Thunderstorms near the North Pole

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

 

The manuscript presents measurement results of lightning in the vicinity of the north pole in area above 80 degrees north from recent years. The authors have been impressed by the shear number of lightnings per year for the last couple of years. They, however, claim prematurely it is an effect of the climate change toward warming which is more sustained in the polar region than in lower latitudes.

The authors analyze data ground-based network of lightning detectors that has developed or been upgraded relatively recently. They report lightning data to be homogeneous and reliable since 2015 only. Individual lightnings that have been detected by not less than 6 instruments have been considered in their analysis. According to them this limits the false detection. There were a number of thunderstorm events in the polar region since 2015. They typically occur in July and August. The total number of lightning is dominated by those produced in a single strong thunderstorm in the corresponding season. The graph of total yearly number of lightnings since 2015 shows an apparent significant increase for the years since 2019 compared to the previous 4 years from 2015 to 2019.

To support their analysis the authors run the WRF model in nested version with increasing horizontal resolution. They illustrate how simulated high radar reflectivity coincidences with real detected thunder activity.

The authors hypothesize the increase in thunder activity in the polar region is linked to the thermal contrast between the overheated Siberian land and the still relatively cool northern oceanic surface water. They claim the overheating of the adjacent to the ocean Siberian land is due to the global warming trend. The strengthened thermal gradient between land and ocean, authors say, increases baroclinicity over ocean which facilitates convection which leads to sustained thunderstorms far beyond the normal latitudes.

The has positive and negative sides. The positive side is that the authors want to bring attention to the lightning activity on the polar region and the publication of data should be welcomed. The effort to simulate by atmospheric model thunderstorms in the polar region and to publish the results should also be welcomed. The negative side is that their hypothesis that the regularity of thunderstorms in the polar region in recent years has become an established trend linked to the global warming is premature. If their hypothesis is to be supported by facts the authors should find a way to demonstrate that the heating over land is well sustained and will be more frequent in the future regardless of the predominant circulation pattern for a given summer season, or that circulation patterns in the polar region have shifted towards more favorable for land heating and thus stronger thermal gradients and more convection. In summary, the authors should make a significant effort to support their hypothesis the recent regularity of strong thunderstorms once per year in the polar region is directly linked to the climate warming. Intuitively it is but objectively it needs stronger proofs.

Comments on the Quality of English Language

 

The English is good but need moderate editing. Examples follow:

line 15: “ … evidence in an increase …” should probably be “ … evidence of increase … “;

line 64: “ … were not taking into account … “ should rather be “ … were not taken into account … “;

line 145: “ … 38C … “ should be “ … 38 °C …” although it is not related to the quality of the English language.

Author Response

Thank you for your comments. Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript presents lightning measurements over the Arctic region provided by WWLLN and simulations of thunderstorms using the atmospheric model WRF. The topic of the increase of lightning in the Arctic is generating a lot of interest in the scientific community. Lightning if the Arctic region can produce wildfires and permafrost soil carbon release.

The manuscript is well written. However, I am struggling to identify the novelty of this work and I do not understand why simulations with WRF are used, as they are not necessary for conducting a synoptic study that can be performed using meteorological data or reanalysis data without resorting to simulations that may introduce uncertainty into the results. In addition, the increase of lightning in the Arctic region from WWLLN data was already reported by previous studies.

Apart from these main concerns, I have other minor points:

- Line 19: In Table I, you show thunderstorms lasting for more than several hours. This is not consistent with this sentence. Maybe you should define what you consider to be 1 thunderstorm.

- Lines 58-60: Please consider showing the WRF options in a table. Please provide an explanation to justify the use of these options.

- Figure 1: What are the numbers provided in the text inside the figure? Are they the most active thunderstorms? But you claimed that the longest thunderstorms lasted about 55 min.

- Line 96: This sentence seems to be cut.

- Figures 3-6: Consider adding a grid in panels 1 and 2 for a better comparison with panel 3.

- Line 103: Where is CAPE shown?

- Lines 109-112: I do not understand why you use WRF to come to these conclusions. You could simply analyze the meteorology from reanalysis.

- Lines 131-132: Can this be due to a bias in the modeling to temperature?

- Lines 151-152: This is somehow known from previous studies [https://doi.org/10.1038/s41558-021-01011-y, https://doi.org/10.1038/s41467-023-36500-5, https://doi.org/10.1038/s41561-023-01322-z]. Could you quantify the proposed increase? This could add value to the study.

Author Response

Thank you for your comments for our work. Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

In the paper interesting calculations are presented for the North-Pole weather system at the time lightning was observed. It is evident that at the time of lighting there was an extreme heatwave in northern Siberia, and it appears logical to relate the two. However, the calculations presented in this work do not clearly prove this relation.

The figures 3-6 show the results of the weather simulations as well as the measured lightning activity. A detailed comparison of the right panels with the middle one shows that sometimes the intense rainfall indeed correlates with lightning, but there are also many cases where lightning is observed and no cloud cover is calculated, or where a dense cloud cover does not show lightning. In this respect, since the focus of the work in on lightning, it would be more useful had a lightning index be calculated (see for example: https://doi.org/10.1029/2023EA003104  for rather different terrain though. Also from other studies there is evidence that the product of CAPE and precipitation might be a good indicator, see also DOI: 10.1029/2021JD035621 for influence of terrain. There may, however, be better, more appropriate, references than these two I found.)

In the conclusion it is argued that: “As a result of strong heating of the land surface in the Siberia region, strong convective systems develop due to the enhanced baroclinic instability that develops between the hot land areas and the cold Arctic ocean.” Again, this seems entirely reasonable, however, I do not see this back in the results that have been presented. Looking in detail at the results presented in Fig 5 for example, I see a wind vortex off the eastern coast of Nova Zembla, indicating a pressure low if I am correct, with some associated lightning, but appears also lightning off the coast between Greenland and Canada, clearly not associated.

In a revised version these seeming discrepancies should be addressed. It would also be good to note if there have been occasions of heatwaves in Northern Siberia without associated lightning activity over the North Pole.

Author Response

We really appreciate your attention paid for our work. Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

 

There is too much hypothesizing in this paper.

Line 141-154 is a good example: „ All storms are associated with a strong warming of the North Siberian Lowland and other northern parts of Eurasia (up to 35°C) in the summer months.”

If all storms are associated with something unusual does not necessarily mean the thing causes them and that such thing will always cause thunderstorms and that this is the only thignn that causes them.

“This is very unusual for such northern territories, however, appear in 4 of 5 summer seasons since 2019.”

That a thing appears in 4 out of 5 summers does not mean it has an increased frequency in climatological sense. It can be due to a natural variability of the climate. Longer time periods are needed to claim a climate trend.

On the other hand, the Arctic Ocean surface still remained very cold. Hence, the electrification processes in clouds over the ocean surface couldn’t be effective at these low temperatures. This extreme warming over the land, with cool temperatures over the Arctic Ocean, produce an enhanced north-south temperature gradient that increases the baroclinic instability over the Arctic ocean north of Siberia.

“This enhanced temperature gradient produces an enhanced polar jet stream over the Arctic”

The statement of this relation needs illustration with data or mdelling or at least a reference to publications.

“ that results in the genesis of storms that produce significant amounts of lightning due to the instability that develops in these storms.”

The electrification of clouds and thus lightning is due to friction of particles in the overturning air parcels in a convective cloud. The convection is due to instability. This is supposed to be a scientific paper and all statements should be scientifically sound and impecable.

“ Hence, the extremely warm temperatures over northern Siberia drive the formation of storms over the Arctic,”

This is a repetition of a statement above.

“and hence impact the polar lightning north of 80 degrees. If such summer warmings continue to appear and expand in the coming years, we predict even more thunderstorms detected in the polar regions, and close to the North Pole.“

This “prediction” is too simplistic. Why more? More in what sense? More by year and season because of more thunderstorms or more by storm because of more intense convection, or both?

Figure 1 shows statistics for 8 year and this is not enough to establish a climate trend. We can not see from the paper how things were before. May there thunderstorms before as well.

The recent warming trend from ERA5 in the new figure does not prove the existence of a trend in thunderstorm and lightning activity.

The paper still needs serious reorganization to clean itself from speculative statements related to climate trends. The authors should remain in analyzing the data they can have and abstain form generalizing to climate level. Elucidating the causes of thunderstorms in the Arctics by model simulations and presenting lightning data from the recent years is enough of contribution for the paper to be published.

Comments on the Quality of English Language

Only minor language editing can still be needed.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have satisfactorily answered to the most of my questions. I recommend this paper for publication. However, the references are removed from the last version of the manuscript and should be added before publication.

Author Response

Thank you very much for the comments. We have checked the references

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have addressed all my comments.

Author Response

Thank you for your comments. It helped us to improve the paper.