Simulations of a cold-air pool associated with elevated wintertime ozone in the Uintah Basin, Utah

Neemann, E. M.; Crosman, E. T.; Horel, J. D.; Avey, L.

doi:https://doi.org/10.5194/acp-15-135-2015

Articles | Volume 15, issue 1

Atmos. Chem. Phys., 15, 135–151, 2015

https://doi.org/10.5194/acp-15-135-2015

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

Special issue: Uintah Basin Winter Ozone Studies (ACP/AMT inter-journal SI)

Atmos. Chem. Phys., 15, 135–151, 2015

https://doi.org/10.5194/acp-15-135-2015

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

Articles | Volume 15, issue 1

Research article 09 Jan 2015

Research article | 09 Jan 2015

Simulations of a cold-air pool associated with elevated wintertime ozone in the Uintah Basin, Utah

E. M. Neemann et al.

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Final revised paper (published on 09 Jan 2015)
Preprint (discussion started on 17 Jun 2014)

Interactive discussion

Status: closed

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

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- Supplement

RC C4635: 'Needs vertical profiles of chemical species', Anonymous Referee #1, 09 Jul 2014
- AC C6338: 'Reply to Reviewer #1', Erik Neemann, 29 Aug 2014
RC C4803: 'review #2', Anonymous Referee #2, 14 Jul 2014
- AC C6339: 'Reply to Reviewer #2', Erik Neemann, 29 Aug 2014
RC C5232: 'clarify intent of study', Anonymous Referee #3, 24 Jul 2014
- AC C6340: 'Reply to Reviewer #3', Erik Neemann, 29 Aug 2014

Peer-review completion

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

AR by Erik Neemann on behalf of the Authors (29 Aug 2014) Author's response Manuscript

ED: Referee Nomination & Report Request started (12 Sep 2014) by Paul B. Shepson

RR by Anonymous Referee #3 (04 Nov 2014)

Suggestions for revision or reasons for rejection

Comments: The revised paper has addressed most of my concerns in the first review. Two issues I was specifically asked to comment on are the ice fog vs. liquid water question and the need to verify the model using chemistry profile data.

1. Supercooled water vs. ice fog. I am still unconvinced that the fog observed in the Uintah basin during the 2013 experiment was essentially (and radiatively) all ice, but I don’t think this point is critical for the paper to be acceptable for publication. Because this is a model sensitivity study, the important point is that, “restricting cloud ice sedimentation and conversion to snow in the lowest model layers resulted in more realistic vertical profiles…” in the model runs (p.17, line 25). What happened in nature is another issue. That issue is described in the text, though, so it does need to be resolved.
The evidence I am most concerned about is the optical remote sensing systems at the Horsepool site, such as the IR and UV lidar systems, which show a complete dropout of the signal when the fog or low stratus are present. I am told by lidar propagation experts that this is a definitive sign of liquid water in the cloud. If the fog and stratus were all ice at Horsepool, the lidars would have very good signal, as the authors pointed out for the ceilometer at Roosevelt. It is well known that liquid-water droplets can exist at cold temperatures below -30C in cirrus clouds, so it is likely that some ice and some liquid both exist in the Uintah fogs, the question being how they are partitioned. The two pieces of evidence for ice in the fog are the picture of the sun provided in the responses and the satellite data. It is my impression that a mostly ice cloud will show halo or “sun-dog” or other optical effects, which can be seen in the textbook image also provided. I don’t see this kind of optical structure in the picture from the Uintah Basin, although maybe it’s just the picture quality. I also don’t see that the picture of the frost on the instrument cover is enough to rule out riming (by supercooled water) as opposed to deposition (hoar frost). I am not familiar with what verification has been done on the satellite determination of liquid vs. water, but this evidence may not be as strong as the Horsepool optical remote sensing. On the one hand, on p.7, line 25, the authors state that the satellite data “confirm” that the low clouds are ice, then later in line 29, they state that the data range observed is, “typically associated with ice-phase stratus and fog,” which sounds short of confirmatory. At any rate, the evidence seems inconclusive as to whether and where the fog was all ice, so I would be satisfied if the authors would modify some of their statements to reflect this:
--p.7, line 25: change “confirm that the low clouds are…” to “provide evidence that the low clouds may be…”
--p.17, line 24: delete “that were not observed during the event” so the phrase reads, “…liquid-phase low clouds and fog, whereas restricting…”

2. Profiles of chemical constituents. Since this is to me a model sensitivity study, I was not thinking that profiles, including chemical profiles, were necessary at locations other than the Roosevelt site to complete this study. As I read the paper, it does stand out that model sensitivity studies are evaluated at the Horsepool site, but no comparisons with measurements are done there, even though a great deal of profile measurement data are available at the location through tethered balloon and lidar datasets. I can see a good case either way, so I’d say… if the reviewers who brought this issue up still feel that species profiles are necessary for the paper to be published, I would support that position, since the data are available.

Other comments:

1. The most significant ‘other comment’ I have is that the “NONE” /no-snow model run is really not a very good representation of what actually happens it the wintertime Uintah Basin when there is no snow on the ground. The winter of 2012 was an entire period of no snow under a wide range of meteorological conditions. There were one or two brief disturbed shortwave periods when an inch or so of snow fell and quickly melted, which is when the “infrequently observed” (p.11, line 16) low cloud and fog would have occurred. But, if one compares like conditions to the undisturbed period being studied in 2013, the undisturbed periods during the 2012 project had no low clouds or fog present. The model, however, did produce low cloud/fog (p11, line 28) under these conditions. I don’t believe that simulating 2012 conditions would be much different from simulating 2013 conditions without snow, because of the wide range of conditions during 2012. I recommend that the authors just matter-of-factly state where these discrepancies occur, such as, specifically:
-- p.11, line 16: instead of “only infrequently observed” it is more accurate to say “were not observed during undisturbed periods” or similar.

2. p.7, last line, to top of p.8: “clouds observed within the basin generally occurred….within a shallow mixed layer.” This would not be true of the clouds/fog at night, when the BL was very stable (no mixed layer).

3. p.10: The discussion on this page (starting line 3) begins with an evaluation of model differences at Horsepool, without observational verification, but this paragraph ends and the next one begins, with comparisons of the model runs with ceilometer data, presumably at Roosevelt. This switch should be made more clearly. What is “integrated cloud amount” in Fig. 10?

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ED: Reconsider after minor revisions (Editor review) (09 Nov 2014) by Paul B. Shepson

Dear authors:
Thank you for your efforts in dealing with the 3 reviewers' concerns. Because of remaining concerns about the ice- vs liquid water- fog, and low clouds, and the fact that your revision does not deal with the issue of the need to show some ozone vertical profile information in the paper, it was sent for re-review to Reviewer #3. As a result of that review and my own read of the paper and the existing reviews, I am returning this manuscript to you to consider the following issues:
1. Please edit the manuscript in light of the new Reviewer #3 concerns and suggestions re the ice vs liquid phase low cloud issue.
2. I think that the paper will be more readable and clear if you, as suggested by Reviewers 1 and 2, add some ozone vertical profile information in support of the discussion of the impact of meteorology on ozone in Figure 6. This could be done for some selected period, like mid-day Feb. 4 and/or 5 at Horsepool. The fact that the ozone data will be available in other manuscripts is inadequate support of the discussion in this paper, in that a high quality paper should be readable and understandable as a stand-alone document. Your reply to the Reviewer 2 comments that you are simulating and focusing on the meteorological aspects of the inversions is not accurate, because this paper is analyzing the impact of the inversions on ozone, specifically, as indicated in the Title of the paper, and within the manuscript. Including some vertical profile information in your paper should not detract from the ability of others to independently and more thoroughly discuss and analyze the chemical species profiles, and assuming you have consent from those who generated those data, and can properly credit their use.
Please also consider the other minor comments from the second review from Reviewer #3.

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AR by John Horel on behalf of the Authors (24 Nov 2014) Author's response Manuscript

ED: Publish as is (25 Nov 2014) by Paul B. Shepson

Short summary

This paper uses numerical model simulations to investigate the meteorological characteristics of the 31 January–6 February 2013 cold-air pool (also know as a temperature 'inversion') in the Uintah Basin, Utah, and the resulting high ozone concentrations. A number of factors that influence cold pools and pollutant concentrations in the Uintah Basin are discussed, including snow cover, ice fog, and thermally driven flows.