Polarity transitions of narrow bipolar events in thundercloud tops reaching the lower stratosphere

Domain: earth_science
Published: 2024-08-26
Editorial outcome: minor_revision

Abstract
Blue corona discharges are often generated in thunderclouds penetrating into the stratosphere and are the optical manifestation of narrow bipolar events (NBEs) observed in radio signals. While their production appears to depend on convection, the cause and nature of such discharges are not well known. Here we show the observations by a lightning detection array of unusual amounts of 982 NBEs during a tropical storm on the coastline of China. NBEs of negative polarity are predominantly observed at the cloud top reaching the stratosphere, and positive NBEs are primarily at lower altitudes. We find that the dominant polarity changes with the typical time of development of thunderstorm cells, suggesting that the polarity depends on the phase of the storm cells. Furthermore, we find that the lightning jump of negative NBEs is associated with above-anvil cirrus plumes of ice crystals and water vapor in the lower stratosphere. We propose that variations in updrafts induce changes in the altitude and charge concentrations of the cloud layers, which lead to the polarity transition. Our results have implications for studies of the chemical perturbations of greenhouse gas concentrations by corona discharges at the tropopause.

Introduction
Blue corona discharges are often observed within, or close to, the top of thunderclouds1–4. They are bursts of streamers from localized, fast (few 10 μs) emissions of blue light in cloud tops (pixies). They may extend upwards forming blue cones of light reaching 25–50 km altitude (blue starters and blue jets)1,5,6 and even reach the ionosphere at about 80 km altitude with a flash of red light (gigantic jets)7–9. They are commonly produced in deep convective storms with cloud tops in the lower stratosphere3,10. Understanding how they are initiated and under what conditions has implications for understanding their perturbations to the concentrations of greenhouse gases such as nitrous oxide (N2O) and ozone (O3) in the tropopause regions11–13 and the influence of these natural processes on Earth’s radiation balance13–15. Recent measurements from space suggest that blue corona discharges in the upper regions of thunderstorm clouds are closely related to high-altitude compact intra-cloud discharges, also named narrow bipolar events (NBEs)16–20. These are 10–30 μs radio pulses in wideband electric field records21–24. NBEs have been described as a short-channel (few 100 m) “bouncing wave” phenomenon from observations and modeling of their broadband signatures and the accompanying fine structure25–28. The production mechanism of NBEs has been associated with the so-called fast positive and negative breakdown in which streamers propagate at speeds of 107–108 m/s29–31. The exact nature of the fast breakdown that produces NBEs and can move at >107 m/s through virgin air remains a mystery. There are recent attempts to model NBEs and explain their electromagnetic signatures that consider fast breakdown and the “rebounding” observed by Rison et al. 29 using modified transmission line models.

NBEs are classified as positive or negative according to the orientation of the associated current pulse22,32,33. Similar to the polarity asymmetry in normal lightning, NBE polarities exhibit distinct differences32,34. The sources of negative NBEs are usually observed to be close to the cloud tops, and the positive ones are deeper in the clouds18,19,23,32. NBEs radiate intense electromagnetic waveforms at very low/low frequencies (VLF/LF) and emit the strongest very high frequency (VHF) radio signals in nature. The VHF pulses can penetrate the ionosphere and therefore be detected both by satellites and ground receivers35,36. In this respect, radio observations of NBEs are now offering a new means to observe the properties and conditions for initiation of blue corona discharges18.

Here we show a storm with a high number of 672 negative and 310 positive NBEs. The storm is at a location that allows for the detection of NBEs by a dedicated ground-based sensor network and cloud parameters by a weather radar close to the center of the electrical activity. With simultaneous observations of cloud properties from space, the data provide an unprecedented opportunity to understand how cloud properties affect the generation of discharges. We find that the dominant polarity of NBEs depends on the phase of the storm cells and propose a conceptual model of the dynamic distribution of charge regions in clouds.

Results
The storm and the NBEs
On 30 July 2017, the tropical storm Haitang arrived on mainland China at 18:00 UTC, with maximum wind speeds reaching 20 ms−1. The infrared (IR) brightness temperature derived from the geostationary meteorological satellite Himawari-837 is shown in Fig. 1a with the storm track. In the rain bands close to the coastline, a convective region with cloud-top temperatures as cold as 185 K and below has developed. The measurements of a weather radar located on the coast presented in Fig. 1b show that the clouds reach several kilometers into the stratosphere. The region has high lightning activity measured by the Global Lightning Detection Network (GLD360), and the domination of negative cloud-to-ground (−CG) and positive intra-cloud (+IC) discharges suggest a normally electrified storm with a main negative charge region and an upper positive region38.

NBEs are observed by ground-based lightning detection sensors at locations marked with green triangles in Fig. 1a. An example of a negative NBE is shown in Fig. 1c. The electric field waveform is recorded by four radio stations that are stacked with distance to the NBE. The radio signal shows the typical NBE signature of negative polarity. It is characterized by a single bipolar pulse propagating directly to the receivers (t = 0) and two ionospheric reflection pulses. The time delay of the main pulses allows estimation of the source location and the delay between the main pulses and the reflected ones helps estimate the source height39,40. The estimated source altitude (19.5 km) and distance lead to the modeled reflection times shown by the solid red lines.

Alternated dominance of negative and positive NBEs
During the storm's landfall from 16:00 UTC to 20:00 UTC, 672 negative and 310 positive NBEs were observed (see “Methods”, subsection E). To our knowledge, there is no literature report of the outburst of negative NBEs observed in a tropical storm. As shown in Fig. 2a, the sources of positive NBEs are primarily between 10 km and 17 km and most of the negative NBEs are above the tropopause, suggesting a normally electrified storm with the main negative and positive charge regions, which is consistent with prior NBE observations32,39. The inferred negative charge layer is close to the −40 °C isotherm as measured by the radiosonde from the non-inductive charging mechanism41,42. The positive charge region was near the tropopause as suggested by the height distribution of NBE polarities shown in Fig. 2b, which is higher than generally found in normally electrified thunderstorm32,43. The high altitude of the upper positive charge region may contribute to the outburst of negative NBEs44.

From Fig. 2a, we note that the dominant polarity of NBEs appears to alternate with time. Positive NBEs occur at the beginning of the storm, peaking just before 17:00 UTC where negative NBEs take over. Between 17:10 UTC and 17:40 UTC, positive NBEs occur sporadically whereas negative NBEs occur at a high rate. This alternating pattern continues throughout the sequence. This is further illustrated in Fig. 2c, which presents the number of NBEs, positive and negative in 5-min bins, with time.

We further quantify the relation between positive and negative NBEs by considering cells of 25 × 25 km2 and 5-min consecutive time bins. In each space-time cell, we determine the proportion of the dominant number of positive or negative Polarity to the total number (PDP), which is defined as the number of NBEs of the dominant polarity to the total number of NBEs43,45. A larger PDP indicates a larger difference in the proportion of positive and negative NBEs, suggesting greater competition between positive and negative NBEs during a given period (see “Methods”, subsection definition of PDP). We require a minimum of five events and at least one negative polarity. In all, 115 values are determined. A histogram of PDP values is shown in Fig. 2d. More than 80% of the space-time cells have PDP-values greater than 0.75, suggesting that one polarity dominates and the strong competition relation of NBEs polarities.

To support that the relation between NBE polarities is general, we have analyzed additional thunderstorms recorded by the Guangzhou (GZ) station for one year in the low-latitude region. On average, the overall occurrence rates of NBEs are about 1.8%. A histogram of PDP values is shown in Supplementary Fig. 1. More than 75% of the space-time grids have PDP values greater than 0.75, suggesting that the competition behavior of NBEs polarities is general.

Above-anvil cirrus plumes (AACPs) with negative NBEs
To understand how the dominant polarity depends on cloud dynamics, we analyze the evolution of the IR brightness temperature provided by the Himawari-8 satellite from 16:50 UTC to 17:50 UTC. This period shows the clearest variations in the dominant polarity and has the least complexity in the evolution of the cloud top temperatures. Fig. 3a shows the cloud top temperature at 17:20 UTC with the NBE activities superimposed. The cloud exhibits a unique temperature shape with a cold ring region encircling a warmer region. This signature suggests that the warm region is an AACP, which is formed by deep convection updrafts lifting clouds into the stratosphere46–50. AACP are indicators of severe storms and stratospheric hydration events49,51. We see that negative NBEs occur near the warm AACP region where only a few positive NBEs are produced as shown in more detail in Fig. 3b. A radar scan at 17:15 UTC and 17:21 UTC by the weather radar is shown in Fig. 3c, d, with the altitudes of NBEs around the scan time superimposed. The measurements confirm that the clouds reach above the tropopause and that the warm region is an AACP.

For each negative NBE, we determine the cloud top temperature at its location and estimate the maximum AACP temperatures. Fig. 3e compares the 10-min averages of the negative NBE altitudes and temperatures to the AACP temperature. We see that when the AACP temperature increases, the height of negative NBEs decreases and their average temperatures also increase, suggesting that they preferentially occur at the edge of the AACP as it expands.

To investigate this further, we consider a region box of 80 × 6 km2 around the NBE activity. We label the region within the box with temperatures below 195 K (the tropopause temperature) as the convection cold ring region and the region with higher temperatures as the warm AACP region. The development of the AACP is illustrated in Fig. 3f, which show the areas of the two regions as functions of time. We note that the rate of negative NBEs increases as the warm AACP region develops, whereas positive NBEs are almost extinguished, suggesting a significant dependence of the dominant polarity on the convective strength and phase of storm cell development.

Convection phase of the dominant polarity NBEs
To understand how the dominant polarity depends on the convective phase inside the cloud, we analyze the S-band Doppler radar (see “Methods”, subsection radar observation). Figure 4a presents the evolution of the maximum height of different reflectivity during the polarity transition from 17:40 UTC to 18:10 UTC. We see that positive NBEs are located relatively deeper in the cloud, and negative NBEs occur in the region with reflectivity 10–15 dBZ near the overshooting top, which is consistent with previous observations44,52. The convection stage of overshooting tops (OT) can be separated into two stages: upwelling and decaying47. We define the overshooting convective phase as an upwelling stage if the maximum altitude of 40 dBZ gradually increases and finally reaches beyond the tropopause and a decaying stage if the maximum altitude of 40 dBZ decreases and is limited to altitudes more than 3 km below the tropopause53.

From Fig. 4a, we note that the convective development inside the cloud shows a significant difference for the outburst of negative NBEs than for positive NBEs. The outburst of positive NBEs occurs while negative NBEs are gradually inhibited during the upwelling stage, where the height of 40 dBZ gradually increases reaching an altitude above the tropopause. In particular, as shown in Fig. 4b, negative NBEs surprisingly disappear when the height of 40 dBZ reaches above 17 km, which is out of the conventional view that negative NBEs are always produced in OT44,52. In contrast, negative NBEs outburst while few positive NBEs occur during the decaying stage, where the height of 40 dBZ decreases and generally stays below 13 km as shown in Fig. 4c. Moreover, we also determine the convection development from the volume of reflectivity above 35 dBZ at altitudes higher than 10 km (V35dBZ) and the area of reflectivity above 35 dBZ (S35dBZ) around NBEs activity (see “Methods”, subsection estimation of the total volume and area of reflectivity). The variations of S35dBZ and V35dBZ as functions of time are illustrated in Fig. 4d. We see that the rate of positive NBEs increases as the S35dBZ and V35dBZ develop, whereas negative NBEs are almost extinguished. This suggests that the variations in altitudes of large reflectivity induced by the convection phase play a major role in the generation of dominant polarity NBE.

We further analyze another supercell thunderstorm associated with dominant negative NBEs (see Supplementary Fig. 5), which has been investigated to reveal the connection between negative NBEs and blue discharges17. The positive and negative NBEs also exhibit a strong competitive relationship. Negative NBEs occur intermittently when the cloud top reaches above the troposphere and is associated with the low altitude of the large reflectivity (see Supplementary Fig. 6), which is similar to the above tropical storm case.

A conceptual model for the polarity transition
The aforementioned analysis suggests that the convection strength and phase play a crucial role in the production of the dominant polarity NBEs. As suggested in radar observations, negative NBEs prefer to occur in the decaying stage with a low altitude of large reflectivity while positive NBEs outburst in the upwelling stage with a high altitude of large reflectivity in overshooting thunderclouds. This is consistent with the IR brightness temperature analysis that the rate of negative NBEs increases with the development of the warm AACP region while positive NBEs outburst with the growth of the cold ring region.

We propose a conceptual model that the charge distribution induced by the convective phase is responsible for this result, as illustrated in Fig. 5. In a normally electrified storm, the updraft carries super-cooled droplets and small ice crystals upwards while graupel, which is considerably larger and denser, tends to fall54. These different motions of the precipitation would cause collisions to occur. When the rising ice crystals collide with the graupel, the ice crystals become positively charged and the graupel becomes negatively charged, which gradually accumulates a main negative charge layer and an upper positive charge layer38,42. Yoshida et al. 55 examined the 3-D lightning location and radar data to investigate the relationship between thunderstorm electrification and storm kinetics. They found that the upper positive charge layer is carried in the updraft with low radar reflectivity while the main negative charge region is located in high reflectivity regions.

Positive NBEs are generally produced between the main negative charge layer and the upper positive charge layer under the stage of the cloud below the tropopause as shown in Fig. 5a. Figure 5b depicts the inferred charge distribution for the outburst of positive NBEs during the upwelling stage of OT. The strong updraft drives the cloud top into the tropopause. Under the intense updraft, the heavier graupel carrying negative charges would be lifted to a higher height as inferred from the high height of large reflectivity in Fig. 4b. Values of reflectivity > 35 dBZ typically indicate the presence of graupel-sized particles56. The higher negative charge layer would narrow the gap with the upper positive charge layer, enhancing the ambient electric field. Moreover, the breakdown threshold is lower because of less pressure and humidity for higher altitudes57, facilitating the initiation of positive NBEs. Meanwhile, ice crystals with the upper positive charge layer are elevated to a very high altitude, which would cause strong mixing with the screening charge layer, making it difficult for the inception of negative NBEs8.

Figure 5c illustrates the inferred charge distribution for the outburst of negative NBEs during the decaying stage of OT. When the convection gradually decays, the larger and heavier graupels, carrying negative charges, fall into the mixed-phase region, while small ice crystals with positive charges can still be at a high altitude58, as inferred from the lower altitude of large reflectivity in Fig. 4c. The distance between the upper positive charge layer and the main negative charge layer would then increase, resulting in a lower electric field in this region. This would make it difficult for positive discharge to occur59. Meanwhile, the cloud top above the tropopause would accelerate the formation of the screening charge layer59,60, and the high upper positive charge and the enhancement of the electric field by the accumulated positive charge make it easier for negative NBEs to win the competition for the upper positive charge layer. Moreover, the reduced intra-cloud discharge may lead to further accumulation of the main negative charge layer, which would make it easier to produce more negative CGs. This may explain why frequent negative CGs were observed during blue discharges17,19.

Methods
Ground-based sferics observations
The VLF/LF sferics signal is measured by receivers of the Jianghuai Area Sferic Array (JASA). Each station is equipped with a vertical E-field antenna (with 3dB bandwidth of 800–300 kHz) and records dEz/dt (time derivative of the vertical electric field) for each lightning event. All recordings are synchronized with a GPS clock with a sampling rate of 5 MHz. The atmospheric electricity sign convention is used for the E-field sensor; that is, positive NBEs occurring between the upper positive charge layer and main negative charge layer produce the negative initial peak while negative NBEs occurring between the upper positive charge layer and screening charge layer produce the positive initial peak.

Identification and detection efficiency of NBEs
NBEs are characterized by a shorter duration (<40 μs) of the VLF/LF signal compared to normal lightning. They are always associated with ionospheric reflected waves at distant stations, especially during the nighttime. We adopt the criterion of the short duration of the pulse (40 μs), the reflected wave in distant stations, and the high signal-to-noise ratio to identify NBEs. Each selected event is manually inspected to avoid misidentification.

Data from a local lightning network consisting of 19 Vaisala LS7000 sensors, the GLD360 network, and JASA are combined to determine each NBE location. We first use the identified NBEs to match the location provided by GLD360 and the local lightning array. Nearly 80% of NBEs in the tropical storm recorded by VLF/LF stations can be matched with the local network and GLD360. If missed, we then use at least four VLF/LF stations to locate the event. Although the VLF/LF stations are located 400–1000 km away from the tropical storm, which would cause considerable attenuation of direct ground waves of NBEs signal, the signal can also be triggered by obvious ionospheric sky-waves (shown in Fig. 1c). The location error for the tropical storm in this paper is within 5 km. To evaluate the detection efficiency for the observed NBEs, we present the peak current distribution of the matched NBEs shown in Supplementary Fig. 7.

Error evaluation for the NBE height
The uncertainty of the estimated NBE heights mainly originated from the error in the source location and the reflected pulses. The sampling rate of the radio receiver is 5 MHz/s (0.2 μs point-to-point), and the identification of the pulses of the reflected signal is manually checked to avoid misidentification. To estimate the error of the NBE height, we performed the following test. For a typical NBE, up to 5 km and 1-μs are intentionally added in 2-D location and reflection pulses. Under such conditions, the results suggest that the maximum error in altitude is no larger than 0.8 km variance from the original altitudes (see Supplementary Fig. 8). It should be noted that the height of NBEs is derived by assuming that it is a point discharge, regardless of the channel length.

IR brightness and radar observation
The IR brightness data are obtained from the geostationary meteorological satellite Himawari-8, which has ten IR channel bands ranging from 3.9 μm to 13.3 μm. The window-IR (IR; 10.4 μm) channel is used to sense the cloud top temperature with a temporal resolution of 10 min.

Radar observations are derived from an S-band radar. The beamwidth of the radar is ≤1°. The detection height includes nine layers of elevation angles ranging from a minimum of 0.5° to a maximum of 19.5° and a volume scan on a 6-min cycle. The distance between the radar and the storm in most of the life cycle varies from 80 km to 120 km, corresponding to measuring the vertical height from 2 km to 22 km. The detection precision for the horizontal reflectivity (ZH) is 1 dB. Radar coordinate data are processed by spline interpolation into a Cartesian grid with a horizontal resolution of 0.25 × 0.25 km and a vertical resolution of 0.25 km.

Definition of PDP
To quantify the relation between positive and negative NBEs, we define the PDP:

PDP = max(+NBE, -NBE) / Σ(+NBE, -NBE)

A larger PDP indicates a larger difference in the proportion of positive and negative NBEs, suggesting greater competition between positive and negative NBEs during a given period. For example: 10 NBEs, including 8 positive and 2 negative NBEs, are generated in a period. The value of PDP in this period is PDP = max(8, 2) / Σ(8, 2) = 8 / 10 = 0.8. Note that we only calculate the value of PDP during the period when the sum of NBEs is larger than five and at least one negative polarity.

Estimation of the total volume and area of reflectivity
According to the size of the storm cell producing NBE activity, we restrict a region box of 30 × 40 km2 around the NBE activities. We first establish a spatial Cartesian coordinate system in the box and project the radar volume scan data onto the coordinate. Then, the spatial area with an altitude between 10 km and 22 km is divided into cubic grids with a resolution of 0.5 km. The nearest neighbor interpolation method is applied to obtain the radar reflectivity of the center point of each grid. Finally, we count the number of grids with radar reflectivity greater than 35 dBZ to calculate the value of V35dBZ and put these grids onto a horizontal plane to obtain the value of S35dBZ.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of the article titled \"Altitude and polarity transitions of corona activity in thundercloud tops reaching the lower stratosphere” submitted for publication in Nature Communications by Liu et al.\nThe article under consideration presents a meteorological analysis of a storm producing copious amounts of Narrow Bipolar Events (NBEs). This reviewer does not find this article suitable for publication in Nature Communications due to the following reasons:\n1. The findings lack novelty and potential impact. The key findings regarding the fact that positive and negative NBEs have a bi-modal altitude distribution, facilitated by distinct storm charge structures, have already been discussed in the peer-reviewed literature. The NBE polarity convention used has to do with the sign of field change.\n2. On the one hand, the mechanism to explain the shift in the dominant NBE polarity due to the evolution of convection during the storm lifetime seems obvious and, thus, quite reasonable. On the other hand, it does not explain the observed temporal dynamics unambiguously. The conclusions are largely dependent on indirect measurements of the storm’s electrical structure.\n3. The conclusions are based on the analysis of a single storm, and there is no evidence to suggest that the conceptual model should be general. The only other example provided in the Supporting Information does not seem to corroborate the conclusions. Additionally, the Extended Data Figure 1 suggests that the model should not be one-dimensional, i.e., that there are two horizontally-separated regions where NBEs of different polarities are produced.\n4. The manuscript lacks a contemporary understanding of NBE physics. NBEs are produced by large and fast streamer systems of both polarities. Particularly, even a single NBE polarity, let’s say positive NBEs, can be produced by streamer discharges of both polarities.\n5. All potential connections between NBEs and jets mentioned in this manuscript have been previously discussed in the peer-reviewed literature. Including in papers overlooked in the authors’ reference list.", "reviewer_2": "Review of the manuscript entitled “Altitude and polarity transitions of corona activity in thundercloud tops reaching the lower stratosphere“ by Liu et al.\nThe authors present a detailed analysis of a thunderstorm producing enormous amount of narrow bipolar events (NBE). The authors show the changing prevalence of one polarity and place it in the changing properties of the storm cells evolution.\nThe manuscript is well written, presents new observations, and a comprehensive analysis. The authors effectively combine electromagnetic measurements with radar observations, the cloud properties are derived from the satellite measurements.\nThe results are original and worth to publish, the work is significant in the field of atmospheric electricity and related meteorology. The methodology is quite well described. There is enough details the authors provided to support their statements.\nI think that the manuscript needs some improvements and clarification before it can be considered for publication in the Nature Communications journal. My comments are listed below.\n1. Title: From the title (and also when looking at the names of co-authors from the ASIM group) I would expect the manuscript contains also the optical data confirming the existence of corona by observation of blue discharges. As this is not the case, and the presence of corona is based on simultaneous previous observations of NBEs and blue discharges, I would ask the authors to consider the modification of the title accordingly.\n2. You state that “high lightning activity measured by the GLD360 network, and the domination of negative cloud to-ground (-CG) and positive intra-cloud (+IC) discharges…” but you did not present anything about these discharges. It would be interesting to see what was their distribution in time and space in comparison with the NBEs of both polarities.\n3. You discuss the changes in the charge arrangement leading to the NBE polarity switching. It would be useful to formulate clearly what generally caused the observed enormous amount of NBEs and how often this can happen, especially if you want to use the presence of radio signals emitted by NBEs as an indicator of an exchange of greenhouse gases between the troposphere and stratosphere.\n4. Methods: The estimation of the altitude of NBE is a crucial part of the work. I would suggest to evaluate more precisely the error in the altitude calculation.¨\n5. Fig.4a: I do not understand why the NBEs appeared only in groups which are separated by approx. 1 min of nothing, this cannot be natural, please explain.\n6. I is quite confusing that you use in different figures different time intervals, if there is a reason for such approach, could you explain it properly?", "reviewer_3": "This is a well-written paper. I have a few comments that the authors need to consider and address in the revised version:\n1. Lines 83-83: Kindly reword this sentence, I recommend inserting: “helps estimate” after “….and the reflected ones”.\n2. Lines 85-86 and Figure 2a. It appears that a significant number of the negative NBEs are above the troposphere. The authors should note here that previous observations have also reported above-tropopause NBEs in “convective surges (plumes) overshooting the tropopause” such as Smith et al. (2004) doi:10.1029/2002RS002790 and Nag et al. (2010) doi:10.1029/2009JD012957.\n3. Line 107: The acronym PDP and its meaning is not introduced before in the text.\n4. Lines 109-111: It is difficult to understand the significance of this finding without further explanation of what PDP means and how values greater than 0.75 is suggestive of \"strong competition\".\n5. Lines 196-197: Need to discuss the reasoning for this statement regarding the lower breakdown threshold. Is it because of changes in atmospheric composition, pressure, and temperature? Perhaps need a reference like Riousset et al. (2020) doi.org/10.1016/j.icarus.2019.113506?\n6. Line 210-213: What about the occurrence of \"regular\" ICs between the main negative and positive charge regions. They would also play a role (perhaps a more dominant one) than the negative CGs in this charge balance? Please clarify here."}, "author_response": "Point-by-point Response to Reviewer’s Comment\nManuscript number: NCOMMS-23-46957-T\nManuscript Title: Altitude and polarity transitions of corona activity in thundercloud\ntops reaching the lower stratosphere\nReviewer #1:\nReview of the article titled \"Altitude and polarity transitions of corona activity in\nthundercloud tops reaching the lower stratosphere” submitted for publication in Nature\nCommunications by Liu et al.\nThe article under consideration presents a meteorological analysis of a storm producing\ncopious amounts of Narrow Bipolar Events (NBEs). This reviewer does not find this\narticle suitable for publication in Nature Communications due to the following reasons:\nWe appreciate the referee’s insight and thoughtful comments. Although the other two\nreferees provided a very positive evaluation on this work and suggested that it could\npotentially have a high impact on the geophysical research community, we now further\nemphasize the novelty of this work in the revised version to clarify and address each of\nthe comments and concerns you have raised. Below is our point-by-point response to\neach comment.\n1. The findings lack novelty and potential impact. The key findings regarding the fact\nthat positive and negative NBEs have a bi-modal altitude distribution, facilitated by\ndistinct storm charge structures, have already been discussed in the peer-reviewed\nliterature. The NBE polarity convention used has to do with the sign of field change.\nResponse: We agree with the referee’s point that several studies have reported the\naltitude distribution of NBEs, and those papers are appropriately cited in the\nintroduction. It should be noted that prior observations, mainly conducted with respect\nto thunderstorms at middle and high latitude, generally focused on positive NBEs in the\ncloud (Lv et al., 2010; Nag et al., 2010; Wu et al., 2011, 2012; Karunarathne et al., 2015;\nBandara et al., 2023; Li et al., 2023). A latest study by Chen et al., (2024) present five\nclusters of upper-level compact intracloud discharges (CIDs) in overshooting tops.\nBased on the data analysis and theoretical work, they infer that upper-level CIDs were\nlikely made possible by the recurring action of energetic cosmic rays and the rapid\nrecovery of the negative screening charge layer. However, what meteorological and\nelectrical conditions of thunderstorms produce the outbreak of negative NBEs on cloud\ntop and the evolution of both polarities at such condition are still unclear.\nTo our knowledge, there has been no literature ever reported NBEs polarities\nrelationship. In our work, we primarily focus on the evolution of NBEs polarities in a\ntropical storm with an extraordinarily high number of negative NBEs. It is the first\nstudy that shows the dominant polarity changes with time during a storm. The novelty\nand broad impact are summarized as follows:\nThe novelty:\n1. It is the first time we observe an unprecedentedly large number of negative corona\ndischarges in a tropical storm cloud top penetrating the stratosphere.\n2. We first show the dominant NBEs polarity changes with time during a storm, and\npresent the strong competition relationship between NBEs polarities in\novershooting thundercloud, as well as their polarity generation depending on the\nstorm cell development phase is reported.\n3. The lightning jump of negative NBEs is found to be closely associated with the\noccurrence of above-anvil cirrus plumes (AACP) in the lower stratosphere.\nThe broad impact:\na) Our results provide new insight into the generation of cloud top discharges from\nthe view of the dynamic development of convection. Theoretical work based on the\nquasi-electrostatic field simulation predicts that the generation of cloud top\ndischarges depends on a sudden charge imbalance by normal lightning. The\ncompetition of NBEs polarities in our finding suggests that the strength degree of\nupdraft would play a significant role in the initiation of cloud top discharges, which\nprovide constraints on their generation mechanism.\nb) The proposed conceptual model provides a unified illustration of the required\nconvection phase for cloud top discharge generation. The implication is that we\nmay, in the future, develop proxies for their generation rate in global storms\nmeasured by the new generation meteorological satellites to predict the regional\nand global effects they have on the lower stratosphere. From that perspective, this\noffers the possibility to evaluate the global impact of cloud top discharge on the\ngreenhouse gas agents at the tropopause.\nc) The close relationship between the occurrence of AACP and the outbreak of NBEs\nin our findings suggests that ground-based observations of radio signals would\nprovide a new means to measure the impacts on the concentrations of greenhouse\ngases in the upper troposphere and lower stratosphere.\nInspired by the reviewer’s comments, we realized that we did not clearly present the\ninnovation. We have now changed the title and revised the text to emphasize the novelty\nand broad impact.\nPlease see revised manuscript (Lines 1-2, Lines 242-246, and Lines 278-284):\nChange of title: Polarity transitions of narrow bipolar events in thundercloud tops\nreaching the lower stratosphere.\nOur results provide new insight into the mechanism of cloud top discharges from the\nview of dynamic development of convection. We emphasize the importance of\nconvection phase on the initiation of cloud top discharges. The competition of NBEs\npolarities in our findings suggests that the height variations of the upper positive charge\nlayer induced by the strength of updrafts would play a significant role in the initiation\nof cloud top discharge types.\nThe conceptual model provides a unified illustration of the required convection phase\nfor cloud top discharge generation. The implication is that we may, in the future,\ndevelop proxies for their generation rate in global storms measured by the new\ngeneration meteorological satellites to predict the regional and global effects they may\nhave on the lower stratosphere. From that perspective, this offers the possibility to\nevaluate the global impact of cloud top discharge on the greenhouse gas agents at the\ntropopause.\nAdd the reference: Chen, S., Rakov, V. A., Zhu, Y., & Ding, Z.Clusters of compact\nintracloud discharges(CIDs) in overshooting convective surges. J. Geophys. Res.\nAtmos.,129, e2023JD040307 (2024).\n2. On the one hand, the mechanism to explain the shift in the dominant NBE polarity\ndue to the evolution of convection during the storm lifetime seems obvious and, thus,\nquite reasonable. On the other hand, it does not explain the observed temporal dynamics\nunambiguously. The conclusions are largely dependent on indirect measurements of the\nstorm’s electrical structure.\nResponse: Thanks for the support of the proposed conceptual model. We agree with\nthe reviewer’s point that the electrical structure in the conceptual model is inferred from\nthe indirect measurements of radar and NBEs distributions. It should be noted that many\nobservation and simulation results have suggested that thunderstorm electrification is\nrelated to storm kinetics parameters such as graupel volume and updraft from the radar\nobservations (Carey and Rutledge,1996; Bruning et al., 2010; Calhoun et al, 2013,2014;\nMacGorman et al., 2017). Yoshida et al., (2017) examined 3-D lightning location\nBOLT data and radar data to investigate the relationship between thunderstorm\nelectrification and storm kinetics. As shown in Figure R1, they found that BOLT\nsources ascribed to the upper positive charge region are located in the radar reflectivity\nregion less than 30 dBZ. These observations imply that the upper positive charge layer\nis carried in the updraft with low radar reflectivity. In contrast, BOLT sources ascribed\nto the main negative charge region are located in high reflectivity regions.\nAs such, based on the relation between storm electrification and kinetics, the electrical\nstructure of our conceptual model can be inferred from the NBEs distributions and radar\nobservations. We agree that there are some limits of the model due to such indirect\nmeasurements. Nevertheless, this provides a simplified and - as the referee has pointed\nout - reasonable illustration on the finding of competition relation between NBEs\npolarities in overshooting tops.\n[redacted]\nThe conceptual model can explain the observed temporal dynamics of NBEs polarities\n.\nAs shown in the radar-inferred conceptual model on the left of Figure R2, the strong\nupdraft driving the heavier graupels with negative charges would be lifted to a higher\nheight. This would narrow the gap with the upper positive charge layer, enhancing the\nambient electric field and making it easier to initiate positive NBEs. Meanwhile, ice\ncrystals with the upper positive charge layer are elevated to a very high altitude, which\nwould cause strong mixing with the upper screening charge layer, making it difficult\nfor the inception of negative NBEs. This can explain the occurrence of the outbreak of\npositive polarity but few negative NBEs.\nWhen the convection gradually decays as shown on the right of Figure R2, the larger\nand heavier graupels, carrying negative charges, fall into the mixed-phase region, while\nsmall ice crystals with positive charges can still be at a high altitude. The distance\nbetween the upper positive charge layer and the main negative charge layer would then\nincrease, resulting in a lower electric field in this region. This would make it difficult\nfor positive discharge to occur. Meanwhile, the high upper positive charge and the\nenhancement of the electric field by the accumulated positive charge make it easier for\nnegative NBEs to win the competition for the upper positive charge layer. This can\nexplain the occurrence of the outbreak of negative polarity but few positive NBEs.\nFigure R2. The conceptual model for illustration of competition of NBEs polarities in\novershooting thundercloud tops\nInspired by the reviewer’s comments, we realized the possible limits of the proposed\nmodel from the radar indirect measurements. We have now added the discussion. It\nmerits further investigation on the relation between storm kinetics and electrical\nstructure of storm producing outbreak negative NBEs based on the direct 3-D lightning\nand high temporal resolution radar data.\nPlease see revised manuscript (Lines 196-199 and Lines 225-233):\nYoshida et al., (2017) examined 3-D lightning location data and radar data to investigate\nthe relationship between thunderstorm electrification and storm kinetics. They found\nthat the upper positive charge layer is carried in the updraft with low radar reflectivity\nwhile the main negative charge region is located in high reflectivity regions.\nThe conceptual model is based on the well-established simplified charge layer\ndistribution of the normally electrified thunderstorm, which is inferred from the radar\nand NBEs distributions based on the relation between storm electrification and kinetics.\nIt should be noted that the conceptual model has some limits as the charge structure in\nsome thunderclouds is more complex, which may result in forming small charge\npockets and initiating sporadic positive NBEs with higher heights than negative polarity.\nThis complexity, though, is outside of the scope of this paper and will require further\ninvestigation.\n3. The conclusions are based on the analysis of a single storm, and there is no evidence\nto suggest that the conceptual model should be general. The only other example\nprovided in the Supporting Information does not seem to corroborate the conclusions.\nAdditionally, the Extended Data Figure 1 suggests that the model should not be one-\ndimensional, i.e., that there are two horizontally-separated regions where NBEs of\ndifferent polarities are produced.\nResponse: Thanks for the comment. The thunderstorm producing outbreak of negative\nNBEs is relatively rare due to the strict conditions for negative NBEs generation. To\nour knowledge, there has not been any literature ever reported such a large number of\nnegative NBEs in a tropical storm top, which makes this study extremely novel and\ntimely.\nTo clearly show the consistency with the other storm, we now give a detailed\ndescription of the other storm shown in Figure R3. First, the storm is dominated by\nnegative NBEs, which can be used to investigate the relation of NBEs polarities. Figure\nR3c shows that the different NBE polarities occur alternatively during the storm\nevolution, also suggesting the strong competition of the polarity. Secondly. the radar\nanalysis shown in Figure R3a suggests that negative NBEs outbreak under the condition\nwith a low altitude of 40 dBZ, which is consistent with the proposed conceptual model.\nFigure R3. Detailed information of the other thunderstorm with outbreak of negative\nNBEs.\nTo support that the competition relation is general, we statistically analyze more\nthunderstorms recorded by the GZ station for one-year in the low-latitude region. By\nconsidering cells of 25 × 25 km2 and 5-minute consecutive time bin, we determine the\nProportion of the Dominant number of positive or negative Polarity to the total number\n(PDP) in a grid required with a minimum of 5 NBEs and both polarities. In all, 1574\ngrids are determined. A histogram of PDP values is shown in Figure R4. Approximately\n80 % of the space-time grids have PDP-value greater than 0.7, suggesting that one\npolarity always dominates and the strong competition relation of polarities is general,\nwhich thus supports the results of the manuscript.\nFigure R4. Histogram of the proportion of competition value PDP in the grids.\nPlease see revised manuscript (Lines 115-119):\nTo support that the relation between NBE polarities is general, we have analyzed\nadditional thunderstorms recorded by GZ station for one-year in the low-latitude region.\nOn average, the overall occurrence rate of NBEs is about 1.8%. A histogram of PDP\nvalues is shown in Extended Data Fig. 1. More than 75 % of the space-time grids have\nPDP-value greater than 0.75, suggesting that the competition relation of NBEs\npolarities is general.\n4. The manuscript lacks a contemporary understanding of NBE physics. NBEs are\nproduced by large and fast streamer systems of both polarities. Particularly, even a\nsingle NBE polarity, let’s say positive NBEs, can be produced by streamer discharges\nof both polarities.\nResponse: Thanks for the comment. Understanding the physical mechanism of NBEs\nrequires high spatial and temporal radio measurements from interferometer and\nlightning mapping imaging. Rison et al., (2016) used a broadband RF interferometer to\ninvestigate NBEs and lightning initiation in New Mexico thunderstorms. They found\nthat NBEs are produced by fast positive breakdown (FPB) that propagates away from\nthundercloud positive charge and toward thundercloud negative charge at a speed of ~5\n× 107 m/s. A later study of Florida thunderstorms by Tilles et al. (2019) suggests that\nNBEs can also be caused by the fast negative breakdown (FNB) that propagates instead\naway from the negative charge and toward the positive charge at a similar speed.\nFurther, Lyu et al., (2019) suggest that lightning initiation can also begin by a process\nwith isolated short VHF pulses of typical duration less than 0.5 μs.\nFor the physical mechanism of NBEs why they can also produce strong HF/VHF\nradiation. Liu et al., (2019) first developed a statistical approach to investigate the radio\nspectrum of NBEs, and suggested that an ensemble of tens of millions of streamers can\nreproduce the current moment, charge transfer, and radio spectrum of fast breakdown.\nThe recent optical measurements by ASIM from ISS confirm their result. Liu et al.,\n(2021) presented both polarities of NBEs associated with the 337 nm but without or\nweak 777.4 nm, suggesting the streamer breakdown.\nOur work focuses on the phenomenal features of NBEs polarities and investigates the\nrelationship between NBE polarity and their dynamic meteorology. NBEs can serve as\nthe initiation of normal lightning and cloud top discharges in deep convection\npenetrating the lower stratosphere, understanding what conditions of thunderstorms can\nproduce NBEs, especially negative polarity has implications not only for understanding\nlightning initiation but also for studying their perturbations to the concentrations of\ngreenhouse gases in the upper troposphere.\nWe observed an unprecedentedly large number of negative NBEs in a tropical storm\ncloud top penetrating the stratosphere, and find the competitive relationship between\nthe occurrence of NBEs of different polarities. The comprehensive electrical and\nmeteorological analysis suggests that the positive charge layers dominated by\nconvection strength contribute to NBEs polarity generation. This would provide the\nconstraints on the generation mechanism of cloud top discharges.\nWe added the contribution of our results to understanding physical insight in the\ndiscussion.\n5. All potential connections between NBEs and jets mentioned in this manuscript have\nbeen previously discussed in the peer-reviewed literature. Including in papers\noverlooked in the authors’ reference list.\nResponse: We agree with the referee that the connections between NBEs and jets have\nbeen discussed in the literature. In this work, we focus on the competition between\nnegative and positive NBEs rather than NBEs and jets. The connection between NBEs\nand jets mentioned in our work is used to discuss what conditions produce blue\ndischarges and gigantic jets as well as their difference.\nAccording to the theoretical prediction from Krehbiel et al. (2008) and Riousset et al.,\n(2010), blue discharges occur as a result of an electrical breakdown between the upper\npositive charge and the screening charge after a CG or IC discharge inducing a sudden\ncharge imbalance. If the upper positive charge layer is strongly mixed into the screening\ncharge, such situation would produce a substantial imbalance between the main\nnegative and positive charges favorable for the occurrence of gigantic jets.\nThe competition of NBEs polarities in our findings suggests that the height variations\nof the upper positive charge layer induced by the strength of updrafts would play a\nsignificant role in the initiation of types of cloud top discharges. The upwelling stage\nof the overshooting cloud with the outburst of positive NBEs is associated with a very\nhigh altitude of the upper positive charge layer causing strong mixing with the screening\nlayer, which potentially triggers negative gigantic jets occurring beneath the upper\npositive charge region. The gigantic-jet condition by Boggs et al., (2018) is usually\nfeatured with 40 dBZ echoes above 14 km, which is consistent with our proposed model.\nIn contrast, the decaying stage of the overshooting tops with outburst of negative NBEs\nin Fig. 5 (right) is associated with a relatively stable charge layer at the cloud top, which\nwould facilitate the production of blue discharges.\nThese results may provide new insight into the mechanism of cloud top discharges from\nthe view of the dynamic development of convection, with implications for studies of\nthe chemical perturbations of greenhouse gas concentrations by cloud top discharges at\nthe tropopause.\nReviewer #2:\nReview of manuscript entitled “Altitude and polarity transitions of corona activity in\nthundercloud tops reaching the lower stratosphere” by Liu et al.\nThe authors present a detailed analysis of a thunderstorm producing enormous amount\nof narrow bipolar events (NBE). The authors show the changing prevalence of one\npolarity and place it in the changing properties of the storm cells evolution.\nThe manuscript is well written, presents new observations, and a comprehensive\nanalysis. The authors effectively combine electromagnetic measurements with radar\nobservations, the cloud properties are derived from the satellite measurements.\nThe results are original and worth to publish, the work is significant in the field of\natmospheric electricity and related meteorology. The methodology is quite well\ndescribed. There is enough details the authors provided to support their statements.\nI think that the manuscript needs some improvements and clarification before it can be\nconsidered for publication in the Nature Communications journal. My comments are\nlisted below.\nResponse: Thanks for your support and constructive comments, which have greatly\nhelped to improve this paper. We have revised the title and analysis sections, taking\ninto consideration your feedback. The detailed point responses are listed in below.\n1. Title: From the title (and also when looking at the names of co-authors from the\nASIM group) I would expect the manuscript contains also the optical data confirming\nthe existence of corona by observation of blue discharges. As this is not the case, and\nthe presence of corona is based on simultaneous previous observations of NBEs and\nblue discharges, I would ask the authors to consider the modification of the title\naccordingly.\nResponse: We agree with the referee’s comment. the term NBEs is usually defined\nfrom radio signal. Owing to the ISUAL and ASIM optical measurements, we now know\nthat the source is a corona discharge in the cloud producing both NBEs in radio signal\nand blues in optical. We have used ‘corona discharge’ in the title based on the close\nrelation between NBEs and corona discharges reported by several literatures from a\nbroader impact.\nAs there are no simultaneous optical observations, we now modify the title to “Polarity\ntransitions of narrow bipolar events in thundercloud tops reaching the lower\nstratosphere” according to the referee’s suggestion.\nPlease see revised title (Lines 1-2):\nPolarity transitions of narrow bipolar events in thundercloud tops reaching the lower\nstratosphere.\n2. You state that “high lightning activity measured by the GLD360 network, and the\ndomination of negative cloud to-ground (-CG) and positive intra-cloud (+IC)\ndischarges…” but you did not present anything about these discharges. It would be\ninteresting to see what was their distribution in time and space in comparison with the\nNBEs of both polarities.\nResponse: The referee brings up an excellent point. Following the referee’s suggestion,\nwe have analyzed the distributions of CGs and ICs with different polarities of NBEs\nwithin a 25×25 km grid in a given 5-min bin. A total of 114 grids with negative NBEs\nand 104 grids with positive NBEs are determined. The results are shown in Figure R5,\nit is interesting to note that negative NBEs are more correlated with -CGs and +ICs\ncompared to positive NBEs.\nFigure R5. Relationship among -CG, +ICs and NBEs polarities\nAs shown in Figure R5b, the occurrence of negative NBEs is correlated with normal\n+ICs and not as similar as competition with positive NBEs. This may be because the\nconditions for the initiation of positive NBE are relatively strict. According to the\ninterferometer observations reported by Lyu et al., (2018), few fast positive breakdowns\n(FPB) were observed in a thunderstorm, but more normal ICs pulses were observed.\n3. You discuss the changes in the charge arrangement leading to the NBE polarity\nswitching. It would be useful to formulate clearly what generally caused the observed\nenormous amount of NBEs and how often this can happen, especially if you want to\nuse the presence of radio signals emitted by NBEs as an indicator of an exchange of\ngreenhouse gases between the troposphere and stratosphere.\nResponse: This is a good comment. The tropical storm in our work is indeed a\nsurprising case associated with an unprecedented number of NBEs. For the reason of\nthe enormous amounts of NBEs. The radar and NBEs distributions suggest that the\nstrong convection lift cloud above 20 km and the inferred upper positive charge region\nis located near and above the tropopause, which is higher than the usual electrified\nthunderstorms reported in previous studies. The high altitude of the upper positive\ncharge region may contribute to the outburst of negative NBEs. We suggest that the\nspecial meteorological and electrical structure with a high altitude of upper positive\ncharge layer in the tropical storm of typhoon would provide favorable conditions for\noutbreak of negative NBEs.\nThe quantitative occurrence of NBEs requires statistical analysis. Following the\nreferee’s suggestion. we analyze the NBEs of thunderstorms observed in low-latitude\nat the GZ station for one year. Figure R6 shows the time series of hourly lightning and\nNBEs polarities counts. On average, the overall occurrence rates of NBEs are about\n1.8% in low latitudes, which is higher than that of 0.5% in Great Plains thunderstorms\nas reported by Wiens et al., (2008).\nFigure R6. Hourly lightning flashes and NBEs for the months of May–August 2019.\nFor the polarity, we see that NBEs are generally dominated with positive polarity in the\nlow-latitudes thunderstorms except for some special days with outbreak of negative\nNBEs (>102 counts/hour). It is interesting to note that negative outbreaks are closely\nassociated with tropical storms in typhoon, which is similar to the results in our work.\nAnother question is raised: does outbreak of NBEs serve as the perfect proxy for\ntyphoon development? It definitely merits further investigation in the future work.\nWe now added the occurrence frequency of enormous NBEs on Lines 116-117.\n4. Methods: The estimation of the altitude of NBE is a crucial part of the work. I would\nsuggest to evaluate more precisely the error in the altitude calculation.\nResponse: Thanks for the comment. To estimate the uncertainness in result of NBEs\ndischarges height due to the inaccuracy in determining location and reflection pulses,\nwe made the following test. For a typical NBE, up to 5 km and 1-µs are intentionally\nadded in 2-D location and reflection pulses. Under such conditions, Figure R7 shows\nthat no larger than 0.8 km variance from the original result. We now supplement the\ndetails of the error estimation in Method and in Supplementary information.\nFigure R7. Variation of NBE discharge altitude with different location error and time error of\nreflected wave for nine NBEs.\nPlease see revised manuscript (Lines 315-319):\nTo estimate the error of the NBE height, we performed the following test. For a typical\nNBE, up to 5 km and 1-µs are intentionally added in 2-D location and reflection pulses.\nUnder such condition, the results suggest that the maximum error is no larger than 0.8\nkm variance from the original altitudes shown in Extended Data Fig. 8.\n5. Fig.4a: I do not understand why the NBEs appeared only in groups which are\nseparated by approx. 1 min of nothing, this cannot be natural, please explain.\nResponse: We sincerely appreciate the referee’s careful review to point out the issue.\nThe 1-minute interval is because for NBEs we have only taken into account minutes\ninfo and ignored the seconds when we plotted the NBEs scatter plots. We have now\ncorrected this in the new Figures 3, 4, S4, and S5.\n6. I am quite confusing that you use in different figures different time intervals, if there\nis a reason for such approach, could you explain it properly?\nResponse: Thanks for the comment. The time interval of the radar and cloud condition\nanalyses in different figures did not fully align because we want to present the different\nconditions of the outbreak and disappearance of negative NBEs: During the period in\nFigure 3, the negative NBEs bursts are produced while the positive NBE rarely occur.\nThe cloud analysis shows that the area of the Above-anvil cirrus plumes (AACP) is\nclosely related to the frequency of the bursts of negative NBEs, suggesting the close\nconnection between the negative NBE and AACP. Since the AACP is a product of\nstrong convection injecting the cloud into the stratosphere, it indicates that the\novershooting cloud is in the decaying stage at this time. The simultaneous radar\nobservations in Figures 3c and 3d show that the heights of its 40 dBZ echo are relatively\nlow, also suggesting the decaying stage of the cloud.\nDuring the period in Figure 4, the negative NBEs suddenly disappear and the positive\nNBEs win the competition and occurs explosively. The cloud top temperature varies\nslightly while the radar observation shows a significant difference in that there is\nextreme updraft in the cloud, suggesting that the extremely convective updraft can\ninhibit the generation of the negative NBEs rather than promote it. The above analysis\ncan provide an illustration of the competition of NBEs polarity in overshooting tops.\nThe simultaneous observations of the overlapping periods of the cloud and radar are\nadded in the supplementary information.\nReviewer #3:\nThis is a well-written paper. I have a few comments that the authors need to consider\nand address in the revised version:\nResponse: Thanks for your positive comments. We have now revised the manuscript\nbased on your suggestions. The detailed point responses are listed below.\n1. Lines 83-83: Kindly reword this sentence, I recommend inserting: “helps estimate”\nafter “….and the reflected ones”.\nResponse: Done.\nPlease see revised manuscript (Lines 83-85):\nThe time delay of the main pulses allows estimation of the source location and the delay\nbetween the main pulses and the reflected ones helps estimate the source height\n2. Lines 85-86 and Figure 2a. It appears that a significant number of the negative NBEs\nare above the troposphere. The authors should note here that previous observations have\nalso reported above-tropopause NBEs in “convective surges (plumes) overshooting the\ntropopause” such as Smith et al. (2004) doi:10.1029/2002RS002790 and Nag et al.\n(2010) doi:10.1029/2009JD012957.\nResponse: Thanks for pointing out this. We have now added previous observations\nfrom the literature.\nPlease see revised manuscript (Lines 91-94):\nThe sources of positive NBEs are primarily between 10 km and 17 km and most of the\nnegative NBEs are above the tropopause, suggesting a normally electrified storm with\nthe main negative and positive charge regions, which is consistent with prior reported\nabove-tropopause NBEs observation.\n3. Line 107: The acronym PDP and its meaning is not introduced before in the text.\nResponse: Thanks for the comment. we have now added the acronym PDP in main text.\nPlease see revised manuscript (Lines 106-109):\nWe determine the Proportion of the Dominant number of positive or negative Polarity\nto the total number (PDP), which is defined as the number of NBEs of the dominant\npolarity to the total number of NBEs.\n4. Lines 109-111: It is difficult to understand the significance of this finding without\nfurther explanation of what PDP means and how values greater than 0.75 is suggestive\nof \"strong competition\".\nResponse: To clearly explain what PDP means, we now move the definition of PDP\nto the main text. The “Proportion of the Dominant number of positive or negative\nPolarity to the total number” (PDP) is used to quantify the relation between positive\nand negative NBEs:\n𝑚𝑎𝑥 (+𝑁𝐵𝐸,−𝑁𝐵𝐸)\n𝑃𝐷𝑃 =\n∑(+𝑁𝐵𝐸,−𝑁𝐵𝐸)\nA larger PDP indicates a larger difference in the proportion of positive and negative\nNBEs, suggesting greater competition between positive and negative NBEs during a\ngiven period. We give an example: 10 NBEs, including 8 negative and 2 positive NBEs,\nmax (8,2) 8\nare generated in a period. The value of PDP in this period is 𝑃𝐷𝑃= = = 0.8.\n𝛴(8,2) 10\n5. Lines 196-197: Need to discuss the reasoning for this statement regarding the lower\nbreakdown threshold. Is it because of changes in atmospheric composition, pressure,\nand temperature? Perhaps need a reference like Riousset et al. (2020)\ndoi.org/10.1016/j.icarus.2019.113506?\nResponse: This is a good comment. According to Figure 1 in Riousset et al. (2020),\nthe lower breakdown threshold is attributed to less pressure and humidity for higher\naltitudes. We have added the reference and supplemented the discussion in the text.\nPlease see revised manuscript (Lines 208-211):\nThe higher negative charge layer would narrow the gap with the upper positive charge\nlayer, enhancing the ambient electric field. Moreover, the breakdown threshold is lower\nbecause of less pressure and humidity for higher altitudes, facilitating the initiation of\npositive NBEs.\n6. Line 210-213: What about the occurrence of \"regular\" ICs between the main negative\nand positive charge regions. They would also play a role (perhaps a more dominant one)\nthan the negative CGs in this charge balance? Please clarify here.\nResponse: The referee brings up an excellent point. Following the referee’s suggestion,\nwe analyzed the distributions of -CGs and +ICs with different polarities of NBEs within\na 25×25 km grid in a given 5-min bin in a tropical storm. A total of 114 grids with\nnegative NBEs and 104 grids with positive NBEs are determined. Figure R8 shows that\nan outbreak of negative NBEs is generally associated with frequent -CGs and ‘regular’\nICs. Moreover, it is interesting to note that the +IC seems to be more correlated with\nnegative NBEs outbreak, suggesting ICs indeed play a more dominant role than -CGs\nin the charge balance for the cloud top discharges.\nFigure R8. Relationship among -CG, +ICs and NBEs polarities"}, {"round": 2, "reviewer_comments": {"reviewer_2": "The authors took into account all of my comments, even if some of them induced quite extensive work including a search for additional similar storm examples. I appreciate the authors' efforts to improve the manuscript accordingly, I do not have additional requests and recommend the manuscript for publication.", "reviewer_3": "The authors have addressed all my comments adequately and I recommend this article for publication.\nAdditionally, in my opinion the authors have mostly addressed Reviewer 1’s comments appropriately. Previous studies of different aspects of CIDs/NBEs have been published in Nature Communications (see references 5 and 6 in item 2 below). This paper discusses the issue of polarity of NBEs/CIDs in detail, which is not found in abundance in the scientific literature.\nI suggest the following additional changes necessary to fully address Reviewer 1's comments.\n1. Comment #1: In order to address this comment from Reviewer 1, I suggest the authors reference the paper by Williams (2006) on polarity asymmetry in lightning as one of the outstanding mysteries along with a brief sentence or two in the introduction.\nWilliams, E.R., (2006), Problems in lightning physics—the role of polarity asymmetry, Plasma Sources Sci. Technol. 15 (2006) S91–S108, doi:10.1088/0963-0252/15/2/S12.\n2. Comment #4: In order to address Reviewer 1’s comment I suggest that the authors introduce a brief paragraph in the introduction on the physics of NBE/CID production along with pertinent references. A few sentences such as those below in the introduction should suffice.\nCIDs/NBEs have been described as a short-channel (few 100 m) “bouncing wave” phenomenon from observations and modified transmission-line modeling [1, 2, 3, 4] of their broadband (in the VLF-HF range) narrow bipolar pulse signatures and the accompanying fine structure. Using data from broadband interferometry, the channel production mechanism of NBEs has been associated with the so-called fast positive and negative breakdown in which streamers propagate at speeds of 10^7-10^8 m/s [5, 6, 7]. The exact nature of the fast breakdown that produces high-power (few tens of GW [4]) NBEs and can move at >10^7 m/s (up to 10^8 m/s, according to [7]) through virgin air remains a mystery. Fast breakdown could be a corona or ionization wave caused by positive corona from ice hydrometeors that is initiated ahead of the wave front, either in a forward or retrograde direction relative to the ambient electric field [7]. There are also recent attempts [e.g., 8] to model CIDs and explain their electromagnetic signatures that consider fast breakdown and the “rebounding” observed by Rison et al. [2016] using modified transmission line models.\n1. Eack, K. B. (2004), Electrical characteristics of narrow bipolar events, Geophys. Res. Lett., 31, L20102, doi:10.1029/2004GL021117.\n2. Nag, A., and V. A. Rakov (2009), Electromagnetic pulses produced by bouncing-wave-type lightning discharges, IEEE Trans. Electromagn. Compat., 50(03), 466–470.\n3. Nag, A., and V. A. Rakov (2010a), Compact intracloud lightning discharges: 1. Mechanism of electromagnetic radiation and modeling, J. Geophys. Res., 115, D20102, doi:10.1029/2010JD014235.\n4. Nag, A., and V. A. Rakov (2010b), Compact intracloud lightning discharges: 2. Estimation of electrical parameters, J. Geophys. Res., 115, D20103, doi:10.1029/2010JD014237.\n5. Rison, W., P. R. Krehbiel, M. G. Stock, H. E. Edens, X. M. Shao, R. J. Thomas, M. A. Stanley, Y. Zhang (2016), Observations of narrow bipolar events reveal how lightning is initiated in thunderstorms, Nature Communications 7, Article number: 10721, doi:10.1038/ncomms10721.\n6. Tilles, J., N. Liu, M.A. Stanley, P.R. Krehbiel, W. Rison, M.G. Stock, J.R. Dwyer, R. Brown, J. Wilson (2019), Fast negative breakdown in storms challenges electrical breakdown theory, Nat. 406 Commun 10(1), 1648 (2019).\n7. Krehbiel, P., C. da Silva, S. Cummer (2018), Continued mysteries of lightning studies, 16th International Conference on Atmospheric Electricity, Nara, Japan, 18-22, June, 2018.\n8. Li, D., Luque, A., Gordillo-Vázquez, F. J., Silva, C. d., Krehbiel, P. R., Rachidi, F., & Rubinstein, M. (2022). Secondary fast breakdown in narrow bipolar events. Geophysical Research Letters, 49, e2021GL097452. https://doi.org/10.1029/2021GL097452."}, "author_response": "Point-by-point Response to Reviewer’s Comment\nManuscript number: NCOMMS-23-46957A\nManuscript Title: Polarity transitions of narrow bipolar events in thundercloud\ntops reaching the lower stratosphere\nReviewer #2:\nThe authors took into account all of my comments, even if some of them induced quite\nextensive work including a search for additional similar storm examples. I appreciate\nthe authors' efforts to improve the manuscript accordingly, I do not have additional\nrequests and recommend the manuscript for publication.\nResponse: Thanks for your constructive comments. We appreciate the reviewer's great\nefforts in improving this work.\nReviewer #3:\nThe authors have addressed all my comments adequately and I recommend this article\nfor publication.\nResponse: We are grateful to the reviewer for the insightful comments, which has\nhighly improved the quality of the manuscript.\nAdditionally, in my opinion the authors have mostly addressed Reviewer 1’s comments\nappropriately. Previous studies of different aspects of CIDs/NBEs have been published\nin Nature Communications (see references 5 and 6 in item 2 below). This paper\ndiscusses the issue of polarity of NBEs/CIDs in detail, which is not found in abundance\nin the scientific literature.\nI suggest the following additional changes necessary to fully address Reviewer 1’s\ncomments.\n1. Comment #1: In order to address this comment from Reviewer 1, I suggest the\nauthors reference the paper by Williams (2006) on polarity asymmetry in lightning as\none of the outstanding mysteries along with a brief sentence or two in the introduction.\n2. Comment #4: In order to address Reviewer 1’s comment, I suggest that the authors\nintroduce a brief paragraph in the introduction on the physics of NBE/CID production\nalong with pertinent references. A few sentences such as those below in the introduction\nshould suffice.\nCIDs/NBEs have been described as a short-channel (few 100 m) “bouncing wave”\nphenomenon from observations and modified transmission-line modelin of their\nbroadband (in the VLF-HF range) narrow bipolar pulse signatures and the\naccompanying fine structure. Using data from broadband interferometry, the channel\nproduction mechanism of NBEs has been associated with the so-called fast positive and\nnegative breakdown in which streamers propagate at speeds of 107-108 m/s. The exact\nnature of the fast breakdown that produces high-power (few tens of GW) NBEs and can\nmove at >107 m/s (up to 108 m/s, according to) through virgin air remains a mystery.\nFast breakdown could be a corona or ionization wave caused by positive corona from\nice hydrometeors that is initiated ahead of the wave front, either in a forward or\nretrograde direction relative to the ambient electric field. There are also recent attempts\nto model CIDs and explain their electromagnetic signatures that consider fast\nbreakdown and the “rebounding” observed by Rison et al. [2016] using modified\ntransmission line models.\nResponse: Thanks for the constructive suggestion. The reviewer’s suggestion brings\nan excellent supplement to addressing the comment. We have now added a brief\nsentence on polarity asymmetry in lightning (Lines 65-66), and a brief paragraph in the\nintroduction on the physics of NBE/CID production accordingly (Lines 56-63)."}]

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