| 13 Aug 2026
Contrasting Arctic Cirrus Clouds: Properties and Processes from Observations and Simulations
Abstract. Cirrus clouds in the Arctic play a central role in the regional radiation budget, yet their formation pathways and properties remain poorly constrained, especially during Warm Air Intrusions (WAI). In this study, we analyse and contrast cirrus observed during WAI events (WAI cirrus) with those formed under undisturbed Arctic Conditions (AC cirrus) using remote sensing measurements from the HALO–(AC)³ campaign, combined with backward trajectory modelling with CLaMS-Ice and LAGRANTO. This allows us to connect the optical, macro- and microphysical properties measured via lidar and radar to the cloud origin and dynamics during their evolution. We find that WAI cirrus encountered stronger vertical transport along their trajectories compared to AC cirrus. At the time of detection, WAI cirrus were geometrically and optically thicker, exhibited higher depolarization ratios and ice supersaturation, and contained more numerous but smaller ice crystals, consistent with homogeneous ice nucleation at colder temperatures. AC cirrus, in contrast, showed larger ice crystals and a comparably greater contribution from heterogeneous ice nucleation. As WAI events are projected to become more frequent and long-lasting, a resulting increase in the formation of WAI cirrus may enhance their influence on the Arctic radiation budget leading to cooling. Our results provide observational constraints on cirrus variability in the Arctic and emphasize the need for their improved representation in weather and climate models.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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Review of 'Contrasting Arctic Cirrus Clouds: Properties and Processes from Observations and Simulations' by Georgios Dekoutsidis et al.
This work contains a detailed analysis of the optical, macro- and microphysical properties of Arctic cirrus clouds. The remote sensing data was recorded during the HALO–(AC)³ airborne measurement campaign using Varcloud retrievals from a combined lidar and radar system. In addition, backward trajectories are analysed using two different models, CLaMS-Ice and LAGRANTO. The analysis is based on separating the cirrus clouds into two groups: The first group are cirrus clouds that formed during warm air intrusions and the second group are cirrus clouds that formed during arctic conditions. For these two groups the formation of the clouds, conditions along their trajectories, the optical, macro- and microphysical properties at the time of detection and their origin and ice nucleation processes are compared. Differences were found for instance in the ice particle size and concentration, the cloud geometrical and optical depth. Possible reasons for and consequences of these differences are thoroughly discussed. The results important for the accurate modeling of Arctic cirrus cloud radiative properties. Therefore I recommend this article for publication in Atmospheric Chemistry and Physics with minor revisions (see comments below).
General comments:
-Please add quantitative numbers (which you have very details in your results section) to the two or three most important findings that you give in the abstract or conclusion. This will provides more details in comparison to only quantitative stating more/higher/stronger/smaller etc.
-Provide the threshold parameters for T and available water vapour or backward trajectories inside Arctic circle to be considered as AC or WAI.
This will provide more transparency on the separation. Are there any mixed/ unclear cases that were excluded in the analysis?
-The description of Varcloud is rather short in comparison to CLAMS-Ice and LAGRANTO. Please add a few sentences on how the combined lidar and radar
retrieval works and which parameters can be retrieved.
-Please add a short comparison how the found micro- and macrophysical properties compare to other studies of Arctic cirrus clouds. E.g. De La Torre Castro et al. (2023) and Järvinen et al. (2025) for N, r_eff, IWC), similar for the macrophysical properties. Are the findings consistent?
-L366-377: During warm air intrusion the airmasses originate from non-polar regions where different INP concentrations are expected in comparion to Arctic cirrus where the airmasses originate from Arctic regions. This should also affect on the prevalence of homogeneous vs. heterogeneous ice nucleation. Please add a few sentences on this as well to the discussion.
Specific comments:
L39 & L44: Reference missing
L150: Please rephrase, simplify and elaborate. I don't understand this sentence.
L154: Please elaborate why liquid origin clouds are assumed to be heterogeneously formed. Why can they not be formed homogeneously without INP when the temperature drops below 235 K?
L178: Grammar incorrect. Also two 'however' in a row.
L185: Grammar incorrect [...] microphysics are [...]
L242 & L344/345: Reference missing
Technical corrections:
-Please double check the first paragraph, e.g. L27,31,35 points and commas are not formatted properly.
-Same goes for paragraph 2.3.2 (e.g. L131,L143,L144)
-Make physical variables like T, RHi, IWC and t0 cursive.
-Write out acronyms the first time they appear (e.g. WALES, Varcloud, DARDAR, HAMP, LAGRANTO, CALIOP, CPR,...)
-Please consistently use or not use Oxford comma (L424)
-L433: 3 should be upper case (L433)