the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Different responses of cold-air outbreak clouds to aerosol and ice production depending on cloud temperature
Abstract. Aerosol-cloud interactions and ice production processes are important factors that influence mixed-phase cold-air outbreak (CAO) clouds and their contribution to cloud-phase feedback. Our current understanding is that increases in ice-nucleating particle (INP) concentrations cause a reduction in cloud total water content and reflectivity. However, no study has compared the sensitivities of the CAO cloud to these processes under different environmental conditions. Here, we use a high-resolution nested model to quantify and compare the responses of cloud microphysics and dynamics in cloud droplet number concentration (Nd), INP concentration and efficiency of the Hallet-Mossop (HM) secondary ice production process in two archetypal CAO events over the Labrador Sea, representing intense (cold, March) and weaker (warmer, October) mixed-phase conditions. Our results show that variations in INP concentrations strongly influence both cases, while changing Nd and the HM process efficiency affect only the warmer October case. With a higher INP concentration, cloud cover and albedo at the top of the atmosphere increase in the cold March case, while the opposite responses were found in the warm October case. We suggest that the CAO cloud response to the parameters is different in ice-dominated and liquid-dominated regimes, and the determination of the regime is strongly controlled by the cloud temperature and the characteristics of ambient INP, which both control the glaciation of clouds. This study provides an instructive perspective to understand how these cloud microphysics affect CAO clouds under different environmental conditions and serves as an important basis for future exploration of cloud microphysics parameter space.
Competing interests: KSC is an executive editor of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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Status: open (until 26 Feb 2025)
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RC1: 'Comment on egusphere-2024-4070', Anonymous Referee #1, 12 Feb 2025
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Summary
This is a modelling study on the sensitivity of different cold air outbreak events to certain aspects of the microphysics package (CASIM) employed in the UK’s unified model (UM). Specifically, this work examines the sensitivity to the droplet number concentration (serving as a proxy for aerosols and CCN), the ice nuclei concentration (through a scaling factor for the Cooper curve) and an efficiency setting for secondary ice production via the Hallett-Mossop process (rime splintering.) The primary objective is to study the relative importance of aerosol-cloud-interactions (ACI) and ice production processes in two cold air outbreaks.
Two extreme cold air outbreaks based on observations over the Gulf of Labrador have been chosen, a relatively warm outbreak (Oct 2022) and a cold outbreak (March 2022). They offer a very nice contrast arising from the difference in cloud temperature. The analysis of the cold case is most novel, given that the cloud resides at temperatures colder than -15°C. The author readily identifies other numerical studies of warmer cold air outbreaks, similar to the Oct 2022 case, where the Hallett-Mossop process is likely to be active.
The paper is well organised and well presented. I commend the author. The discussion of the response/evolution of the simulated cloud field and the development of precipitation to the various microphysics variations is particularly done well. The authors also do a very nice job in discussing the range selection for the sensitivity study.
Recommendation – Major revisions
I have reached this decision as I do not think the evaluation of the control simulations are nearly adequate. There is one evaluation figure of the cloud water path (CWP) (Fig 4), which is immediately dismissed because there is little confidence in the CWP from MODIS. While that may be true, that means there is no meaningful evaluation. It is simply unfair to ask me as the reviewer, and the wider scientific community to accept “a full model-observation comparison for this case is in preparation and will be shown in a subsequent paper.” This is putting the cart before the horse.
I am particularly concerned because I don’t think the ‘stratocumulus’ section of the March 15 simulation (the western portion of the simulation domain) shows sufficient skill to allow for the analysis presented – at least not without major caveats. From the RGB imagery (Figure 3a), I see relatively shallow roll clouds that grow quickly, which are common at the start of cold air outbreaks, when dry Arctic air first passes over warmer water. The strong winds and latent heat flux produce these rolls which progress to open MCC. The simulation, on the other hand, shows a thick, very cold stratocumulus, with the CWP two orders of magnitude more than MODIS CWP (Figure 4a and 4b). This is far more than ‘within one order of magnitude’ stated in the manuscript. The boundary layer in the simulated profile for this region (Figure 11, top row) extends to nearly 3 km. This is a region renowned for multilayer clouds (e.g., Mace et al., 2009 https://doi.org/10.1029/2007JD009755), yet the CONTROL simulation has a single, well-mixed boundary layer with a 3 km thick cloud? I need further evaluation work here.
Are there any aircraft profiles through this region from the M-Phase field campaign? Are there any upper air soundings from the coast of Canada?
At the least, I require an evaluation of cloud top temperature and cloud top height against MODIS products with a discussion focussed on the stratocumulus region of 15 March. In a perfect world, we would have a CALIPSO overpass to tell us the true cloud-top height and structure.
I am requesting this, given that so much of the stratocumulus-to-cumulus transition (SCT) discussion is underpinned on there being genuine stratocumulus in this portion of the domain. If the CONTROL simulation evolves from something rather inaccurate to something accurate, then aerosols and microphysics might not have anything to do with it, rather it’s simply the synoptic-scale dynamics in the operational analysis being downscaled from something poor to something accurate.
While the Oct case study also needs a more rigorous evaluation, I am more comfortable with the quality of the CONTROL simulation and the ensuing discussion.
Minor revision
Given the detailed discussion on the SCT, it is worthwhile to establish the environment beyond the un-evaluated cloud top temperature. It would be worthwhile to consider other aspects of the boundary layer environment and the role they may play in this transition. Please comment on the SST, downstream SST gradient, the boundary layer stability, the M parameter and the estimated inversion strength (EIS). As this manuscript reads, one might think that the only thing that matter are the cloud temperature and the microphysics.
Typos
Figure 1 legend: 2022 instead of 2024
Figure 8 legend: 24 October
Citation: https://doi.org/10.5194/egusphere-2024-4070-RC1
Data sets
Model data used for figures in paper submitted to ACP "Different responses of cold-air outbreak clouds to aerosol and ice production depending on cloud temperature" Xinyi Huang https://doi.org/10.5281/zenodo.14536461
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