Feedbacks of dust and boundary layer meteorology during a dust storm in the eastern Mediterranean

Rémy, S.; Benedetti, A.; Bozzo, A.; Haiden, T.; Jones, L.; Razinger, M.; Flemming, J.; Engelen, R. J.; Peuch, V. H.; Thepaut, J. N.

doi:https://doi.org/10.5194/acp-15-12909-2015

Articles | Volume 15, issue 22

https://doi.org/10.5194/acp-15-12909-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

https://doi.org/10.5194/acp-15-12909-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 15, issue 22

Research article

|

20 Nov 2015

Research article |

| 20 Nov 2015

Feedbacks of dust and boundary layer meteorology during a dust storm in the eastern Mediterranean

S. Rémy, A. Benedetti, A. Bozzo, T. Haiden, L. Jones, M. Razinger, J. Flemming, R. J. Engelen, V. H. Peuch, and J. N. Thepaut

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Final revised paper (published on 20 Nov 2015)
Preprint (discussion started on 13 Nov 2014)

Interactive discussion

Status: closed

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

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RC C11568: 'Technical corrections for MS entitled: ''Positive feedback of dust aerosol via its impact on atmospheric stability during dust storms in the Eastern Mediterranean''', Anonymous Referee #1, 26 Jan 2015
RC C11570: 'Full review of the paper entitled ''Positive feedback of dust aerosol via its impact on atmospheric stability during dust storms in the Eastern Mediterranean''', Anonymous Referee #1, 26 Jan 2015
RC C11800: 'Review: Positive feedback of dust aerosol via its impact on atmospheric stability during dust storms in the Eastern Mediterranean', Slobodan Nickovic, 02 Feb 2015
AC C13046: 'New version of the article', Samuel Remy, 24 Apr 2015
- AC C13076: 'More details on the differences between the old and the new version', Samuel Remy, 30 Apr 2015

Peer-review completion

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

AR by Samuel Remy on behalf of the Authors (24 Apr 2015) Author's response Manuscript

ED: Reconsider after major revisions (28 Apr 2015) by Michael Schulz

AR by Samuel Remy on behalf of the Authors (29 Apr 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (30 Apr 2015) by Michael Schulz

RR by Anonymous Referee #1 (26 May 2015)

Suggestions for revision or reasons for rejection

The influence of dust on the forecasts is a topic that has not been treated fully and this paper is an interesting contribution to it. I fear that the authors have not harvested all the results they could have from such study. Here are the questions I still have after reading the paper in full two times:

1/ Is it necessary to simulate dust and its interactions with climate to have an accurate forecast in the tropics and subtopics? Is the influence of dust local (in space) and short in time or is it synoptically of great importance and can last for a season?

2/ How does the influence of dust for the two short periods and at the two sites (Tamanrasset and Cairo) relate to the broad synoptic weather pattern of the subtropics.

3/ Given the problems encountered with the optical properties entered in the original manuscript, a good documentation of the ones used in the paper is needed. You should draw the refractive index used for these experiments and compute the radiative efficiency for both SW and LW over the region of interest. Note that Yu et al. 2006 (see Tables 13 through 17) have published values of dust radiative efficiencies for clear sky (SW) that you could put in regards to your results.

Since the paper has many sections, the summaries of results that are included at the end of these sections are very useful. I recommend that the sections 9 and 10 be merged together since they both summarize the main findings from this study.

Sections 4 and the first half of section 5 are not well written and need re-writing. Overall most sections would gain by having and English-speaker try to write them in a more concise form and extract the most important messages. Therefore I think that the authors should spend time improving the text from this manuscript.

Minor points:

Page 3, lines 138 through 145: when you cite Perez et al. (2006) and Miller et al. (2014) you could have more centered your comments on how these papers relate to your study and summarize their main results that are very relevant to this work!

Page 3, lines 205 to 209: the optical properties were not obtained by E. Highwood and S; Woodward, please trace from which publications these were taken as it is important to document how absorbing dust is.

Page 4 , lin 307 and 308: when you mention wind intensities, for how long were these intensities averaged? I suppose that these are not instantaneous winds. Please provide the information in order for the reader to know if the peak wind intensities have been smooth out by a time average.

Page 5 line 424: ‘slightly effected’ is not proper English, did you mean ‘slightly affected’?

Page 6, section 5, first and second paragraphs need to be re-written, please focus on what we lean about the influence of dust on the forecasts.

Paeg 6, line 575 and all along the paper: please replace ‘W’ with ‘West’; ‘E’ with ‘East’ when referring to the position relative to a location.

Page 7, lines 593 to 595: I did not understand how you reached the following conclusion: ‘’ They are less impacted by SW as compared to surface wind speeds, which shows that synoptic factors are less important than the local factors to explain the lower wind speeds of SW.’’

Page 7 line 685 : replace ‘translatesinto’ with ‘translates into’.

Page 8 line 706 : replace ‘Most this layer lies…’ with ‘Most of this layer lies…’

Starting page 9 and all along the rest of the text : replace ‘0UTC’ with ’00 UTC’
‘3UTC’ with ‘3 UTC’
‘12UTC’ with ’12 UTC’
‘15YTC’ with ’15 UTC’

Page 10 line 901 : replace ‘TOTAl’ with ‘TOTAL’

Page 10 line 938 : replace ‘rue’ with ‘true’

Page 10 line 959 : replace ‘fist’ with ‘first’

Page 10, lines 979-891 : ‘’ This sections aims to assess whether using prognostic aerosols to compute the aerosol radiative effect is beneficial for the forecasts of these two parameters.’’
It is unclear to me which two parameters you are referring to.

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RR by Anonymous Referee #3 (23 Jun 2015)

Suggestions for revision or reasons for rejection

The paper analyses dust radiative feedbacks and their impacts upon weather and dust forecasts with the MACC-II forecasting system during a series of dust storms in the Sahara and the Eastern Mediterranean in April 2012.

The paper identifies and analyses in detail two types of feedbacks upon dust emission: A primary feedback that is related to the negative (positive) forcing at the surface leading to a decrease (increase) of wind speed and dust emission by altering the thermal stability of the boundary layer; and a secondary one that is explained by the changes in the horizontal gradient of temperatures leading to local increases or decreases in wind speed and dust emission at the edge of the dust layer. With regards to the improvement of weather forecasts, the paper concludes that taking into account radiative effects in assimilation runs leads to improvements in surface temperature and dust AOD forecasts.

The results are sound and many of them confirm previous studies. The paper is interesting and deserves to be published. I have some comments, which should be addressed before the paper is accepted for publication.

General comments:

- With regards to the feedbacks upon dust emission: the main feedback identified and analyzed in the paper is the one that relates the decrease in emission to a reduction of eddy mixing within the boundary layer. This feedback was identified and extensively studied in Miller et al. (2004) and Pérez et al. (2006). Negative radiative forcing at the surface reduces the flux of sensible heat from the ground that powers eddy mixing in the arid regions that are dust sources. The dependence of wind speed upon surface forcing predicts that dust emission should increase in models with positive forcing (Miller et al. 2004). Other studies have shown that dust radiative forcing alters the regional distribution of surface pressure, increasing mobilization where the surface wind is increased (Heinold et al. 2008; Ahn et al. 2007).

What I believe it represents a novelty in the paper is the identification and interpretation of the secondary feedback related to the changes in the horizontal gradient of temperatures.

This should be highlighted in the paper. The original references need to be properly cited and described in the introduction and during the discussion of the results and conclusions. A better job needs to be done to distinguish when the paper is confirming previous results or identifying a new mechanism.

- The first study showing an improvement in short-term weather forecasts by introducing online dust radiative effects is Pérez et al. (2006).

Specific comments:

Abstract:
Please identify more clearly the two mechanisms. The first mechanism confirms previous studies. The second is more of a novelty and should be highlighted. The results concerning the assimilation runs are another novelty and should be highlighted as well.

Section 1.1

Lines 73 to 90: I feel this paragraph is both confusing and inaccurate. Are the numbers reported related to surface forcing? Aren’t those numbers related to radiative forcing at TOA?

More generally, why do the authors emphasize on the climate effect when the focus is to study short-tem effects. Dust short-term effects and climate effects at the surface substantially differ (and this is why I consider the previous paragraphs as confusing). A dust event interrupts the daily cycle of solar heating by dimming the surface. Over land, the ground cools with the passage of the dust layer overhead, reducing the upward transfer of heat by the longwave and sensible fluxes. But the surface air temperature is quickly restored to its unperturbed value following the passage of the dust layer. On time scales longer than a few days, the atmospheric temperature adjusts to the dust forcing at TOA and the perturbed energy exchange at the lateral margins of the dust layer (please see Miller et al., 2014). In regions of frequent vertical mixing, forcing at TOA is an especially strong constraint upon temperature at the surface.

Lines 91 to 111: As I explained above, proper references and a specific description of the results of these references on the specific focus of the paper are missing.

Lines 138 to 145: The studies cited in this paragraph do not look at mineral dust impact on climate. They rather look at short-term effects (weather time scales). This should be reconciled with my previous comments.

Section 1.2
Line 183: there is no wet deposition in the model?
Line 200: objectives instead of objective

Section 2.2
Last paragraph: Is there any reference for the WGNE model inter-comparison?

Section 4
Lind 461 to 466: I do not necessarily agree with this statement. If the decrease in LW downwards between the two periods was due to dust, why does the REF simulation (with a constant monthly dust climatology) show the decrease in LW as well?

Lines 483 to 485 (and lines 510 to 514 in section 5): You don’t need to follow the internal model convention to plot the results. If you do so, you should also describe the convention in the captions of the figures.

Section 6:

Lines 619 to 633: The interpretation of the secondary feedback (through the thermal wind equation) is not vey clear. In what do you base that the gradient of surface temperature is the predominant factor? Have you quantified it? Changes in pressure gradient seem to follow the same pattern in Fig 9.

Section 8:
Line 938: typo (“true”)
Line 955: Please avoid the expression “it seems that”. Nevertheless, I believe that the interpretation is correct concerning the differences between TOTAL_ASSIM and TOTAL with respect to surface temperature and winds.

Section 9:
Line 1009: missing end of sentence.

Sections 9 and 10:
Please wrap up by highlighting the new results compared to previous studies.

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ED: Reconsider after major revisions (03 Jul 2015) by Michael Schulz

AR by Samuel Remy on behalf of the Authors (14 Sep 2015) Author's response Manuscript

ED: Publish as is (05 Nov 2015) by Michael Schulz

AR by Samuel Remy on behalf of the Authors (06 Nov 2015)

Short summary

In this paper we report on the feedbacks between dust and boundary layer meteorology during a dust storm over Egypt and Libya in April 2012, using an atmospheric composition forecasting system. Dust was found to act on atmospheric stability, leading to an increase (night) or a decrease (day) in dust production. Horizontal gradients of temperature were modified by the radiative impact of the dust layer, leading to changes in wind patterns at the edge of the storm due to the thermal wind effect.