Reassessment of shortwave surface cloud radiative forcing in the Arctic: consideration of surface-albedo–cloud interactions

Stapf, Johannes; Ehrlich, André; Jäkel, Evelyn; Lüpkes, Christof; Wendisch, Manfred

doi:https://doi.org/10.5194/acp-20-9895-2020

Articles | Volume 20, issue 16

https://doi.org/10.5194/acp-20-9895-2020

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

Special issue:

Arctic mixed-phase clouds as studied during the ACLOUD/PASCAL...

https://doi.org/10.5194/acp-20-9895-2020

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

Articles | Volume 20, issue 16

Research article

|

26 Aug 2020

Research article |

| 26 Aug 2020

Reassessment of shortwave surface cloud radiative forcing in the Arctic: consideration of surface-albedo–cloud interactions

Johannes Stapf, André Ehrlich, Evelyn Jäkel, Christof Lüpkes, and Manfred Wendisch

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Final revised paper (published on 26 Aug 2020)
Supplement to the final revised paper
Preprint (discussion started on 23 Jul 2019)

Interactive discussion

Status: closed

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

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RC1: 'Referee Comment', Anonymous Referee #1, 21 Aug 2019
- AC1: 'Replies to Referee comment 1', Johannes Stapf, 19 Nov 2019
RC2: 'Review summary', Anonymous Referee #2, 21 Aug 2019
- AC2: 'Replies to Referee comment 2', Johannes Stapf, 19 Nov 2019
RC3: 'Referee Comment acp-2019-534', Anonymous Referee #3, 21 Aug 2019
- AC3: 'Replies to Referee comment 3', Johannes Stapf, 19 Nov 2019

Peer-review completion

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

AR by Johannes Stapf on behalf of the Authors (19 Nov 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (20 Jan 2020) by Von Walden

RR by Anonymous Referee #3 (31 Jan 2020)

Suggestions for revision or reasons for rejection

The authors have completed a thorough revision, though I do have some remaining suggestions after consideration of which I think the manuscript will be ready for publication.

(1) I am pleased to see the inclusion of a new literature review on CRF in section 3.1. Though nitpicking, the authors may consider working in the perspective of Allen et al. 2003 and references therein into this section too, which is relevant framing for their overall discussion, in particular for the interpretation of LW CRF described in that section and page 7 (L17-19).

(2) I still have philosophical differences with the authors that prevent me from seeing how this study is a “reassessment” of a fundamental concept. The authors are correct that there are a variety of methods in use and that these methods are not necessarily comparable, but this is the nature of CRF, which is a hypothetical quantity. Here, the authors devote attention to a more sophisticated treatment of the albedo and I think reached some interesting conclusions. If they insist that this is a reassessment then I suppose I can live with it as it is largely stylistic point.

(3) In their response to my previous comments, the authors provide a sensitivity study to show the errors in CRF based on the height of the instruments. However, in the main text, there continues to be some handwaving on the treatment of the data collected at altitude, especially the LW. If the authors will not implement atmospheric corrections, I suggest that this analysis be included as supplementary material and referenced in the appropriate section.

(4) In the revision the authors argue that this is a study on SW processes. This is clearly stated now in the title and abstract where the focus is entirely on the SW and the LW is absent. However, within the text there is still consideration of the LW, which becomes important only at the end where the authors argue that their treatment of the SW causes the net CRF to go from strongly positive (LW dominant) to “near neutral” (perhaps “weakly positive” would be better). This is an important statement in the manuscript and it does not appear in the abstract, but is an interesting result that gives motivation to present LW calculations. I suggest including this finding in the abstract and perhaps working the concept of the balance of warming and cooling processes as a hypothesis in the introduction that warrants testing the sensitivity of SW CRF methods on net CRF results. This would set up the scope nicely for the reader.

(5) P21L19-20: Is this so? Nominally, if the boundary-layer lapse rate and surface temperature response under clearing skies is accounted for in the clear-sky term, the value of the clear-sky term should become more positive. Given that your LWD is measured at altitude, it is a bit difficult to say how things would change if the response were included.

 (6) I am not convinced that the brightness temperature measured by the KT15 at flight altitude is representative of a surface skin temperature without applying atmospheric and emissivity corrections, the latter being even more critical over the heterogeneous surface and the former being necessary despite the narrow-band (though I know of know proven methods). I suggest at minimum drawing more attention to this assumption in the text: While the Hori article does show nadir emissivity near unity at the center of the instrument band, the range is more like ~0.96-0.995 across even the sensitivity of this narrow-band instrument and even this assumes a consistent surface type.

(7) P5L26: “location and time” instead of “location”; P21L7: “enables” to “enables us to”

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RR by Anonymous Referee #1 (27 Feb 2020)

Suggestions for revision or reasons for rejection

Overall: The manuscript is significantly improved by the authors’ edits and it is worth publishing. However, it still requires major revision. Although the writing is generally understandable, the entire manuscript should be reviewed and edited for grammar and clarity. Overall, the main issue with the manuscript is the organization, which makes it extremely difficult to follow. It should be possible to easily find the results of the paper and discussion of those results, which is currently very difficult since they are intermixed with introductory material and methods.

Please rewrite the statement of what this work does on page 3, lines 8-10, to be more general and relate better to what is actually done in the paper. The current lines suggest that this work specifically focusses on the two processes that change the surface albedo in cloudy conditions (1) cloud-induced weighting of the transmitted downward irradiance to smaller wavelengths and 2) a shift from direct to diffuse irradiance in cloudy conditions). However, this seems to be too specific based on the results presented. A better statement would be that this work evaluates the effects of surface albedo-cloud interactions on CRF, including the impact of areal vs local albedo and exploration of the dependence of surface albedo on cloud LWP for different surface types, and during ACLOUD; and that this work presents the effect of surface albedo-cloud interactions on CRF during ACLOUD. This statement should be parallel with what is in the abstract and conclusions.

Sections 3 and 4: These sections intermix introductory material, methods, and results. To my read, Section 3.1 is introductory material, Section 3.2 is definitions, Section 3.3 is an uncertainty estimate for a particular source of error, and Section 3.4 presents an interesting result that is perhaps novel? Section 3.4.1 seems to give an overall estimation of uncertainty and it is unclear to me why it is a subsection of 3.4. Section 3.5 refers to the processes (1) and (2) above and, as such, would seem to be a main result of the paper, so why is it buried here? Why are all these subsections grouped together in one section? Section 4.1 seems to be methods again. Section 4.1.1 is entitled “Application to the observations,” but various things have already been applied to various observations, so a more descriptive title is needed; perhaps “Dependence of surface albedo on cloud LWP for ACLOUD measurements.” The Section 4.2 title, “Correction of CRF,” is also insufficiently descriptive and should be changed.

Please put introductory material (Section 3.1) into the introduction, or its own section. Please consider having a section on methods, that groups all methods together (e.g. 2.2, 3.2, 4.1), and a section on uncertainty (which would include Sections 3.3 and 3.4.1.) The results seem to be contained in Sections 3.4, 3.5, 4.1.1 and 4.2, so these should be grouped together. Furthermore, section titles should make clear that 3.5 represents a model study related to the two processes showing how broadband albedo changes with LWP for different surface types (related to process 2) and attenuation of incident irradiance by clouds (related to process 1), whereas 4.1.1 shows how broadband albedo changes with LWP for the ACLOUD measurements (related to process 2).

Specific comments:

Please put a horizontal space (extra line break) between paragraphs or begin new paragraphs with an indent so the reader can tell when a paragraph ends.

Page 6 last line – Page 7 lines 1-2: Please delete these sentences. I do not see the point of them, as the current work would also fall into the category of being limited in comparability to previous results due to a different means of computing CRF. Furthermore, the current work relies on aircraft flights, which are not available to many of the previous studies and therefore could not be applied to them.

Page 7 lines 4-6: Please delete these lines or change them to “Below, we derive the … CRF. Our approach is unique in that we use a continuous … cloudy conditions.” Please add to the sentence any aspect of using the areal-averaged albedo that is unique to this work. E.g, if it’s true you could say, “ Below, we derive the … CRF. Our approach is unique in that we use a continuous … cloudy conditions, and in that we account for horizontal photon transport through using the areal-averaged albedo to compute downward shortwave fluxes, as discussed below.”

Pages 7-8
In these equations, the two alphas in Eqn. 7 are later replaced with two non-equal terms, which is not mathematically correct. You can avoid this and simplify by adding the subscripts “all” and “cf” to the relevant alphas in Eqn (8). With this change, Eqn (10) could then replace Eqn (9), which would no longer be needed. The text from lines 19-22 could then be moved to just before Eqn (8) to provide the necessary context, after revising it to read “The surface albedo changes for … Warren (1982). Thus, the measured surface albedo has been separated … alpha_cf.”

Page 11, line 12: reference needed after “(increasing grain size).” Lines 16-19: references are needed. Also, this text through page 14 line 2 seems to be an expansion of information that was given in the introduction and should therefore be moved to the introduction. Retain here only enough information needed to introduce and discuss the results (e.g. “Fig. 5 shows the effect of …”).

Section 4.2 seems like an important section but the results shown in Fig. 10 are not described sufficiently. The caption/text and the legend do not agree. What is “Corrected delta F?” This phrase occurs nowhere else in the paper.

The conclusion (page 20) slowly transitions from a description of the literature, which is not always referenced (lines 12-14, line 17) to results from the current work. Is line 18-20 from the literature or current work? Please reference all statements that are already known and start a new paragraph for the findings of this work, which should begin with a sentence using language such as “we find” or “our results show” to make this clear.

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ED: Reconsider after major revisions (10 Mar 2020) by Von Walden

AR by Johannes Stapf on behalf of the Authors (24 Apr 2020) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (25 May 2020) by Von Walden

The Authors have done a good job responding to the Reviewer's comments in round 2 of this review process. The Editor notes especially the reorganization of the manuscript to make it more readable. This is a very interesting paper and illuminates important aspects of how sea ice is forced by shortwave radiation, and the importance of the definition and application of cloud radiative forcing. Therefore, this paper should be published subject to the minor revisions listed below:

1) First, it is important to correct the error in the definition of cloud radiative forcing (CRF), as originally defined by Ramanathan et al (1989). The Authors are using "all" to represent cloudy-sky conditions, whereas Ramanathan et al's definition states that "all" does truly represent all sky conditions (cloudy plus clear). The reason why Ramanathan et al defined CRF in this manner was so that their definition would not require the determination of the cloud fraction to compute CRF; users must merely estimate the SW and LW clear-sky fluxes and subtract these from measurements under all conditions. This definition does not impact the research or conclusions in this manuscript, but the use of "all" to represent "cloudy" conditions must be corrected. The alternative is to present values of the cloud radiative effect (CRE), which is defined to be cloudy minus clear. If the Authors choose to report values of CRF rather than CRE, then they must re-compute their CRF values using (all minus clear) instead of (cloudy minus clear). Depending on what the Authors choose, they must also carefully edit the text (and figures; e.g. Figure 6!) to change all instances of "all" to "cloudy", or all CRF values to CRE values.

2) This manuscript has evolved significantly as a result of this review process. In round 1, multiple reviewers expressed concern that there was a lack of previous literature cited in this manuscript. The Authors then provided additional citations and references on Arctic cloud radiative forcing. But as the manuscript evolved to focus on shortwave radiation, I believe that there are many relevant papers related shortwave radiation that the Authors should seriously consider citing. This is especially true in section 4 and for Figures 3, 4, 5 and 6. In particular, there is a wealth of literature on the interaction of shortwave radiation with Arctic and Antarctic sea ice, pioneered by Tom Grenfell, Don Perovich, Richard Brandt, Steve Warren and others. Below is a list of references that you should consider citing and referencing. In addition, I would encourage the Authors to add a paragraph at the beginning of section 4 that places their current manuscript in context of the body of work from others in this field, including the authors listed above.

Brandt, R. E., Warren, S. G., Worby, A. P., & Grenfell, T. C. (2005). Surface Albedo of the Antarctic Sea Ice Zone. Journal of Climate, 18(17), 3606–3622. https://doi.org/10.1175/JCLI3489.1

Fitzpatrick, M. F., & Warren, S. G. (2005). Transmission of Solar Radiation by Clouds over Snow and Ice Surfaces. Part II: Cloud Optical Depth and Shortwave Radiative Forcing from Pyranometer Measurements in the Southern Ocean. Journal of Climate, 18(22), 4637–4648. https://doi.org/10.1175/JCLI3562.1

Fitzpatrick, M. F., & Warren, S. G. (2007). The Relative Importance of Clouds and Sea Ice for the Solar Energy Budget of the Southern Ocean. Journal of Climate, 20(6), 941–954. https://doi.org/10.1175/JCLI4040.1

Fitzpatrick, M. F., Brandt, R. E., & Warren, S. G. (2004). Transmission of Solar Radiation by Clouds over Snow and Ice Surfaces: A Parameterization in Terms of Optical Depth, Solar Zenith Angle, and Surface Albedo. JOURNAL OF CLIMATE, 17, 10.

Grenfell, T. C., & Perovich, D. K. (1984). Spectral albedos of sea ice and incident solar irradiance in the southern Beaufort Sea. Journal of Geophysical Research: Oceans, 89(C3), 3573–3580. https://doi.org/10.1029/JC089iC03p03573

Grenfell, T. C., & Perovich, D. K. (2004). Seasonal and spatial evolution of albedo in a snow-ice-land-ocean environment. Journal of Geophysical Research: Oceans, 109(C1). https://doi.org/10.1029/2003JC001866

Shine, K. P. (1984). Parametrization of the shortwave flux over high albedo surfaces as a function of cloud thickness and surface albedo. Quarterly Journal of the Royal Meteorological Society, 110(465), 747–764. https://doi.org/10.1002/qj.49711046511

Wiscombe, W. J., 1975: Solar radiation calculations for Arctic sum- mer stratus conditions. Climate of the Arctic. G. Weller and S. A. Bowling, Eds., University of Alaska Press, 245–254.

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AR by Johannes Stapf on behalf of the Authors (03 Jun 2020) Author's response Manuscript

ED: Publish as is (06 Jul 2020) by Von Walden

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