Response of Arctic mixed-phase clouds to aerosol perturbations under different surface forcings

Eirund, Gesa K.; Possner, Anna; Lohmann, Ulrike

doi:https://doi.org/10.5194/acp-19-9847-2019

Articles | Volume 19, issue 15

https://doi.org/10.5194/acp-19-9847-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue:

BACCHUS – Impact of Biogenic versus Anthropogenic emissions...

https://doi.org/10.5194/acp-19-9847-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 15

Research article

|

02 Aug 2019

Research article |

| 02 Aug 2019

Response of Arctic mixed-phase clouds to aerosol perturbations under different surface forcings

Gesa K. Eirund, Anna Possner, and Ulrike Lohmann

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

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RC1: 'Review of Eirund et al.', Anonymous Referee #1, 03 Dec 2018
- AC1: 'Response to reviewer 1', Gesa Eirund, 03 May 2019
RC2: 'Review of Eirund et al.', Anonymous Referee #2, 03 Dec 2018
- AC2: 'Response to reviewer 2', Gesa Eirund, 03 May 2019

Peer-review completion

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

AR by Gesa Eirund on behalf of the Authors (03 May 2019) Manuscript

ED: Referee Nomination & Report Request started (17 May 2019) by Holger Tost

RR by Anonymous Referee #3 (13 Jun 2019)

Suggestions for revision or reasons for rejection

In this study, the authors perform LES modelling of an Arctic mixed-phase cloud case, with the surface fluxes changed to represent either open ocean or a sea-ice-covered surface. They perform sensitivity studies to an instantaneous increase in either CCN or INP concentrations. The paper is generally well-written and well-organized, and the results are clearly presented. The paper merits publication, provided that the following issues are addressed.

General comments:

In both the abstract and the conclusions section, the authors state that for the perturbed open ocean cases, “Increased ice and precipitation formation relax the liquid cloud properties back to their unperturbed range.” However, the cloud droplet number concentration remains elevated, the cloud droplet radii remain reduced, and the precipitation rate remains elevated. The LWP in the perturbed simulations only reaches the range of the control simulation during the last couple of hours of the simulation. It is not at all obvious that this new cloud state with an LWP similar to that of the unperturbed case is stable. The authors should comment on whether they expect this to be a robust result for longer simulations, or whether a thicker cloud may re-form or if the cloud may even dissipate due to the increased precipitation rate in the perturbed simulations if the duration of the simulations was extended by a few more hours.

When discussing the change in the net surface LW, the authors do not mention whether the LW emission by the surface is different between the open ocean surface and the ice-covered surface. Presumably, the ocean surface is at a higher temperature than the ice-covered surface. This should be mentioned and discussed when contrasting the open ocean and sea-ice simulations, as all differences in the net surface LW between these simulations currently seem to be attributed only to differences in the cloud properties.

Additionally, if the authors have the data available to estimate changes in net surface shortwave (SW) radiation between simulations, it would be helpful to include them where net surface LW is discussed. If negligible, a single sentence would be sufficient, if not, the authors should consider adding an additional row to Table 3. If the authors cannot estimate changes in net SW, they should very briefly discuss how the changes in net surface LW would be expected to compare to changes in net surface SW.

Specific comments and technical corrections:

p1, line 14: “once doubled as compared to the background concentration and once increased by a factor of 3” It would be clearer to use consistent phrasing here, e.g. “doubled [...] and quadrupled” or “increased by 100% [...] and by 300%”

p3, line 3: “aerosol concentration” -> “aerosol concentrations”

p3, line 6: “both, sea salt” -> “both sea salt”

p3, line 19: “Turbulent mixing and de- and entrainment” In this case, it would be much clearer to write the whole word: “Turbulent mixing, detrainment and entrainment”

p4, line 15: “ammoniumbisulfate” -> “ammonium bisulfate”

p4, line 21: The authors should more explicitly state somewhere here that the background CCN concentration is fixed, not prognostic like the INP concentrations and the CCN perturbations.

p4, line 23: “the CCN concentrations were chosen to match the observed Ndrop over the ocean” Do the authors mean over ocean and sea ice? This value is greater than the observed value over the open ocean.

p4, line 28: “aeorsol” -> “aerosol”

p5, line 17: “high latitude” -> “high-latitude”

p7, line 9: please add “smaller” after “5 μm”.

p7, line 10: “We relate these differences”: The current phrasing seems to imply that the authors will discuss this further later in the text, but they do not. Would the authors care to comment on how the differences in observed temperature and relative humidity profiles over the sea ice and open ocean are related to the differences between the ice_control case and observed cloud properties over sea ice?

p7, line 15: “domain wide” -> “domain-wide”

p12, lines 3-4: This sentence is confusing. The Twomey effect describes an increase in cloud reflectivity as cloud droplet number increases and cloud droplet radii decrease, neglecting changes in liquid water content. However, the authors seem to be relating the decrease in radius to an increase in liquid water content. Please explain.

p12, lines 5-6: “a further increase in perturbation strength from 500 cm−3 to 1000 cm−3 does not induce an additional increase in LWP.” According to Fig. 7, the maximum mean LWP increases from ~215 g m-3 to >230 g m-3. So this sentence seems to be incorrect.

p13, line 20: Are the authors trying to relate the decreased variability in LWP to a more significant effect of the perturbation on LWP, despite the magnitude of the effect being similar? This sentence isn’t very clear.

p13, line 33: “(Fig. S5)” Do the authors mean to refer to Fig. S1a here?

p15, line 4: “double the background concentration” -> “an increase equal to the background concentration”

p15, line 7: “400%” -> “300%”. The IWP increases by 300% to attain a value that is 400% of its original value.

p15, line 10: “Investigating” -> “After investigating”

p15, line 11: “clouds perturbed” -> “clouds with perturbed”

p15, line 11: “on the expense” -> “at the expense”

p19, line 11: “Earth surface” -> “the Earth’s surface”

p20, line 8: In the open ocean case, the authors have repeatedly stated that after a CCN perturbation, the cloud relaxes to its initial state. Based on Fig. 9, this is not the case for INP perturbations over open ocean. The statement that “the longer-term cloud response is more affected by CCN perturbations” therefore seems to be in error.

Table 1: “simulation” -> “simulations”

Table 2: Does the condition that cloud ice content q_i>0.001 g m−3 apply to all values listed in the table, or only N_ice and R_ice?

Fig. 4: Please move “(qc >0.01 g m−3)” after “in-cloud”.

Fig. 5: Please specify that it is the vertical range that is restricted to where immersion freezing occurs.

Fig. 11: In the lower-left subplot, it appears that one of the cloud droplets has escaped from the cloud.

Fig. S2: Please re-label the y-axes to Ndrop and Rdrop for consistency with the text.

Figs. S3, S4, and S5: Please adjust the captions from “all sensitivity simulations” to “all CCN perturbation simulations”

Fig. S4: It is difficult to see the control line, because of the overlap with the other simulations. From hour 7 to hour 10, I do not know what the cloud top height for the control simulation is. I greatly appreciate the authors’ consistency in colouring and line style across figures, but perhaps it would be best to increase the thickness of the line for the control simulation in this figure, to make it visible.

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ED: Publish subject to minor revisions (review by editor) (24 Jun 2019) by Holger Tost

AR by Gesa Eirund on behalf of the Authors (03 Jul 2019) Author's response Manuscript

ED: Publish as is (08 Jul 2019) by Holger Tost

AR by Gesa Eirund on behalf of the Authors (10 Jul 2019) Manuscript

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Short summary

Low-level mixed-phase cloud (MPC) properties can be highly affected by the ambient aerosol concentration, especially in pristine environments like the Arctic. By employing high-resolution model simulations we investigate the response of a MPC over an open ocean and a sea ice surface to aerosol perturbations. While we find a strong initial sensitivity to changes in aerosol concentration in both cloud regimes, the magnitude as well as the long-term cloud response depends on the surface condition.