the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Jun 2026
A closed-to-open cell mixed-phase cloud transition observed over the Nordic Seas under high aerosol loading and strong surface fluxes
Abstract. Closed-to-open cell mixed-phase cloud transitions within marine cold air outbreaks subjected to strong turbulent surface fluxes remain poorly understood despite their importance to high-latitude climate. The Cold-Air outbreak Experiment in the Sub-Arctic Region (CAESAR) research aircraft sampled closed-cells with cloud condensation nuclei concentrations surpassing 680 cm-3, decreasing to 90 cm-3 across a transition to open-cells. The aerosol likely originated from Siberian industrial emissions. With fetch, liquid water paths (LWPs) increase from 120 g m-2 to 270 g m-2 and cloud-top effective diameters increase from 10 μm to 16 μm, coincident with more riming. Ice particle number concentrations (Ni) are generally 2 L-1 or less, but exceed ice nucleating particle number concentrations by 100x. As the cloud-top inversion weakens and the boundary layer deepens further, ice precipitation co-exists with lidar-observed surface cold pools, modulated by entrainment events, juxtaposed with surface-based plumes of warm moist air. Open-cells contain isolated LWP peaks surpassing 500 g m-2 collocated with strong updrafts, adjacent to glaciated cloud. Ni surpasses 10 L-1 at cloud temperatures < -15 °C. Precipitation shafts contain abundant large graupel (> 5 mm diameter) with liquid-equivalent precipitation intensities reaching 3 mm hr-1 developing cold pools with virtual potential temperature depressions reaching 1.3 K. Nonetheless, buoyancy fluxes of 200-250 W m-2 prevent sub-cloud decoupling. The updrafts supporting liquid water production occur at the upwind edge of the cold pools. This case expands the observations needed to better understand mixed-phase Arctic cloud processes.
Competing interests: At least one of the (co-)authors is a member of the editorial board 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 paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 30 Jul 2026)
- RC1: 'Comment on egusphere-2026-2940', Anonymous Referee #1, 11 Jul 2026 reply
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- 1
Samuel Ephraim
Aaron Bansemer
Lintong Cai
Markus Petters
Elise Rosky
Jefferson R. Snider
Zhien Wang
Sarah Woods
Kevin Barry
Theresa Campos
Owen Cruikshank
Sabine Eckhardt
Nikolaos Evangeliou
Romanos Foskinis
Jeffrey R. French
Bart Geerts
Coltin Grasmick
Silvia Henning
Varun Kumar
Andreas Massling
Camille Mavis
Greg M. McFarquhar
Athanasios Nenes
Gunnar Noer
Ryan Patnaude
Russell Perkins
Lise Lotte Sorensen
Henrik Skov
Tyler Tatro
Florian Tornow
The processes that establish how mixed-phase closed-cell clouds transition to more open cellular structures are poorly known. First-of-its kind aircraft observations document such a transition in the presence of anomalously high aerosol concentrations over the Nordic Seas at cloud temperatures < -15 °C. The reduces the drop size, discouraging riming. Eventually, ice precipitation produces surface cold pools that drive the convective transition, despite strong counteracting surface fluxes.
The processes that establish how mixed-phase closed-cell clouds transition to more open...
Review “A closed-to-open cell mixed-phase cloud transition observed over the Nordic Seas under high aerosol loading and strong surface fluxes”
The authors present a well-written manuscript that analyzes one specific closed-to-open cell transition in a marine cold air outbreak. The analysis appears thorough and detailed and contains appropriate figures. My comments below are mostly of minor character.
General comments
Specific/minor comments
Line 187: Typo: should be “Scandinavia”, not Scandanavia.
Fig. 4, 13, 14, 15 and others: The “rainbow”/jet colormap is not perceptually uniform. Please consider shifting to a colormap that is perceptually uniform (this refers to the gradients inside the colormap).
Line 224: What do you mean by “not yet shown”?
Line 249: Could you add information here, what causes the higher aerosol concentrations downwind? Which sources are there between Villum and the flight path?
Fig. 7, 11 and 17: Do you need the color bar? If the curves are of only one color each, you could also consider to just give the distance in the legend or caption.
Line 263: Consider adding that you talk about “vertical” profiles here.
Line 272/3 and 290: How certain are the altitude estimations for inversion and cloud top height? Is the overshoot robust?
Fig. 9: For the sake of colorblind-friendliness, I recommend changing one of the colors red or green.
Fig. 13/14(f): Could you use the same color scale for both figures anyway? SL1 goes hardly ever below 1 g kg-1 anyway, as far as I can see.
Fig. 14: Typo in caption: Locations where cold pools (entrainment) are identified “are” underlined in blue (gold) in (d)-(f).
Line 411: Typo “Aitken” mode, not Aiken.
Section 7: The answer to the question proposed in the section title seems hidden in line 435-437. You could consider repeating that at the end of the section.
Line 485-489: Do you mean that collisional breakup is followed by increased transfer from the liquid to the ice phase through the Wegener-Bergeron-Findeisen process? The ice-ice collisions can’t be causing the depletion of LWC themselves.
Line 493/4: This is an interesting suggestion. Do you have observational evidence for how frequent the recirculation of falling into updrafts occurs or on which scales this process is relevant in Arctic marine cold-air outbreaks? Does this e.g. depend on the width or intensity of the cold pool?
Line 537: Typo “THREDDS” server, not TREDDS.
Line 570/1: There seems to be a slight underestimation of droplet number concentration in the model compared to observations, even though the ranges overlap (as you correctly state). Could you comment on that possible underestimation?