the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Response of cirrus clouds to idealised perturbations from aviation
Abstract. Aviation is a rapidly growing source of climate forcing, and the non-CO2 effective radiative forcing of aviation is approximately twice that of aviation CO2. However, considerable uncertainty remains regarding aviation’s non-CO2 effects because the radiative forcing of aviation aerosol-cloud interactions, especially with cirrus clouds, is poorly known. Here, we use a large eddy simulation model to quantify the impact of ice crystal number concentration (ICNC) perturbations on the water budget and microphysics of pre-existing cirrus clouds. These perturbations aim to represent the second half of the chain of effects linking aircraft aerosol emissions to changes in ICNC and ice water path. We examine two types of cirrus: warm conveyor belt outflow and gravity wave cirrus, which represent different updraft regimes and formation mechanisms. In both cases, the primary effect of an idealised increase in ICNC is to extend cloud lifetime, with the increase proportional to the magnitude of the ICNC perturbation applied. The effect is more pronounced in the gravity wave cirrus case than in the warm conveyor belt outflow cirrus case because the latter has lower initial ICNC and ice water contents. Quantitatively, the sensitivity of ice water path (IWP) to changes in ICNC, expressed as ∆ln(IWP)/∆ln(ICNC), is 0.06 for gravity wave cirrus and 0.35 for warm conveyor belt outflow cirrus when calculated 45 minutes after imposing the ICNC perturbation. These results suggest that aviation has the potential to increase the lifetime and radiative effects of pre-existing cirrus clouds.
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Interactive discussion
Status: closed
- RC1: 'Comment on egusphere-2024-821', Anonymous Referee #1, 11 Apr 2024
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RC2: 'Comment on egusphere-2024-821', Anonymous Referee #2, 07 May 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-821/egusphere-2024-821-RC2-supplement.pdf
- RC3: 'Reply on RC2', Anonymous Referee #3, 21 May 2024
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AC1: 'Comment on egusphere-2024-821', Ella Gilbert, 10 Sep 2024
We are grateful to the three anonymous reviewers for the time and thought they have evidently put into their reviews of our manuscript. We agree with their comments that the simulations presented in the manuscript have shortcomings that challenge their degree of realism. Our simulations also do not allow us to test our assumption that the response of cirrus ice water path to ice crystal number perturbations can be studied separately from the preceding response of ice crystal number to aerosol perturbations. To address those shortcomings, we have started new simulations, now perturbing aerosols in more relevant parts of the model domain. Analysing those new simulations will take some time and involve extensive changes to the manuscript, so we feel it is best not to pursue publication of the present manuscript, and resubmit a new manuscript when the new analysis is ready.
We take this opportunity to clarify doubts that reviewers had on the radiative forcing mechanism studied by the manuscript. We study aerosol-cloud interactions (termed "mechanism A" by Reviewer 1), not contrails or embedded contrails. The two processes, although both involving aerosols interacting with clouds, are distinguished by the degree of influence of the aircraft. Contrails are formed a few seconds behind the aircraft, in the warm exhaust air from by-products of combustion, water vapour and, in the case of kerosene, soot aerosols. In contrast, aviation aerosol-cloud interactions refer to clouds that form naturally and are influenced by aviation aerosols. Aviation aerosol-cloud interactions influence clouds long after the aircraft and its dynamical and thermodynamical perturbation have gone, many hours to many days later. Some of the aviation aerosols that exert aerosol-cloud interactions may have formed contrails first, but have been returned to the atmosphere when the contrail sublimated. Aviation aerosols stay a long time in the atmosphere because they are emitted at high altitudes, so have opportunities to interact with clouds that form at commercial aircraft cruise altitudes and lower. We will present the subject of our study more carefully and less ambiguously in future manuscripts.
Citation: https://doi.org/10.5194/egusphere-2024-821-AC1
Interactive discussion
Status: closed
- RC1: 'Comment on egusphere-2024-821', Anonymous Referee #1, 11 Apr 2024
-
RC2: 'Comment on egusphere-2024-821', Anonymous Referee #2, 07 May 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-821/egusphere-2024-821-RC2-supplement.pdf
- RC3: 'Reply on RC2', Anonymous Referee #3, 21 May 2024
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AC1: 'Comment on egusphere-2024-821', Ella Gilbert, 10 Sep 2024
We are grateful to the three anonymous reviewers for the time and thought they have evidently put into their reviews of our manuscript. We agree with their comments that the simulations presented in the manuscript have shortcomings that challenge their degree of realism. Our simulations also do not allow us to test our assumption that the response of cirrus ice water path to ice crystal number perturbations can be studied separately from the preceding response of ice crystal number to aerosol perturbations. To address those shortcomings, we have started new simulations, now perturbing aerosols in more relevant parts of the model domain. Analysing those new simulations will take some time and involve extensive changes to the manuscript, so we feel it is best not to pursue publication of the present manuscript, and resubmit a new manuscript when the new analysis is ready.
We take this opportunity to clarify doubts that reviewers had on the radiative forcing mechanism studied by the manuscript. We study aerosol-cloud interactions (termed "mechanism A" by Reviewer 1), not contrails or embedded contrails. The two processes, although both involving aerosols interacting with clouds, are distinguished by the degree of influence of the aircraft. Contrails are formed a few seconds behind the aircraft, in the warm exhaust air from by-products of combustion, water vapour and, in the case of kerosene, soot aerosols. In contrast, aviation aerosol-cloud interactions refer to clouds that form naturally and are influenced by aviation aerosols. Aviation aerosol-cloud interactions influence clouds long after the aircraft and its dynamical and thermodynamical perturbation have gone, many hours to many days later. Some of the aviation aerosols that exert aerosol-cloud interactions may have formed contrails first, but have been returned to the atmosphere when the contrail sublimated. Aviation aerosols stay a long time in the atmosphere because they are emitted at high altitudes, so have opportunities to interact with clouds that form at commercial aircraft cruise altitudes and lower. We will present the subject of our study more carefully and less ambiguously in future manuscripts.
Citation: https://doi.org/10.5194/egusphere-2024-821-AC1
Data sets
LES simulations Ella Gilbert, Jhaswantsing Purseed, and Nicolas Bellouin https://zenodo.org/records/10845637
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Jhaswantsing Purseed
Martina Krämer
Beatrice Altamura
Nicolas Bellouin
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