Optical emissions associated with narrow bipolar events from thunderstorm clouds penetrating into the stratosphere

Narrow bipolar events (NBEs) are signatures in radio signals from thunderstorms observed by ground-based receivers. NBEs may occur at the onset of lightning, but the discharge process is not well understood. Here, we present spectral measurements by the Atmosphere‐Space Interactions Monitor (ASIM) on the International Space Station that are associated with nine negative and three positive NBEs observed by a ground‐based array of receivers. We found that both polarities NBEs are associated with emissions at 337 nm with weak or no detectable emissions at 777.4 nm, suggesting that NBEs are associated with streamer breakdown. The rise times of the emissions for negative NBEs are about 10 μs, consistent with source locations at cloud tops where photons undergo little scattering by cloud particles, and for positive NBEs are ~1 ms, consistent with locations deeper in the clouds. For negative NBEs, the emission strength is almost linearly correlated with the peak current of the associated NBEs. Our findings suggest that ground-based observations of radio signals provide a new means to measure the occurrences and strength of cloud-top discharges near the tropopause.

Lines 277-281. I think this conclusion is too far-fetched, because negative NBE are not that common (like blue jets). While there may be a link with greenhouse gas concentrations, in my opinion it would be very hard to get meaningful observations that would tell us more about those gases, because there are so many unknowns. I suggest removing this conclusion from the text and also the abstract, since it isn't needed. The NBE observations themselves are impactful enough. If the authors stand by including this conclusion, I suggest expanding on the text and explain how studies relating greenhouse gases and RF detections of cloud-top discharges would to be done. Line 353. Add "data" after "lightning" Line 354. This website is some kind of portal, in Chinese, and it asks for some code or login, which I do not have. I recommend the authors put the data used for this manuscript in a depository that is in English and does not require a login.
Line 355. Correct spelling of "website" Line 433. Correct misspelled word "limpact" Table 1. Suggest sorting all events by increasing time, not decreasing, and not separating out the negative and positive events.  Figure 1a. The radar site should be marked in the top view, because it is very close to being in view of the current plot limits. Suggest making the zoom a little wider, so the radar site can be included and the ISS track is shown longer as well. In addition, add a line in this plan view plot indicating the direction of the radar scan shown in panel b.
Line 532. Correct spelling of "lightning" Line 537. How is the height of the tropopause inferred? Say it is an estimate, or give a reference or other supporting data. Figures 2 and 3. I suggest using a color other than light yellow-green for the VLF waveforms. For instance, use black, dark green, medium blue, and red, for the various traces so the traces are better visible on a white background.
Lines 543-544 and 551-552. "is in line with the pink". No idea what is meant by this. I see no pink lines.

Reviewer #2 (Remarks to the Author):
Review of "Optical emissions associated with narrow bipolar events....stratosphere" This is a good paper which reports new results addressing a long-standing puzzle in lightning science, viz, are NBEs relatively devoid of light emissions (relative to ordinary lightning)? The feat of performing ISS-based multispectral observations of emissions from the tops of thunderclouds is not only impressive but is also tremendously important for the research. Since the negative NBEs take place at the cloud's upper screening layer, only airborne or space-borne optical observations can succeed in viewing the emission region with minimal cloud scattering of light. This would be hard, or likely impossible, to reproduce with ground-based observations. The ground-based observer would see only a temporally-smeared-out and attenuated version of the source emission. It would not be easy from the ground to make a compelling association of that highly stretchedout light with the prompt NBE radio signal. By comparison, the ISS-borne observations reported here have an ideal viewing location, which preserves the fast rise time of the blue emissions from the top of the cloud.
That ability to see the emission region without debilitating cloud scattering has also resulted in the lovely result of Figure 4b, a linear relationship between optical output power and estimated NBE current. That would be impossible without a clear view of the emission region, free of cloud scattering.
A few minor quibbles: Figure 1 caption: "Green dots present the GZ lightning VLF/ELF receiver". Actually not "green dots", but rather a single green triangle. "The pink cross-present..." should be "The pink X's present..." Figure 2 caption: "Sferic E-waveform is in line with the pink..." needs some more explanation. Ditto in Figure 3 caption.
Reviewed by: Abram R. Jacobson (who wishes not to be anonymous) The paper "Optical emissions associated with narrow bipolar events in radio signals from thunderstorm clouds penetrating into the stratosphere" provides some useful information on optical signatures of NBEs. The subject under study is new and the data presented is indeed of interest to the lightning community. The paper confirms some previous theories on the optical emissions from NBEs. There are, however, a few issues that need to be addressed, before the manuscript may be acceptable for publication.
Introduction: Please, mention that NBE is known as Compact intracloud discharges (CIDs) as well. Many papers refer to NBE as Compact intracloud discharges (CIDs) e.g. [1]- [5]. It is important to mention what is the sign convention used in the field records (Atmosferic electricity or Physics sign convention) to say that the NBE is Positive or Negative.
Results: Line 86. This reviewer thinks that the way that is presented the time is confusing. The authors should consider 13:05:56. The same thing happens in other parts of the manuscript. Line 104. Probably, it is a missing character after "Himawari" Line 113. Confusing timestamp Line 137. The first time that the acronym (GZ) is mentioned in the text. Please write up. Discussions: Line 229: Would be "shape" instead of "sharpe"?
Methods: Electric field (E-field) change measurements It is not clear for this reviewer if the NBE electric field waveforms presented in the paper are indeed E or dE/dt. If the multi-station electric-field sensors record dE/dt, how the integration of dE/dt is performed? Throughout the paper, the authors give NBE waveforms as being E-fields, not dE/dt.

Reviewer #1:
This manuscript reports on the simultaneous observation via VLF/VHF and ASIM of positive and negative NBE events in a single thunderstorm. The main conclusion of the paper is a confirmation of earlier studies that both polarities NBE are associated with shorter optical wavelengths, supporting the idea that both phenomena are cold streamer breakdown processes. The data and methods presented in the manuscript are generally sound and convincing. The effect of in-cloud scattering and time-delayed/expanded light emissions is a nice result, as is the simultaneous observation of both polarities NBE. While a nice observation, these results are incremental to our understanding of the physics behind NBE-type breakdown and mostly confirm earlier observations by others. Response: Thanks for your positive comments. We have now thoroughly revised the manuscript based on your suggestions.
Suggestions for improvement are below.
Grammar. The manuscript will benefit from a careful re-read by a native English speaking person, to correct use of articles and other words. Response: We have corrected the typos and rephrased some sentences to improve the readability.
Time stamps. Suggest writing times as "13:06:20.000" etc (with the additional colon) Response: Corrected as suggested.
Do a find and replace all of "lighting" with "lightning" (but check for those cases where you mean to say "lighting", I think there are none in this manuscript.) Response: we replaced all of "lighting" with "lightning".
Lines 72-72. This sentence is vaguely worded and does not convey the point of the studies well. Response: Thanks for pointing this out. We now revised this sentence as "Positive NBEs inside thunderclouds were found to be associated with the 337 nm emissions rather than the 777.4 nm emissions, suggesting they are corona-like discharges formed by many cold streamers".

Response: Corrected
Line 104 (and one other occurrence). Suggest avoiding putting a reference after a number (8). Perhaps add this reference in relation to the satellite in the methods section only. Response: Corrected as suggested Line 107. Add "the" in two places Response: Done Line 108. Intense convective surges: I would really like to see an altitude versus time evolution plot of the radar reflectivies through the core of the storm, with the NBE events overplotted, that clearly shows the strong convective surges. This in itself would be a nice result. Can such a figure be added, perhaps in the supplement section? Response: This is a good suggestion. The evolution plot of the radar reflectivity through the parent thunderstorm can show the detailed info of the parent thunderstorm. We have added it in the supplement section. As shown in Figure R1, it can be seen that the cloud top develops intermittently above the tropopause penetrating into the stratosphere. Interesting to note that negative NBEs are always produced at the overshooting tops (OT) under the strong convective surges. These cloud top discharges are likely related to the blue emissions (e.g., pixies and gnomes) reported by Lyons et al.  Line 136. Correct spelling of "calculable". Or preferably, rephrase as "... and their height also cannot be inferred from the ..." Response: Done Table 1. Suggest sorting all events by increasing time, not decreasing, and not separating out the negative and positive events. Response: We have modified the arrangement of all events by increasing time in Table 1.  Line 532. Correct spelling of "lightning" Response: Corrected.
Line 537. How is the height of the tropopause inferred? Say it is an estimate, or give a reference or other supporting data. Response: The height of the tropopause is inferred from the sounding data. We now add it in the supplementary information. Figures 2 and 3. I suggest using a color other than light yellow-green for the VLF waveforms. For instance, use black, dark green, medium blue, and red, for the various traces so the traces are better visible on a white background. Response: This is a good suggestion. Figures 2 and 3 have been modified for the better visible.
Lines 543-544 and 551-552. "is in line with the pink". No idea what is meant by this. I see no pink lines. Response: Thanks for pointing this out. We have revised the description of the caption. This is a good paper which reports new results addressing a long-standing puzzle in lightning science, viz, are NBEs relatively devoid of light emissions (relative to ordinary lightning)? The feat of performing ISS-based multispectral observations of emissions from the tops of thunderclouds is not only impressive but is also tremendously important for the research. Since the negative NBEs take place at the cloud's upper screening layer, only airborne or space-borne optical observations can succeed in viewing the emission region with minimal cloud scattering of light. This would be hard, or likely impossible, to reproduce with ground-based observations. The ground-based observer would see only a temporally-smeared-out and attenuated version of the source emission. It would not be easy from the ground to make a compelling association of that highly stretched-out light with the prompt NBE radio signal. By comparison, the ISS-borne observations reported here have an ideal viewing location, which preserves the fast rise time of the blue emissions from the top of the cloud.
That ability to see the emission region without debilitating cloud scattering has also resulted in the lovely result of Figure 4b, a linear relationship between optical output power and estimated NBE current. That would be impossible without a clear view of the emission region, free of cloud scattering. Response: Thanks for your great efforts and positive feedback to our work.
A few minor quibbles: Figure 1 caption: "Green dots present the GZ lightning VLF/ELF receiver". Actually not "green dots", but rather a single green triangle. "The pink cross-present..." should be "The pink X's present..." Response: We have revised the caption of Figure 1 under your suggestion. Reviewed by: Abram R. Jacobson (who wishes not to be anonymous)