EAPE but mainly for stationary eddies and not necessarily for transient eddies that form the storms. My point is that there exists previous studies that showed that diabatic heating acts to damp baroclinic waves instead of being a source of energy in storm development (lines 33, 337-338).

3) Equation 1. Here the authors should discuss why they used 850 hPa T' rather than 2m T'. This is discussed below (starting line 148) but in my opinion should be discussed here instead.

4) Lines 112-114: I still don't understand how the effects of ocean eddies on F at the resolved scales would be capture by the reanalysis system. As far as I know, F is model generated, hence if the model does not resolve the ocean eddies in the SST, how can F contain the effects of that? You could argue that the effects of the ocean eddies on the resolved scale temperature structure may be captured since temperature is an observed input that is assimilated, but I am not convinced that the effects of ocean eddies on F can be captured by the model. Either remove this discussion or explain more clearly.

5) Line 127: The heat flux anomalies seems to be skewed towards positive values to me?

6) Lines 155-156: Perhaps the near surface temperature anomalies are more strongly damped by the underlying SST may also contribute?

7) Line 162: "derives from a gain/loss of signal due to averaging" - perhaps add something like "partially"? Not clear to me that all of the signal is due to that as the authors also argued below.

8) Lines 174-175: I don't quite understand the phrase "it is reasonable to interpret any positive instances or moderately negative values as indicative of a relatively weak heat exchange" means.

9) Fig. 3 shows that the covariance between F and T is positive along the Canadian coast. Can the authors explain that?

10) Line 182: phases should be phase?

11) Lines 189-190: The Eady growth rate maximum should probably be defined?

12) Line 196: "to reduce" should be changed to "to decrease"

13) Line 254: "consistently" should be "consistent"?

14) Line 317: As discussed in item 1), I'm not convinced that this is the case.

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RR by Anonymous Referee #1 (16 Oct 2020)

For final publication, the manuscript should be

accepted as is

accepted subject to technical corrections

accepted subject to minor revisions

reconsidered after major revisions

I would be willing to review the revised paper, if the editor considers it necessary.

I am not willing to review the revised paper.

rejected

Suggestions for revision or reasons for rejection

The authors have responded to my comments and added relevant figures and discussion. I have still some minor comments before the paper to be published.

My recommendation is Minor revision

1) Lagged composites (as discussed on lines 157-166) showing F' and T' (or other quantities) might be insightful to better understand the counter-intuitive results of your study. Maybe it would be useful to include them.

2) Discussion lines 189-200 and related Figure 5.
It seems to me, by looking at the contours and the arrows, that when the FT index is strengthening from -100 to -400Wm-2K, baroclinicity stays the same, around 0.6day-1, contrary to what you state line 194.
The change of baroclinicity would depend on the initial value of it?
My problem may be due that you change the figure with the revision.

3) You should check the paper of Dacre et al. (in WCD, 2020) who examined cyclone-relative heat fluxes. This may be relevant to your study.

4) Line 201. You discuss the findings of Hotta and Nakamura, without saying what they are.

5) Line 291. Do you refer to surface 10m or 850hPa winds?

6) The first sentence of the introduction is awkward:
... **storms tracks** are longitudinally localized, with the main regions of intense **storm track** activity ...

Again, Line 81 -> Given that **storm tracks** are by definition the main reservoirs of eddy potential energy...

I am confused. Do you refer to the same entity? i.e. paths of individual storm and regions where the eddy kinetic energy or potential energy is high? You should be consistent throughout the paper: storm tracks should be defined from the start.

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ED: Publish subject to minor revisions (review by editor) (18 Oct 2020) by Sebastian Schemm

AR by Andrea Marcheggiani on behalf of the Authors (23 Oct 2020) Author's response Manuscript

ED: Publish as is (04 Nov 2020) by Sebastian Schemm

AR by Andrea Marcheggiani on behalf of the Authors (04 Nov 2020)

Short summary

In this paper, we investigate air–sea interaction by looking at the relationship between spatial variability in surface heat flux and air temperature. We observe that their interaction characterises different stages of storm evolution, thus providing a new perspective on the role played by air–sea heat exchange.