Review of:

Case study of a long-lived Siberian summer cyclone that evolved from a heat low into an Arctic cyclone

Authors:  Franziska Schnyder, Ming Hon Franco Lee, and Heini Wernli

This study investigates a rare, long-lived Siberian summer cyclone that formed as a heat low in Kazakhstan during a heatwave in July 2021. Using ERA5 reanalysis data, the authors identify and analyze 95 Siberian summer cyclones, including 9 that originated as heat lows. The featured case traveled over 4000 km into the Arctic, underwent rapid intensification, and produced a warm conveyor belt (WCB) that contributed to a potential vorticity (PV) cutoff near the North Pole. The study documents the cyclone's transition from a thermally driven heat low into a baroclinic system, providing new insight into the dynamics of high-latitude summer cyclogenesis and potential climate change implications.  Overall, the manuscript presents a novel topic and thorough investigation into a relatively rare class of cyclones. I found the analysis and results to be very interesting. The observation of a heat low transitioning into an Arctic cyclone, with documented impacts on PV structures and WCB formation, is important for understanding high-latitude weather systems, as well as the climate system. The manuscript is well written and the figures are well presented. Overall, I had a few mostly minor suggestions for the authors to consider.

Recommendation: Minor revision

Comments

1. Line 108-116. How sensitive are the results to the specified radius of 100 km that represents the environment near the cyclone?  The low-level PV is calculated in the lowest 30 levels.  Would it not be more reasonable to select levels above or in the upper part of the PBL?  Since RH and theta are averaged only up to 300 m, how sensitive are the results to this threshold?
2. Lines 149-155: The “subjective threshold” (RH < 20%) for defining Siberian heat lows could be better justified.  Can you provide more information on how this threshold was chosen.  Can you provide more information on the sensitivity of the selected cases to this threshold.  Could you use RH and PV together to better define the subset of cyclones?
3. Table 1: The inclusion of 8 other Siberian heat lows provides a broader perspective, which is great. However, the discussion could benefit from a more quantitative comparison; perhaps including the PV and track length/maximum latitude in Table 1 would help.
4. Section 4.2 and 4.3. What is the role of the rather deep PBL in the cyclogenesis stage.  Are the low-level PV and theta diagnostics representative of the cyclone environment since the boundary layer is so deep?
5. Section 4.4. It would be worthwhile to quantify the role baroclinicity through the evolution, perhaps through a bulk Eady index, to better understand the rapid intensification period.
6. Lines 270-272. What are the important characteristics that allow the WCB to reach the high latitudes in this case?
7. Lines 295-298. Is there any significance to the regions of negative PV aloft in these cases?  Is there any relationship between the high PBLs and buoyancy, and low stability in the early stage of cyclogenesis that might influence the very low values of PV aloft?

Technical Comments:

1. Abstract line 11: Perhaps rephrasing "a warm conveyor belt whose outflow almost reaches the North Pole" to "a warm conveyor belt with poleward outflow approaching the North Pole" would improve clarity.
2. Line 63: "Examples are the Arabian heat low..." could be clearer as "Examples include the Arabian heat low..."
3. Figure 1. The scatter plot could use some improvements, the inset map is a little small, for example, and the light blue and dark blue are not that distinct.
4. Lines 230-231: "the precipitation that occurs here along the fronts is significantly weaker...". could be more clearer as: “Frontal precipitation here is notably weaker…”
5. Line 250: Perhaps rephrasing “Concomitantly, diabatic production of PV intensifies…” to “Simultaneously, latent heating leads to diabatic PV production that intensifies...”
6. Line 322: “are generally not reported for subtropical heat lows” could be clearer as: “are generally absent in subtropical heat lows.

The paper investigates the dynamics of several summertime extratropical cyclones initiated in Siberia with heat low structure. The whole paper uses ERA5 reanalysis datasets. After a brief analysis of the climatology of Siberian summer cyclones, 9 particular cyclones having heat low properties (dry and hot at low levels) are selected. The choice of those cyclones is well explained: first, such cyclones have not been the subject of fully documented case studies and second, these cyclones might become more frequent in the future as they are initiated during heat waves. Among them, one of particular interest having a long duration is deeply analysed. The cyclone was formed in Central Siberia, starting from a heat low structure. It moved to the Arctic and strongly deepened during its baroclinic interaction with an upper-level anomaly when leaving the continent. Interestingly, the end of its life cyclone is associated with an intense Warm Conveyor Belt (WCB) activity which is quite unusual in that region. At the end, the paper comes back to the 8 other "heat low" cyclones and investigates why some of them rapidly deepened while some others did not rapidly intensify.The tools used in the paper includes a cyclone tracking algorithm, a WCB detection algorithm and classical weather maps analysis. The paper is very well written, well organized and the figures are nicely chosen. I particularly enjoyed reading the paper. My only major concern is about the discussion of why some cyclones deepen and some others do not. Figure 10 shows there was systematically a PV anomaly west of the surface cyclone at some time during their life cycle but the question remains open about the reasons why some of them deepened whereas others did not. I would suggest to more deeply analyse baroclinicity environments for the 8 cases to be able to provide some potential reasons for these different behaviors. Despite this major comment, all my remarks are minor and are there to improve the clarity of the text and/or to provide additional information to the reader.

Main comments:

1) Figure 10 shows that all the 9 selected surface cyclones developed and propagated east of an upper-level PV anomaly but some of them deepened and some others did not. Such a westward tilt with height of the anomalies is a favourable configuration for baroclinic growth but does not seem to be sufficient. On line 318 an interpretation is provided: "the absence of a strong PV tower may explain why this cyclone does not intensify as strongly." Do you mean that the low-level PV anomaly is too shallow to baroclinically interact with the upper-level PV anomaly ? In my mind, an alternative explanation could be the difference in the strength of the baroclinicity. How are the mean temperature gradients or the Eady growth rate during the evolution of the surface cyclones ?

2) About the position of the cyclone center. In several panels, the stars which are symbols indicating the cyclones centers are not located inside a close contour of SLP. Since the cyclone center is identified as a SLP minimum, I do not understand this mismatch. In figs. 4b and 10f this is particularly obvious. In Fig.4e it seems that the SLP minimum is further north than the star and should be closer to the high PV.

Minor comments:

- Figure 1: maybe recall in the caption over which levels PV and potential temperature are averaged.

- Figure 1 / line 103 about the choice of the two geographical areas. How do you know that the most eastern box is not related to the initiation of cyclones moving into the North Pacific storm track ? More discussions on the choice of the two boxes would be good

- Figure 2: it would be good to show the time in days rather than in hours to more easily connect the time evolution of the cyclone of Figure 2 with the weather maps shown for cyclone 9.

- Line 178: please refer to Fig.3 at the end of the sentence after "environment of the cyclone".

- Figures 3 and 4 about the synoptic situation at the time of cyclogenesis. Figure 4f (3 July) suggests baroclinic interaction with the first PV anomaly that could explain the slight decrease in SLP during genesis. But the text says "this anomaly is most likely too remote to influence cyclogenesis". I am not entirely convinced by the statement because the distance between the two anomalies is about the size of the anomalies, roughly 1000 km, i.e the radius of deformation.

- Figure 7: please indicate more longitudes.

- Figure 8: please indicate more latitudes and more loingitudes.

- Figures 5 and 9: please indicate subtitles for the x- and y-axes (pressure and longitude).

- Line 208: "the very small to negative PV values troughout the lower troposphere at the cyclone center correspond to the very low static stability". It seems to me that in Fig4e the cyclone center is more to the north than the star indicates and closer to the positive PV (see second main comment).

- Line 214: the text says that "the presence of the upper-level PV anomaly in the vicinity of the cyclone is unusual for subtropical heat lows". If I understood correcly, the text says an upper-level PV anomaly is present, it is unusual in the vicinity of the heat low but has no effect on the cyclone dynamics. And line 217 "another approaching upper-level PV anomaly was most likely decisive". Don't you think that the effect of an approaching upper-level PV anomaly on the surface cyclone depends on the environmental baroclinicity ? See main comment 1. It would be interesting to show the cyclone trajectory superimposed onto the mean low-level baroclinicity. My opinion is that an approaching upper level anomaly will help to advect the surface cyclone northward until it reaches a zone of strong baroclinicity (here the Arctic frontal zone) where the two anomalies can then interact with each oher. So maybe the first PV anomaly is useful to advect the surface cyclone further north and has an impact on the track more than on the cyclone intensity because they are far from the main baroclinic zone at that time.

- Line 276-277. I do not understand why the presence of PV cutoff enhances net surface shortwave radiation. Are you talking about anticyclonic cutoffs or cyclonic cutoffs ?

- Line 308: "cyclone 2 decays before the PV streamer can catch up with the surface cyclone". Hereagain, I am a bit sceptical about such a statement. In Fig.10b, I do see a surface cyclone to the west of the PV streamer, almost in phase quadrature (the edge of the PV streamer is above the SLP minimum). It is a classical feature of the beginning of rapid baroclinic growth. If cyclone 2 still decays with such a configuration, it means that baroclinicity is weak. Here again I suggest computation of the mean baroclinicity.

This paper presents a case study of a long-duration synoptic event in summer of 2021 which began as a heat low over Kazakhstan and ended as an Arctic cyclone over the central Arctic Ocean. The low-pressure system was also notable because of its track length of nearly 4000 km whereas subtropical heat lows often tend to be relatively stationary or travel far shorter distances relative to their areas of cyclogenesis. The authors document key dynamics of the system over its full lifecycle, including its interaction with an upper-level tough, production of a warm conveyor belt that yielded outflow that propagated into the high Arctic, and its eventual formation of a potential vorticity cutoff in the Arctic marine troposphere.

This case study is nicely detailed, well-illustrated and written, and was a pleasure to read. Comparing the 2021 case against other cyclones whose lifecycles began as heat lows in a similar geographic region is an effective means of putting the case in context. It would be interesting to reconstruct the database further back in time with ERA5 to further add historical perspective to this isolated event, but this is likely beyond the scope of the work.

Given the quality of the overall presentation, comments below are very minor in nature.

Minor Comments

Figure 1: It would be good to list the domain coordinates of the two red polygons, or refer the reader back to the Section 2.1 text, to reiterate those specific areas and what they represent in the caption.

Figure 2: Consider putting number labels also in the map for ease of referencing events 1-5 in particular.

Line 199: Suggest change to “…already exceeds 305K.”

Line 208: Do you mean “very small positive”? Please clarify.

Line 273: Do you have a sense this system had an impact on the early July Arctic sea ice conditions along/near its track (at least in the near term)?

Lines 321-322: Is it remarkable that heat lows propagate away from their genesis points or propagate nearly as far as the 2021 case in particular? Please clarify and consider adding a reference in support of the statement.