Aerosol scattering effects on water vapor retrievals over the Los Angeles Basin

Zeng, Zhao-Cheng; Zhang, Qiong; Natraj, Vijay; Margolis, Jack S.; Shia, Run-Lie; Newman, Sally; Fu, Dejian; Pongetti, Thomas J.; Wong, Kam W.; Sander, Stanley P.; Wennberg, Paul O.; Yung, Yuk L.

doi:https://doi.org/10.5194/acp-17-2495-2017

Articles | Volume 17, issue 4

https://doi.org/10.5194/acp-17-2495-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-17-2495-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 17, issue 4

Research article

|

17 Feb 2017

Research article |

| 17 Feb 2017

Aerosol scattering effects on water vapor retrievals over the Los Angeles Basin

Zhao-Cheng Zeng, Qiong Zhang, Vijay Natraj, Jack S. Margolis, Run-Lie Shia, Sally Newman, Dejian Fu, Thomas J. Pongetti, Kam W. Wong, Stanley P. Sander, Paul O. Wennberg, and Yuk L. Yung

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Final revised paper (published on 17 Feb 2017)
Supplement to the final revised paper
Preprint (discussion started on 20 Jun 2016)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Investigating Wavelength-Dependent Aerosol Optical Properties Using Water Vapor Slant Column Retrievals from CLARS over the Los Angeles Basin', Anonymous Referee #1, 20 Jul 2016
- AC1: 'Response to Referee #1', Zhao-Cheng Zeng, 28 Oct 2016
RC2: 'CLARS measurements of H2O slant columns', Anonymous Referee #2, 22 Jul 2016
- AC2: 'Response to Referee #2', Zhao-Cheng Zeng, 28 Oct 2016

Peer-review completion

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

AR by Zhao-Cheng Zeng on behalf of the Authors (28 Oct 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (07 Nov 2016) by Anne Perring

RR by Anonymous Referee #1 (11 Nov 2016)

RR by Anonymous Referee #2 (18 Nov 2016)

Suggestions for revision or reasons for rejection

Review of “Aerosol Scattering Effects on Water Vapor Retrievals over the Los Angeles Basin” by Zeng et al.

This paper uses retrievals of H2O SCDs from CLARS observations to demonstrate that standard deviations of retrieved H2O SCDs amongst 15 H2O bands are due to the effects of scattering aerosol. This fact is established by the calculations.

The results in Figure 3 are very reasonable. The panel 3a (SVO observing path from Mt. Wilson to the sun) H2O SCDs have little wavelength dependence since little aerosol impacts these observations. The panel 3b (LABS LA basin observing path) H2O SCDs display much wavelength dependence, since aerosol is not included in the retrieval, and aerosol is an important contributor to the total optical depths. The retrieval of the LABS data therefore needs to add additional H2O in lieu of aerosol that is present in the LA basin. Panel 3d indicates that the AERONET-Caltech aerosol increases during the day, and therefore the H2O SCD standard deviations in panel 3c also increase during the day.

This paper, however, is very problematic since the methodology does not go far enough in its analyses. Yes, the data in Figure 3 is consistent with the fact that aerosol impacts the total optical depths in the H2O bands. This is expected, and by itself is not a sufficient reason for publication. The current paper does not demonstrate that accurate optical properties (e.g. AODs) can be readily retrieved from the observations.

The paper should be published after additional calculations are carried out by the authors.

Major comments

The following suggested calculations would bring the paper to a level of completion that fully warrants publication:
(a) The retrieval program retrieves H2O SCDs and aerosol properties (e.g. vertical AOD) simultaneously as a function of wavelength. Representative AERONET-MERRA-GOCART SSA and g could be specified (fixed) in a daily basis in the forward model of the retrieval.
(b) Compare the retrieved aerosol properties (e.g. AOD) to those derived from a combined analysis of AERONET, MERRA, and GOCART data,
(c) The retrieval program retrieves H2O SCDs with specified AERONET-MERRA-GOCART wavelength dependent AOD, SSA, and g
(d) Demonstrate that RT forward model calculations including the retrieved H2O and aerosol properties (from (a)) reduce the scatter in a Figure 3b type graph, and
(e) Demonstrate that RT forward model calculations including the retrieved H2O (from (a)) and AERONET-MERRA-GOCART aerosol properties reduce the scatter in a Figure 3b type graph.

The sentence in the Abstract “The understanding of aerosol scattering effects on H2O retrievals provides a sensitive way to quantify the effect of aerosol scattering on greenhouse gas retrievals …” indicates that this fact is established by the work in this paper. The phrase “provides a sensitive way” is not demonstrated by the work in this paper. This sentence needs to be removed from the abstract. The language on page 2, lines 19-20 is appropriate and can be retained. The language on Page 4, line 29 “shows the potential” is appropriate. The final sentence (page 10, lines 23-26) with the phrases “evidence justify our approach” and “providing a sensitive way” is not demonstrated by the paper’s calculations.

Page 3, line 12. Is wavelength dependent surface reflection included in the CLARS-FTS GFIT retrieval algorithm?

Page 3, line 12. Why is wavelength dependent aerosol AOD (and possibly mean SSA and g) not included in the retrieval?

Page 3, line 21. How is Figure 2 constructed? The Figure caption refers to “normalized radiance”, yet it is not stated in the paper how the normalized radiance is calculated. Please do so. Is aerosol included in the RT model calculations that are presented in Figure 2?

Page 3, lines 24-25. A full information analysis would follow the Rodgers methodology and include (a) calculations using a retrieval state vector that includes both H2O and aerosol (with little influence by the a priori aerosol) and (b) calculations using a state vector that includes H2O and aerosol heavily constrained by the AERONET-MERRA-GOCART data.

Page 6 and Figure 6. It is requested that panel (a) also be presented with the means included. The results in panels (a) and (c) are opposite to what is expected. If one adds the AERONET data to the forward model RT calculation, I would expect that the scaling factors would be closer to unity (and closer to 0 when the means are subtracted from the scaling factors) than for the case when no aerosol (panel (c)) is included in the forward model RT calculations. Yet the opposite is apparent. Please clarify.

Page 6 and Figure 6. If the standard deviations are not normalized, what does panel 6d look like? Again, the Aerosol-Free and Clear day curves seem to indicate that less a priori information (and/or a less complete inclusion of all contributors to the forward model) produces a better result. Please clarify.

Minor comments

Page 3, line 27. It is important to mention that aerosol is also included in the forward model RT.
Page 4, line 1 Briefy mention how IC is calculated.
Page 4, line 3. State which variables are retrieved.
Page 4, lines 17-18. Indicate (in %) the representative “small differences” and “larger variation” values.
Page 5, line 11. State the wavelength range “wavelengths (i.e. from 1288 nm to 2190 mn))”
Page 5, line 12. The sentence is not clear. Is the “AOD data” a vertical optical depth, and the AOP is simply this value scaled by a SZA and CLARS viewing angle geometric airmass factor?
Page 5, line 21. Why should the PBLH be similar throughout the year? Is the PBLH information available from nearby airport radiosonde temperature-pressure profiles or from any camera images taken at CLARS? On page 6 the PBLH data is from spring 2010, yet the observations are in winter-spring and summer-autumn 2013. Explain why (and if) the PBLH values are so uniform. On page 7 (line 1) it is stated that the PBLH is an important parameter. Should it not be included in the retrieval state vector?
Page 5, line 23. The “other factors” are not discussed in the text. What are the “other factors”?
Page 6, line 13. Why is HITRAN 2008 (instead of HITRAN 2012) used in the RT calculations?
Page 6, line 14. Is the surface albedo of 0.23 really wavelength independent from 1288 nm to 2190 nm?
Page 8, line 27. Is annual average MERRA data or month-specific MERRA data used for each individual day of the 68 day set?

Referee Report: PDF

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ED: Reconsider after major revisions (22 Nov 2016) by Anne Perring

AR by Zhao-Cheng Zeng on behalf of the Authors (28 Dec 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (11 Jan 2017) by Anne Perring

RR by Anonymous Referee #2 (17 Jan 2017)

Suggestions for revision or reasons for rejection

Review of “Aerosol Scattering Effects on Water Vapor Retrievals over the Los Angeles Basin” by Zeng et al.

The authors have made a strong effort to add to the paper in response to the suggestions of my 2nd review. Figure 8 of the revised paper addresses the major portion of the previous suggestions. The new lines added to the text, e.g. lines 1-8 on page 11, nicely bring a sense of closure to the paper. The paper can now be published subject to minor revisions.

Minor comments

Page 3, lines 5-7 This sentence needs to be revised i.e. “The CLARS measurement technique from Mt. Wilson mimics geostationary satellite observations of reflected sunlight, which are governed by a sun to surface and surface to instrument optical path geometry. The geostationary observations can be used to retrieve GHG mixing ratios (Xi et al., 2015).”

Page 3, line 23. “efficiency of band channels” is an odd phrase to use. Perhaps revise to “is a powerful tool which can be used in the channel selection process”

Page 5, lines 17-19. The sentence needs to be rewritten i.e. “Furthermore, aerosol optical path (AOP) values are calculated by multiplying the vertical path AOD data by air mass factors, which are derived from the SZA and viewing zenith angle of CLARS at West Pasadena (83.1).”

Pages 8 and 9. When I read these pages I naturally searched for a way to make sense of the writing. The current text has run-on paragraphs. The pages should be broken up into coherent paragraphs. Once I did this, the writing was easier to assimilate. Suggested paragraph breaks:
Page 8, line 6 “We perform three experiments..”
Page 8, line 14. “The results for simulated H2O..”
Page 8, line 24. “Further confirmation of this …”
Page 9, line 14. “The result is shown in..”
Page 9, line 22. “Similar control experiments..”

Page 10, line 12. Delete “for the purpose of providing comparable results for future satellite study” or revise. The phrase “comparable results” is not clear.

Referee Report: PDF

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ED: Reconsider after minor revisions (Editor review) (26 Jan 2017) by Anne Perring

AR by Zhao-Cheng Zeng on behalf of the Authors (27 Jan 2017) Author's response Manuscript

ED: Publish as is (27 Jan 2017) by Anne Perring

AR by Zhao-Cheng Zeng on behalf of the Authors (02 Feb 2017)

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Short summary

We propose a novel approach to describing the scattering effects of atmospheric aerosols using H₂O retrievals in the near infrared. We found that the aerosol scattering effect is the primary contributor to the variations in the wavelength dependence of the H₂O SCD retrievals and the scattering effects can be derived using H₂O retrievals from multiple bands. This proposed method could potentially contribute towards reducing biases in greenhouse gas retrievals from space.