the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Jan 2024
Air mass history linked to the development of Arctic mixed-phase clouds
Abstract. Clouds formed during marine cold-air outbreaks (MCAOs) exhibit a distinct transition from stratocumulus decks near the ice edge to broken cumuliform fields further downwind. The mechanisms associated with ice formation are believed to be crucial in driving this transition, yet the factors influencing such formation remain unclear. Through Lagrangian trajectories co-located with satellite data, this study investigates into the development of mixed-phase clouds using these outbreaks. Cloud formed in MCAOs are characterized by a swift shift from liquid to ice-containing states, contrasting with non-MCAO clouds also moving off the ice edge. These mixed-phase clouds are predominantly observed at temperatures below -20 °C near the ice edge. However, further into the outbreak, they become the dominant at temperatures as high as -13 °C. This shift is consistent with the influence of biological ice nucleating particles (INPs), which become more prevalent as the air mass ages over the ocean. The evolution of these clouds is closely linked to the history of the air mass, especially the length of time it spends over snow- and ice-covered surfaces, terrains may that be deficient in INPs. This connection also accounts for the observed seasonal variations in the development of Arctic clouds, both within and outside of MCAO events. The findings highlight the importance of understanding both local marine aerosol sources near the ice edge and the overarching INP distribution in the Arctic for modelling of cloud phase in the region.
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CC1: 'Comment on egusphere-2024-129', Xinyi Huang, 16 Feb 2024
This manuscript uses Lagrangian trajectories with extensive satellite observations to understand how air mass history affects mixed-phase clouds during marine cold-air outbreaks. The importance of local, marine biological INPs is highlighted by linking the time over ice- and snow-covered surfaces with the cloud phase fraction. This paper is well written, and the findings are vital to the understanding of marine CAO clouds. Here are two questions that would be nice to discuss with the authors.
(1). The Lagrangian trajectories were generated by pixels being advected with 1000 hPa wind field. As cloud formation and development also include vertical air motion, with rapid changes in cloud phase on these timescales, how representative are the trajectories of indicating the air mass history involved in both the MCAO and no-MCAO clouds?
(2). How important are other cloud microphysics processes to the change and evolution of the cloud phase? For example, the removal of liquid water by autoconversion and accretion which could also potentially increase the ice and mixed-phase fractions during the lifetime of MCAO clouds. Do the authors have any evidence to eliminate these alternative explanations?
Citation: https://doi.org/10.5194/egusphere-2024-129-CC1 -
RC1: 'Comment on egusphere-2024-129', Anonymous Referee #1, 04 Mar 2024
Review of “Air mass history linked to the development of Arctic mixed-phase clouds” by Murray-Watson and Gryspeerdt
This paper uses trajectory analysis combined with various satellite retrievals to examine the evolution of Arctic mixed-phase clouds, differentiating between marine cold-air outbreak (MCAO) and non-MCAO clouds. The mixed-phase clouds under these two conditions (essentially, convective versus non-convective) display different evolutions; namely, the mixed-phase cloud fraction and the cloud top temperature (CTT) regimes under which they exist. The authors link these differences to the time that the airmasses linger over snow- and ice-covered surfaces, as these areas are devoid of biological ice nucleating particles, which are thought to play an important role at warmer CTTs. As a result of their findings, the authors conclude that the prevalence of mixed-phase clouds at relatively warm CTTs cannot be fully explained by the satellite data used in their study. Thus, they discuss three potential causes: secondary ice production, presence of active ice nucleating particles at warmer temperatures, and retrieval biases. They also highlight the need for additional in-situ measurements of Arctic aerosols and mixed-phase clouds to improve fundamental understanding and inform climate models, the latter of which lack proper representation of these components in the Arctic.
I find this is a nice study overall. The manuscript is well written, and the explanations throughout the paper are informative. I think the authors provide a good discussion of the limitations of their study, and I appreciate its systematic nature; the logic was easy to follow. This study should be a useful contribution for the literature covering Arctic mixed-phase clouds. I have just one major comment and otherwise some minor comments that should be addressed before the manuscript is suitable for publication.
Major comment:
- In my opinion, Section 2 (Methods) does not provide sufficient detail. I understand that this study uses methods from two other papers that are cited, but I think some basic information is not reported. For example, which years are considered in the analysis? Which region do you use to generate the forward trajectories? For the MODIS retrievals described in Section 2.3, you filter out quite a bit of data; what fraction of the data do you end up considering? What does the so-called heterogeneity index tell you and why filter using it? Moreover, I see that you later cite Khanal and Wang (2018), so you’re aware of the study, but I wonder how confident you are that MODIS is properly classifying “single-layer, liquid clouds” in this rather challenging environment. It would be good to say something about that.
Specific comments:
- L133: Please define “TSI”.
- L137-139: Presumably warm air intrusion clouds make up an important fraction of these “other” clouds when the flow is poleward?
- L157: “The temperature data are obtained from ERA5 and processed as the wind data”; what does this mean?
- L198: I see an increase in liquid cloud fraction over time in the “Towards” clouds, not a shift toward mixed or ice phases. Am I missing something?
- Figure 1: Is the x-axis showing time since tracking the airmass? Also, what does the solid line and shading represent?
- Figures 2, 9, and 10: I think it should be possible to determine which bins are statistically significant and which are not. Please add hatching or something similar to indicate this.
- Section 4: You discuss secondary ice production and link it to previous studies that have examined the role of strong updrafts. Considering that MCAOs are largely surface-driven, the M value provides a good proxy for surface turbulent heat fluxes. I wonder if you could filter your data by M value (e.g., bin positive values by increments of 3 or 4) to address your hypothesis.
- L364: COMBLES --> COMBLE
- L364-366: Please also note that COMBLE was measuring CAO conditions ~100s-1000 km downstream of the ice edge.
- L445: We know that secondary ice production can be at play; I suggest changing the phrasing to something like, “if the mechanism is ubiquitous”.
Citation: https://doi.org/10.5194/egusphere-2024-129-RC1 -
RC2: 'Comment on egusphere-2024-129', Abhay Devasthale, 04 Mar 2024
Murray-Watson and Gryspeerdt use pseudo-Lagrangian framework to investigate the air-mass history during the development of mixed-phase MCAO clouds. While doing so, they elucidate on possible explanation behind the observed development by predominantly tying it to the role played by the biological INPs. The study seems to have a clear focus. It is also clearly presented and argued. The analysis and the use of satellite data are done appropriately. In general, I think it is a very good contribution and it should eventually be accepted.
There are a couple of aspects that I wish the authors would hopefully elaborate on.
- The meteorological context seems to be lacking. I believe providing the meteorological context (showing and contrasting temperature (not just the cloud top temp), humidity, dynamics) would be very useful for the potential readers. This would also help to indirectly evaluate if the role of changing INPs makes physically sense. I understand that this can be a manuscript on its own. So, I am certainly not expecting a detailed analysis, but I think at least a brief overview is warranted to fully grasp what is going on as the air masses are transported large distances off the sea ice edges.
- Another thing that got me thinking is the use of AOD as a proxy of INPs here. Given the persistence of clouds, high solar zenith angles and challenging meteorological and surface conditions, it is to be honest difficult to believe AOD changes that are in the order of 0.01 based on CAMS reanalysis. It would be better if the authors show a bit more information on aerosol variability. For example, are the changes in AOD really consistent with the hypothesis along the trajectories?
- Furthermore, it is to be noted that the atmospheric circulation and the aerosol vertical structure in itself (at the cloud base, free troposphere) could be different under non-MCAO and MCAO scenarios, which could complicate the use of AOD.
Citation: https://doi.org/10.5194/egusphere-2024-129-RC2
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