| 16 Dec 2025
Quasi-Lagrangian observations of cloud transitions during the initial phase of marine cold air outbreaks in the Arctic – Part 2: Vertical cloud structure
Abstract. The aim of this work is to study the vertical distribution of microphysical cloud properties, in particular the thermodynamic phase partitioning and the cloud droplet size, in low-level mixed-phase clouds during marine cold air outbreaks in the Arctic. For this purpose, high resolution observations of the initial phase of a strong marine cold air outbreak in the Fram Strait collected with the hyperspectral and polarized imaging systems specMACS during the airborne HALO–(𝒜𝒞)3 campaign are analyzed. Pseudo-vertical profiles of the cloud thermodynamic phase generally showed increasing ice fractions with increasing height and decreasing temperature, except for a geometrically thin layer at the cloud top, which was more liquid-dominated. The measurements indicated that ice formation occurred preferentially at the coldest temperatures. In addition, the effective radius of the liquid cloud droplets increased with height, as expected. The observed vertical evolution of the liquid cloud droplets could be successfully modeled by an entraining parcel model. The good agreement between measured and calculated vertical profiles of the cloud droplet effect radius and additional information based on in situ measurements indicated that the influence of collision-coalescence and ice processes, such as riming, the Wegener-Bergeron-Findeisen mechanism, and ice formation through heterogeneous freezing, on the liquid cloud droplets was small for the observed clouds. The presented analyses and data can help to improve the representation of low-level Arctic mixed-phase clouds in models and to further our understanding of these clouds and the related microphysical processes.
Competing interests: Bernhard Mayer is member of the editorial board of AMT
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This study by Weber et al. investigates the vertical distribution of microphysical cloud properties and its evolution during the early stage of a marine cloud air outbreak observed during the HALO-(AC)3 measurement campaign. The manuscript is interesting to read, well aligned with the scope of the journal, and presents clear results and conclusions that are highly relevant and useful to the community investigating mixed-phase clouds and their evolution during marine cold air outbreaks. Overall, the manuscript is well written and well structured.
I recommend this paper for publication after a minor revision. Below there is a list of comments for the authors to consider.
Comments:
Line 113: “had to used” change to “had to be used”
Line 139: I think it is good to add here that “the backward trajectories were computed from ERA5 wind fields using Lagranto”, so that readers do not need to consult Weber et al.(2025a) to obtain this information.
Section 3.1: The discussion about the ice fraction shown in Figure 2 (Section 3.1) is focused on the mean values. However, there is a large spread in the values of the ice fraction (from 0 to 1) for a given brightness temperature. Can the authors comment on this feature? Is it possible to give some estimation regarding the cloud ice characteristics (i.e. number concentration, size or effective radius) when the ice fraction is 1. It is mentioned that there is a four orders of magnitude difference between the measured ice and cloud droplet number concentrations. I was wondering whether this difference is reduced when the ice fraction is 1.
Line 241: “contraction” change to “contradiction”?
Line 268: “combing” change to “combining”
Figure 5: What is WGS84 (in the y-axis label)?
Section 3.2: In this section, the order in which the results are presented feels somewhat strange to me. After introducing Figure 5 and describing the main features of the measured profiles of the effective radius of liquid cloud droplets, line 264 states that the measured and modelled profiles shown in Figure 5 will be compared. However, the discussion then shifts to explaining the results shown in Figure 6, before finally returning to the comparison of the profiles in Figure 5 at the end of the section. I personally didn’t like very much these transitions between Figures 5 and 6.
I think it would be better to briefly state at the beginning of the section that the goal is to analyze and compare the vertical and temporal evolution of the effective radius of liquid cloud droplets based on measurements and parcel model calculations. Then you can clarify that the parcel model calculations require knowledge of the cloud droplet number concentration. In consequence you want to introduce first the results shown in Figure 6, followed later by the description and comparison of the profiles in Figure 5.
Line 273: Are these decoupled clouds associated to the synoptic situation and possibly forming before the air mass is advected over the ocean, or are they related with some other local atmospheric conditions?
Line 345: “Collision and coalescence is relevant” change to “Collision and coalescence are relevant”
Lines 435-439 Can the authors comment on whether the results and conclusions presented in these lines are likely to be specific to this case or representative of cold air outbreaks in general? Related to this, are there previous studies on cold air outbreaks that have reported results and reached conclusions that are consistent with, or in contrast to, those presented here?