Microphysical, macrophysical and radiative responses of subtropical marine clouds to aerosol injections

Chun, Je-Yun; Wood, Robert; Blossey, Peter; Doherty, Sarah J.

doi:https://doi.org/10.5194/acp-2022-351

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https://doi.org/10.5194/acp-2022-351

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30 May 2022

Status: this preprint is currently under review for the journal ACP.

Microphysical, macrophysical and radiative responses of subtropical marine clouds to aerosol injections

Je-Yun Chun¹, Robert Wood¹, Peter Blossey¹, and Sarah J. Doherty^1,2 Je-Yun Chun et al.

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¹Department of Atmospheric Sciences, University of Washington, Seattle, USA
²Cooperative Institute for Climate, Ocean and Ecosystem Studies, University of Washington, Seattle, USA

Received: 19 May 2022 – Discussion started: 30 May 2022

Abstract. Ship tracks in subtropical marine low clouds are simulated and investigated using large eddy simulations. Five variants of a shallow subtropical stratocumulus-topped marine boundary layer (MBL) are chosen to span a range of background aerosol concentrations and variations in free-tropospheric (FT) moisture. Idealized time-invariant meteorological forcings and approximately steady-state aerosol concentrations constitute the background conditions. We investigate processes controlling cloud microphysical, macrophysical and radiative responses to aerosol injections. For the analysis, we use novel methods to decompose the liquid water path (LWP) adjustment into changes in cloud properties, and the cloud radiative effect (CRE) into contributions by cloud macro- and microphysics. The key results are that (a) the cloud top entrainment rate increases in all cases, with stronger increases for thicker than thinner clouds; (b) the drying and warming induced by increased entrainment is offset to differing degrees by corresponding responses in surface fluxes, precipitation and radiation; (c) MBL turbulence responds to changes caused by the aerosol perturbation, and this significantly affects cloud macrophysics; (d) across two days' simulation, clouds were brightened in all cases. In a pristine MBL, significant drizzle suppression by aerosol injections results not only in greater water retention, but also in turbulence intensification, leading to a significant increase in cloud amount. In this case, Twomey brightening is strongly augmented by an increase in cloud thickness and cover. In addition, a reduction in the loss of aerosol through coalescence scavenging more than offsets the entrainment dilution. This interplay precludes estimation of the lifetime of the aerosol perturbation. The combined responses of cloud macro- and microphysics lead to 10–100 times more effective cloud brightening in these cases relative to those in the non-precipitating MBL cases. In moderate and polluted MBLs entrainment enhancement makes the boundary layer drier, warmer and more stratified, leading to a decrease in cloud thickness. Counterintuitively, this LWP response offsets the greatest fraction of the Twomey brightening in a moderately moist free troposphere. This finding differs from previous studies which found larger offsets in a drier free troposphere, and results from a greater entrainment enhancement in initially thicker clouds, so the offsetting effects are weaker. The injected aerosol lifetime in cases with polluted MBLs is estimated as 2–3 days, which is longer than the estimates from satellite images.

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RC1:
'Comment on acp-2022-351', Anonymous Referee #1, 24 Jun 2022 reply
Review comments on “Microphysical, Macrophysical and radiative responses of subtropical marine clouds to aerosol injections” by Chun et al.,

General comments:

In this manuscript, the authors investigated the processes controlling the microphysical, macrophysical, and radiative response of clouds to aerosol injections from ship tracks through 5 simulations with various aerosols and free tropospheric moisture. Two novel methods are presented in the analysis. The first one decomposes the response of LWP to aerosols into the contribution from boundary layer processes, MBL decoupling processes, the response of cloud fraction, and the response of adiabaticity to aerosol injection. The second one decomposes the response of CRE to aerosol injection to the contribution from the response of droplet number concentration, LWP, and cloud fraction to aerosols. Both methods facilitate better analysis to improve the understanding of aerosol-cloud interaction processes associated with marine clouds. Overall, I think this manuscript meets the requirement of ACP and I recommend publishing it after addressing the following comments.

Major comments:

The three polluted cases use a smaller domain than the “pristine” and “middle” cases. The authors explain in lines 146-149 that the wider domain for the pristine and middle case is for mesoscale circulation, which is more significant in the precipitation cases. By comparing the roll cloud size between the middle and polluted case, it looks similar if the “mesoscale circulation” refers to the mesoscale circulation which maintains the formation of roll structure. So, I am not quite convinced that a different domain size is necessary. I recommend using 96 km X 9.6 km for all the simulations so that many analyses using domain and run averaged values can be more robust.

Section 3.2.4 shows the budget analysis of the impacts of 5 different processes on the cloud number concentration. But the method to decompose the droplet number concentration is omitted. Given the importance of the information, I recommend including the method in detail either in the main context or in the appendix.

Line 215-216: “the low A may be attributed to the low qc,inv caused by the high sedimentation velocity of large cloud droplets….” How about the role of cloud thickness in low A?

Minor comments:

Line 109: above? Below?

Lines 138-139: how do you adjust the free-tropospheric aerosol and divergence?

Line 152: any reference for the number 10.5 m s-1?

Line 177: “this reduces the primary source of turbulence in the marine low clouds (TABLE 2)”. Which variable from table 2 do you use to analyze turbulence? I don’t find TKE or other variables that represent turbulence.

Equation 1: please include the mathematical equation for entrainment efficiency A to show how A relates to cloud liquid water amount.

Lines 267-270: reduction in re leads to the changes in dA/A for all the cases. Are the processes associated with this relationship the same for all the cases?

Lines 280-281: explain why stronger entrainment tends to sharpen the inversion. Entrainment leads to the mixing between the air above and below the inversion, which is supposed to smooth the boundary at the inversion level.

Line 303-305: I assume this is referred from Figure 8, which has not been explained yet. Add “(Figure 8)” to help the readers to digest.

Is it better to exchange the order of section 3.2.3 and section 3.2.2 for the purpose of organizing the paper? The contribution from decoupling is introduced in 3.2.3 but already discussed in 3.2.2.

Line 594: clarify how to calculate fad the adiabaticity.

Line 650: cloud optical thickness? Albedo?

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Citation: https://doi.org/10.5194/acp-2022-351-RC1

Je-Yun Chun et al.

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Short summary

We investigate the impact of injected aerosol on subtropical low marine clouds under a variety of meteorological conditions using high-resolution model simulations. This study illustrates processes perturbed by aerosol injections and their impact on cloud properties (e.g., cloud number concentration, thickness and cover). We show that those responses are highly sensitive to background meteorological conditions, such as precipitation, background cloud properties.


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