| 28 Jul 2026
How do clouds and turbulence shape the northeasterly low-level jet of a summertime Arctic cyclone during its cold core transition?
Abstract. Synoptic-scale Arctic cyclones play a major role in the polar climate variability by bringing heat and humidity from lower latitudes. However, they have not been as intensively studied as mid-latitudes cyclones, and some questions about their development processes remain unresolved. The international THINICE campaign which occurred in August 2022 near Svalbard archipelago aimed at studying Arctic cyclones and in particular low-level jets (LLJs) along fronts that can favour sea ice breakup and contribute the heat and moisture meridional atmospheric transport. By comparing ERA5 reanalysis with radar observations from the campaign, we show that ERA5 reasonably captures the LLJs correctly, in particular several cold-conveyor-belt (CCB) jets, but tends to slightly underestimate their intensity and to position them not close enough to the surface. It is therefore essential to understand the processes that drive the structure and intensity of these LLJs among which diabaticand frictional processes associated with clouds and turbulence that is, processes that are fully parameterised in numerical weather prediction and climate models. The objective of the present work is to understand how clouds and turbulence shape an intense LLJ during the typical warm-core to cold-core transition of a well-observed Arctic cyclone during THINICE. The analysis is conducted using the ERA5 analyses and short-term forecasts. It is based on the computation of backward Lagrangian trajectories initialised in a narrow band of high potential vorticity (PV) associated with the northeasterly LLJ. A PV budgetis performed in which all the diabatic – mostly associated with cloud condensation and radiative effects – and frictional PV tendency terms are computed and investigated. Results show that both diabatic and frictional terms reinforce the potential vorticity gradient across the jet and therefore its intensity when the system is baroclinic. Under such a baroclinic growth phase, the northeasterly LLJ is well identified as a CCB jet along the bent-back warm front. However, when the system is not baroclinic anymore and reaches a cold-core structure, friction generates much less PV and has almost no effect. Radiative cooling above boundary layer clouds no longer generates PV either, unlike during the baroclinic phase, and even slightly destroys it. Heating from condensation inside thick frontal clouds generate the same amount of PV in both phases (baroclinic and cold-core phases). Overall there is less PV generation in the second phase than in the first phase, which overall leads to a less intense LLJ.
Review of ‘How do clouds and turbulence shape the northeasterly low-level jet of a summertime Arctic cyclone during its cold core transition’ by Chiabrando et al.
General Comments
This paper presents a very comprehensive analysis of the representation of low-level jets at two different stages in the evolution of an Arctic cyclone. The authors make use of a variety of research tools and techniques, including PV modification and trajectory analysis, to demonstrate the differences in the processes (radiative, other heating and friction) that are important during these two phases. I think the science is sound and my only suggestion is to try to improve the way in which it is presented. At the moment, there are details that if ironed out would make the paper a very clean, clear contribution to the field. I detail these suggestions in the following lists of specific and technical comments. I think to some extent all my comment belong in the technical comment section, but I highlight those that I feel carry more weight by adding them to the specific comments section.
Specific Comments
Title: I’m not convinced that the low-level jets that are the focus of this paper can be reliably described as ‘northeasterly’ or ‘northwesterly’. I’d like the authors to reconsider this choice here and throughout the paper. I don’t think this adjective is essential for the paper.
Abstract: The abstract ends a little too abruptly. You could end with a short description of the wider implications of the research. Why does it matter?
L34: Axisymmetry is also one of the features that evolve in Arctic cyclones and a distinctive characteristic in front of mid-latitude cyclones. The authors could include this in the discussion simply by saying something like ‘The long Arctic cyclones usually transition from an asymmetric warm core structure toward an axisymmetric cold core…’.
L75-76: In the title and the abstract you were describing the jet as northeasterly but here you describe it as northwesterly. Looking at the rest of the paper I think it is not necessary for the jet to be described in these terms and so I’d recommend removing these adjectives.
L95: Are these the same radiative effects mentioned in L93? This is confusing at this point, but it can be improved by simply pointing to Table 1.
L177-178: I’m not clear what is meant by “Overall, both ERA5 analysis and short-term forecasts correctly captures the intensities of the LLJs in most of the flights. Still, as for flights F46 and F49, ERA5 underestimates on average the maximum wind speed…” To me these two sentences contradict each other. I think some clarification is needed. Additionally, I’m not sure an average over flights as a function of pressure tells us anything useful as each line corresponds to different conditions.
L275: If the trajectories are initialised between 950 and 850 hPa the mid-tropospheric feature is not sampled. Is this feature not important? Also, the jet core is located at 800 hPa and therefore out of the initialisation layer.
L326-327: A better description is needed. The vertical components will always be co-linear as will the horizontal components.
L334-337: To what extent could we expect these results to be general?
Figure 1: Panel (a): This panel is very busy. Maybe it can be simplified by not showing all the time points. It is said that the points are every 6-hours but there is no indication of what is the time of the initial point. It is not easy to discern between the different circle sizes. There are points with black edges but this is also very difficult to see especially over the sea, which might not need to be coloured. The panel includes what appears to be the flight track but there is no mention of it and it can perhaps be removed. Panel (b): This panel could include the times at which the cyclone is over sea ice by a single horizontal line. Caption: Remove ‘with a width of 100km’.
Figure 2: Panel a: The cyan and dark blue lines representing sea ice fractions are very difficult to see. Panel b: Looking at the ERA5 section I wonder whether the jet is there but it is slightly displaced, possibly toward the east, in the reanalysis. This leads to the question as to how systematic the drawbacks found in the paper are. It would be helpful to include a discussion along those lines.
Figure 5: Panel a: The wind contours make sense, but I’m finding the arrows confusing and I don’t think they depict a westward jet, but a southeasterly jet. Also, in panel a the dominant component seems to be the meridional component but that does not correspond to the values in panel b. I wonder whether the two wind components are plotted at the same scale or not. Any effort to make this clearer will be appreciated. Caption: Are the black contours temperature or potential temperature? Can you please double-check the edges of the red box? In the caption it says that the cross sections are a zonal mean between 15degE and 25degE but the box western edge seems to be 20degE.
Figure 15: What is the basis to claim different advection in each panel? If the flow is following the same velocity field, the advection would be the same in
Technical Comments
L4: ‘archipielago’ is not needed and can be deleted.
L5: Add ‘to’ between ‘contribute’ and ‘the heat’. Further, I suggest rewriting this sentence as ‘...contribute to the meridional atmospheric transport of heat and moisture’.
L6: Delete ‘several’ as a quantifier is not needed here.
L8: Substitute ‘among which’ by ‘including’. Otherwise, you need to add a verb somewhere.
L24-25: Highlight similarities and differences between Arctic cyclones and polar lows. At the moment Arctic cyclones are described as travelling inside the Arctic circle while polar lows are said to travel at high latitudes which to me sounds confusingly similar. Which is higher, high latitudes or the Arctic circle?
L27: Change ‘forming’ to ‘generating’.
L51: It should read ‘literature’.
L54: It should read ‘Ertel’. Change ‘non-conserved’ to ‘non-conservative’.
L57: Change ‘... with the CCB’ to ‘...to the CCB’.
L106: It might read better if it says “...with pressure as vertical coordinate.”
L117: What is RALI in RALI-THINICE?
L134: The wind drag vector was already defined as frictional acceleration. Is it necessary to change the name?
L134-135: It should read ‘the first of those three terms…’
L142: I suggest writing ‘Trajectories are initialised from high-PV regions, where the PV is above…’.
L143: What is the reason for having different spacings in longitude and latitude?
L148: Try to discuss the figures and panels in order. In this case you start by discussing Fig. 1c
L157: ‘... at the beginning of its transition toward the cold-core structure’. Does this mean that the red dashed line in Fig. 1b represents the end of the baroclinic phase. If so, please state this explicitly as it was not clear to me when I read the text.
L158: Instead of ‘the left panel of Fig. 2’ you could write ‘Fig. 2a’. The same when referring to Fig. 2b in L161.
L161: The sentence after the comma lacks a subject. Move ‘is shown’ to the end of the sentence.
L163: How can we see that the jet is located more to the north in ERA5?
L167: Looking at the arrows in Fig. 2a I’m just not convinced that we can characterise this jet as northeasterly. See my comment about the title as well.
Figure 3: Panel b: The circles in the RASTA plot are barely visible. Perhaps you could add a rim or some other device to highlight them.
L179: ‘The bias’ Are we really talking about a bias here or an error?
L189: What is the centre of a warm front if this is a linear feature?
L200-203: There are three types of trajectories mentioned in the text but it’s quite difficult to see these three types in Fig. 6a. Perhaps showing less trajectories would help with this.
L217: The shape of the ‘plus’ and ‘minus’ symbols is very difficult to see. I suggest using other, more solid, symbols such as circles or squares.
Figure 7: Re-scale the colour bar as only a very small range of colours is used. What is the grey shading?
L239-240 “The boundary layer clouds are now associated with non-radiative heating…” What part of the section is being discussed at this point?
L270: To me it looks like the maximum is at 800 hPa and not at 900 hPa. In fact, in L272 800 hPa is quoted.
Figure 10: I wonder if using log(p) instead of p as the vertical coordinate is better for the vertical sections. In that way the reader would get a better sense of the vertical extent of the features shown. The ice cover contours are not visible. The red box is supposed to be between 30degW and 20degE but this is not what the figure shows.
L290: Add ‘is higher’ between ‘cooling’ and ‘too’.
L298: Change ‘by’ to ‘to’.
L316: In what way are non-conservative processes ‘efficient’?
L317: Discuss figure panels in order. The discussion of Fig. 14 starts with its last two panels.
L321: Did you mean Eq. (2)? Also the vectors do not need to be co-linear to generate PV, they only need to be at an angle between 0 and 90deg.
Figure 14: Caption: Change ‘wind intensity’ to ‘wind speed’.