Environmental Conditions Associated with Active Lightning Events in April around the Nansei Islands

Environmental Conditions Associated with Active Lightning Events in April around the Nansei Islands | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Environmental Conditions Associated with Active Lightning Events in April around the Nansei Islands Hana Hirano, Kosuke Ito This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9565181/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract A lot of cloud-to-ground lightning strikes sometimes occur around the Nansei Islands in April, while those April lightning bursts (ALBs) have not been investigated. This work aims at elucidating the environment contributing to ALBs through a case study and composite analysis. Two-thirds of the ALBs cases are characterized by a carrot-shaped clouds, indicating a mesoscale convective system known as the back-and-side building. The composite analysis was conducted for cases with daily lightning strikes > 1000 d − 1 (L1000) and with no daily lightning strikes (NL). In both cases, westerly wind blows in the middle-troposphere. In L1000, the Nansei Islands were typically located to the west of a synoptic-scale migratory high at the surface and southerly or south-westerly wind advected the air of high equivalent potential temperature that makes the atmospheric condition more unstable to the Nansei Islands. Low-level south-to-southerly and middle-level westerly consisted of the vertical wind shear favorable for the carrot-shaped clouds. Geographically, the ALBs distribution well corresponded to the local-warm Kuroshio current, implying that the pressure adjustment mechanism over the Kuroshio current helps trigger the ALBs through the convergence. As in L1000, the pressure adjustment mechanism was active in NL; however, synoptic-scale downward motion and higher stability likely prevented the active convection. Lightning the Nansei Islands carrot-shaped clouds Kuroshio pressure adjustment mechanism April Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1 Introduction Lightning activities suggest active convection, severe weather and abundant ice nuclei. They sometimes bring disasters such as aircraft damage and the loss of human lives. Toward better understanding of global atmospheric electrical circuits, preventing disasters and utilizing the lightning information for weather prediction (Slocum et al. 2023 ), it is important to know their distributions and physical backgrounds. Lightning around Japan has been investigated by several researchers so far, most of which focus on the lightning in summer or winter. For example, ground heating caused by strong solar radiation is a primary factor contributing to active convection with lightning over land in summer (Fujibe 1988 ; Ishii et al. 2014 ). Lightning strikes occur even in winter along the coast of the Sea of Japan (Hayashi and Marui 2016 ; Ishii et al. 2014 ). On the other hand, several studies have pointed out the large number of lightning strikes in the vicinity of the Nansei Islands. Iwasaki ( 2014 ) showed that the annual-mean cloud-to-ground flash (CG) strike distribution exhibits the high frequency over the Kuroshio current along its path around the Nansei Islands and in the east coast of Japan. Kato et al. ( 2025 ) also showed that lightning occurrence corresponds to the position of the Kuroshio, and that the number of lightning strikes increases in spring around the Nansei Islands. Hayashi and Marui ( 2016 ) showed that Nansei Islands are the regions characterized by the largest number of lightning days around Japan in spring (March-May). However, as far as the authors’ knowledge, the environmental conditions that bring enhanced lightning activity around the Nansei Islands in spring have not been investigated in detail. In this study, we begin with a description of the monthly distribution of lightning strikes following the method section. To our surprise, very active lightning events were detected in April, which is much earlier than the Baiu season and day-time heating in summer. Thus, the main topic of this paper is to reveal the atmospheric and oceanic background conditions leading to the April lightning bursts (ALBs) around the Nansei Islands. Finally, we summarize the proposed mechanism with some discussion. 2 Methods 2.1. Lightning data Lightning data were observed by lightning detection network system (LIDEN). This system consists of 30 stations equipped with detection sensors, and each station approximately 200 km apart and is operated by the Japan Meteorological Agency (JMA). We used longitudes, latitudes (lightning location) and the kind of discharge observed by LIDEN. LIDEN measures CG flash and cloud-to-cloud flash by low frequency (LF) waves. In this study, we focused on CG lightning same as in Ishii et al. ( 2014 ). LIDEN data are available from 2006 to 2023, but we used LIDEN data from 2016 to 2023 because observation instruments in western Japan were replaced around 2015 (Drs. Shugo Hayashi and Satoshi Yoshida, personal communication on 29 December 2020). We grouped the data into bins with the intervals of 0.5° in longitude and latitude. 2.2. Atmospheric and oceanic data To investigate the daily-mean atmospheric condition, we employed the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis of the global climate (ERA5; Hersbach et al. 2020 ). As for sea surface temperature (SST) data, we employed the 1/4° daily optimum interpolation sea surface temperature (OISST), which is a long-term Climate Data Record developed by National Oceanic and Atmospheric Administration (NOAA) (Huang et al. 2021 ). In this study, OISST data is used from 2016 to 2023. The target region around the Nansei Islands is defined as \(\:22^\circ\:\text{N}\) to \(\:31^\circ\:\text{N}\) and \(\:122^\circ\:\text{E}\) to \(\:131^\circ\:\text{E}\) . To eliminate the effect of typhoons on the atmosphere and ocean, analyses are not conducted when a typhoon is within 800 km of the target region based on the JMA best-track data. To compare the environmental condition of many lightning strikes with that of no lightning strikes in April, we used daily mean environmental data for composite analysis. The criterion for a day with a large number of lightning strikes was defined as 1,000 or more CG lightning strikes. For presentation purposes, these cases are referred to as L1000 and the days with no lightning strikes are referred to as NL. The threshold of 1,000 lightning strikes was selected to ensure comparable sample sizes between the two groups (58 cases in L1000 and 87 cases in NL). In this study, we do not show the composite-mean atmospheric state of events with 1 to 999 lightning strikes. However, the composite-mean of environmental conditions is intermediate between L1000 and NL (figure not shown). 3. Results 3.1. Feature of lightning strikes in April in the Nansei Islands Figure 1 shows the distributions of number of CG lightning around Japan for 8 years (2016 to 2023) per day from March to August observed by LIDEN. The first notable feature is that high-frequency CG lightning area is located around the Nansei Islands, as indicated by the red box in Fig. 1 b (22°N–31°N, 122°E–131°E), and it expands from April as June approaches and shifts to mainland Japan in July and August. One question is why lightning activity was so active in the adjacent seas of the Nansei Islands as early as April. High frequency lightning from May to August can be explained by active convection by the Baiu front and daytime heating in summer. However, the high density of CG strikes around Nansei Islands in April has not been explained so far. Furthermore, Fig. 1 shows that high-frequency lightning area in April is more specially concentrated and localized compared to that in June and July. When each CG day is ranked by the number of lightning strikes from 2016 to 2023, 17 days out of the top 100 days occurred in April. This number is comparable to that in June–August and substantially larger than in May (5 days). In addition, more than 30,000 strikes per day occurred twice in April during this period, which ranks within the top 5%. Thus, ALBs are particularly significant, and their environmental condition needs to be investigated. 3.2 Case study To exemplify the specific environmental conditions during periods of ALBs, the environment condition of 23 April 2019 that had 11,193 lightning strikes was chosen. Figure 2 shows brightness temperature from Himawari-8/9 (Bessho et al. 2016 ). Low brightness temperature area is triangular and spreads from 28°N, 127°E to northeast. Well-known carrot-shaped clouds accompanied with heavy rainfall and lightning activity at its tip. We made sure the heavy rainfall (> 100 mm/day) for this event based on Global Satellite Mapping of Precipitation dataset (figure not shown). Carrot-shaped clouds tend to persist for up to 10 hours (Itoh et al. 1992 ) and are appear when low-level wind is perpendicular to mid-level wind and air with high equivalent potential temperature ( \(\:{\theta\:}_{e}\) ) flows into lower levels (Itoh et al. 1992 ; Seko et al. 1999 ). As shown in Fig. 2 , cloud-top temperature of the carrot-shaped clouds is much lower than − 20°C, that satisfies the temperature requirement for charge separation mechanism (Takahashi 1978 ). Figure 3 shows a daily-mean \(\:{\theta\:}_{e}\) and wind at 950 hPa and 500 hPa on the day. Bolton ( 1980 )’s formula was used to calculate \(\:{\theta\:}_{e}\) . Target region was characterized by high- \(\:{\theta\:}_{e}\) air, with southwesterly winds at 950 hPa, and westerly winds at 500 hPa. The vertical wind shear and the inflow of high- \(\:{\theta\:}_{e}\) air into lower levels are conditions favorable for the development of a back-and side-building (BSB) type mesoscale convective system, in which new convective cells develop on the upstream side of the mid-level flow. Carrot-shaped clouds are thought to form in association with this type of convective system (Seko and Nakamura 2003 ). The Nansei islands were under southerly-wind condition in the lower troposphere because they were at the western edge of a high-pressure system migrating to the east. It was underneath the southern portion of midlatitude westerly jet. In other words, the migrating high-pressure system provides vertical wind shear and transports the high- \(\:{\theta\:}_{e}\) air from the south in its western edge. Our subjective analysis with Himawari-8/9 indicated that carrot-shaped clouds were observed in about two-thirds of L1000. 3.3 Composite analysis In this section, composite analyses of L1000 and NL are compared to reveal features of the environmental conditions. Figure 4 shows the composite-mean of geopotential height and horizontal wind vector in L1000 and NL groups at 300 hPa, 500 hPa, and 950 hPa. At 300 hPa and 500 hPa, westerly wind blows around the Nansei Islands both in L1000 and NL. One noticeable difference between L1000 and NL is that a relatively deep trough was seen in the northeast of the Nansei Islands in NL than in L1000. At 950hPa, south-westerly wind is seen around the Nansei islands in L1000. Checking all 58 cases in L1000, we found that the southerly or southwesterly wind is robust around the Nansei Islands, and they are associated with southerly in the west of a migrating high-pressure system in most cases. Vertical wind shear is one of conditions of the BSB-type mesoscale convective system (Seko and Nakamura 2003 ). In contrast, an anticyclone was typically over the Nansei islands area at 950 hPa and horizontal divergence was observed near the surface in NL. Figure 5 shows a potential temperature ( \(\:\theta\:\) ) and \(\:{\theta\:}_{e}\) along \(\:125^\circ\:\text{E}\:\) in both groups overplotted by the meridional and vertical components of wind. The aspect ratio of a wind vector is set to the same as the aspect ratio of a plot domain. Red and orange contours show the relative humidity of 90% and 80%, respectively. In L1000, wind vectors show the southerly and gentle upward motion from 15 \(\:^\circ\:\) N to 30 \(\:^\circ\:\) N below 900 hPa (Fig. 5 a, c). Considering the gentle positive gradient of \(\:\theta\:\) with increasing latitude in the baroclinic region, the southerly wind naturally undergoes upward motion and thus the air is humidified. Around 25 \(\:^\circ\:\) N–30 \(\:^\circ\:\) N, the strong upright upward motion is nearly along the line with \(\:{\theta\:}_{e}\) and crosses the line with \(\:\theta\:\) . It suggests that the humid air mass moves along keeping certain \(\:\theta\:\) until condensation and then it exhibits strong upward motion along with \(\:{\theta\:}_{e}\) after condensation. This profile underlies the active convection associated with ALBs in L1000. On the other hand, the composite mean profile shows that a flow vector aligned from 40 \(\:^\circ\:\) N–50 \(\:^\circ\:\) N in the upper troposphere to 30 \(\:^\circ\:\) N in the lower troposphere nearly along the line with \(\:\theta\:\) (Fig. 5 b, d). It implies that the descending flow originated from the upper-level convergence to the west of an upper-level trough as shown in Fig. 4 b, d suppresses active convection around the Nansei Islands. Convective available potential energy (CAPE) is an important metric for revealing potential of convection. Figure 6 shows CAPE in both L1000 and NL. In L1000, CAPE is larger near the southwest of the target region. The composite-mean of CAPE was approximately 600 J kg − 1 off the coast of east Taiwan. CAPE around the Nansei Islands is as small as 100 J kg − 1 . As such, high-CAPE area in Fig. 6 a did not correspond to high-CG-lightning areas ranging from southwest to northeast of the target region in April (Fig. 1 b). This is probably because in the southwestern portion outside of the target domain, the CAPE is not activated for strong upward motion until the air condensates and reaches the level of free convection (see Fig. 5 ). Also, active convections make atmospheric stability closer to the neutral condition. So far, we have shown atmospheric environmental conditions relevant to the ALBs. Another perspective might come from the oceanic environment. In particular, the Kuroshio current is one of the major currents in world oceans that flow clockwise from around the north of equator into south of Japan through the Philippine Sea and the East China sea in North Pacific (Steele et al. 2010 ). The Kuroshio carries warm water from low-latitude ocean. Minobe et al. ( 2008 ) shows that the Gulf Stream affects the marine boundary layer on it and that causes convection of entire troposphere over the Gulf Stream by pressure adjustment mechanism that leads to surface wind convergence. Minobe et al. ( 2008 ) shows that surface wind convergence over the Gulf Stream along its path corresponds SST Laplacian using a combination of operational weather analyses, satellite observations and an atmospheric general circulation model. Sasaki et al. ( 2012 ) shows that a rain band in the East China sea in June is intensified on the Kuroshio because of the same mechanisms of Minobe et al. ( 2008 ). Following Minobe et al. ( 2008 ), Sasaki et al. ( 2012 ), we calculated a contribution from SST Laplacian related to Kuroshio. Figure 7 shows sea surface temperature (SST), SST Laplacian, and sea surface pressure Laplacian in April, June, and August. As consistent with Minobe et al. ( 2008 ), SST Laplacian well corresponds to sea surface pressure Laplacian, particularly over Kuroshio in April and June. Sea surface pressure Laplacian is expected to be related to divergence through the following formula (Minobe et al. 2008 ): $$\:-\left(\:\frac{\partial\:u}{\partial\:x}+\frac{\partial\:v}{\partial\:y}\right){\rho\:}_{0}=\left(\:\frac{{\partial\:}^{2}P}{\partial\:{x}^{2}}+\frac{{\partial\:}^{2}P}{\partial\:{y}^{2}}\right)\left(\frac{\epsilon\:}{{\epsilon\:}^{2}+{f}_{0}^{2}}\right)\:$$ 1 where \(\:u,\:v\) is zonal and meridional wind, \(\:{\rho\:}_{0}\) is the density of the marine boundary layer. \(\:P\) is sea-surface pressure, \(\:\epsilon\:\equiv\:\frac{{C}_{D}\left|{V}_{c}\right|}{H}\) where \(\:{C}_{D}\) is a coefficient of friction, \(\:{V}_{c}\) is the scale of typical lower wind, \(\:H\) is a depth of boundary layer, and \(\:{f}_{0}\) is Coriolis parameter. There is a sharp variation of SST between the Kuroshio and adjacent regions in April as warm boundary current intervenes the cold northern ocean, and therefore SST Laplacian in April is much clearer than all the other months shown in Fig. 7 . Sea surface pressure Laplacian magnitude associated with the Kuroshio is up to 1.0 hPa in April. From the Eq. ( 1 ), since sea surface pressure Laplacian is in proportion of the opposite sign of wind divergence, sea surface wind convergence on the Kuroshio in April is strongest of all the other months (Fig. 7 g). It is because the SST Laplacian on the Kuroshio is getting unclear as SST rises in summer (Fig. 7 d, e, f). Horizontal distributions of SST Laplacian and wind convergence over the Kuroshio along its path are similar to those of Minobe et al. ( 2008 ) and Sasaki et al. ( 2012 ), who indicated that convection can be driven by pressure adjustment mechanism over the Kuroshio. It is notable that the distribution of sea surface pressure Laplacian well corresponds to the distribution of ALBs (Fig. 1 b). It suggests that the oceanic condition also plays an important role on active convection in this region. In the same analysis of Fig. 7 , SST Laplacian and sea surface pressure Laplacian analysis have been conducted in L1000 and NL separately. However, the results were almost the same value in April (figures not shown). Therefore, the pressure adjustment mechanism can help the active convections continually in April, although above-mentioned atmospheric conditions are also prerequisite. 3.4 Proposed mechanisms Here we summarize the atmospheric and oceanic environmental conditions when ALBs were observed. Around the Nansei Islands, the combination of mid-level westerlies and low-level southwesterly or southerly winds results in vertical wind shear. In most cases, the Nansei Islands are located along the western periphery of a synoptic-scale high-pressure system that transports high- \(\:{\theta\:}_{e}\) air into the lower troposphere, thereby enhancing atmospheric instability over the region. In addition, there is a meridional gradient of \(\:\theta\:\) value to the south of the Nansei Islands that makes the southerly wind have upward components. Under those environments, the pressure adjustment mechanism (Minobe et al. 2008 ) over the Kuroshio further helps the active convections with lightning. In contrast, the lower-tropospheric air around the Nansei Island is linked to a downward motion originating from the upper-level convergence in the west of a trough along the nearly constant \(\:\theta\:\) value in NL. Therefore, the horizontal divergence is around the Nansei islands at lower troposphere. As a result, convections with lightning are hardly observed, implying that the pressure adjustment mechanism over the Kuroshio does not beat the large-scale suppression. 4. Conclusion In this study, we first show that very active lightning strikes have been frequently observed around the Nansei Islands in April, which we call ALBs. Then, we proceed to clarify the atmospheric and oceaninc environmental conditions for those ALBs around the Nansei Islands with composite analysis of the environments of L1000 and NL case between 2016 to 2023 in April. The composite analysis of the L1000 case revealed westerly flow at the mid- and upper levels and southwesterly inflow of high- \(\:{\theta\:}_{e}\) air at low levels, resulting in an unstable atmospheric environment. Furthermore, the mechanisms of convergence of a marine boundary layer over the Kuroshio triggers the convection over the Kuroshio and occurring high frequency lightning around it in April. Further work is needed to check the distribution of seasonal march, polarity and peak current of lightning, aerosols and radar-based observations. Also, we need to clarify the importance of each component by using numerical models. Finally, Virts et al. ( 2015 ) reported that the lightning is active over Gulf Stream. The similarities and differences should be checked. Abbreviations ALB April Lightning Burst BSB Back-and-side building L1000 Cases with daily lightning strikes > 1000 d − 1 NL Cases with no daily lightning strikes CG Cloud-to-ground flash LIDEN Lightning detection network system JMA Japan Meteorological Agency LF Low frequency ECMWF European center for medium-range weather forecasts SST Sea surface temperature OISST Optimum interpolation sea surface temperature NOAA National oceanic and atmospheric administration CAPE Convective available potential energy Declarations Competing interests The authors declare that they have no competing interests. Funding This work was supported by JSPS KAKENHI Grant Numbers JP24H02226 and JP25K01073. Author Contribution HH analyzed the lightning and reanalysis data and was a major contributor in writing the manuscript. KI created figures and suggested the relevant mechanisms. All authors read and approved of the final manuscript. Acknowledgement We are grateful to Dr. Shugo Hayashi, Dr. Satoru Yoshida, and Dr. Takuya Kawabata for providing LIDEN data and giving us a lot of advice. We also thank Dr. Hiroyuki Yamada for discussions and giving valuable comments. Finally, we would like to express our sincere gratitude to Dr. Soichiro Hirano for encouragement throughout this study. Availability of data and materials LIDEN data was provided by Meteorological Research Institute. It is also available from Japan Meteorological Business Support Center ( https://www.jmbsc.or.jp/jp/online/n-online0.html ). The ERA5 and OISST datasets generated and/or analyzed during the current study are respectively available in the Climate Data Store repository, https://www.ecmwf.int/en/forecasts/dataset/ecmwf-reanalysis-v5 and National Centers for Environmental Information, https://www.ncei.noaa.gov/products/optimum-interpolation-sst . Equivalent blackbody temperature (produced from Himawari data) was provided by Japan Aerospace Exploration Agency (JAXA), https://earth.jaxa.jp/en/data/2529/index.html . References Bessho K et al (2016) An introduction to Himawari-8/9—Japan’s new-generation geostationary meteorological satellites. J Meteorol Soc Jpn 94:151–183. https://doi.org/10.2151/jmsj.2016-009 Bolton D (1980) The computation of equivalent potential temperature. Mon Wea Rev 108:1046–1053. https://doi.org/10.1175/1520-0493(1980)108%3C1046:TCOEPT%3E2.0.CO;2 Fujibe F (1988) Diurnal variations of precipitation and thunderstorm frequency in Japan in the warm season. 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J Atmos Sci 35:1536–1548. https://doi.org/10.1175/1520-0469(1978)035%3C1536:REAACG%3E2.0.CO;2 Virts KS, Wallace JM, Hutchins ML, Holzworth RH (2015) Diurnal and seasonal lightning variability over the Gulf Stream and the Gulf of Mexico. J Atmos Sci 72:2657–2665. https://doi.org/10.1175/JAS-D-14-0233.1 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 08 May, 2026 Reviewers invited by journal 04 May, 2026 Editor assigned by journal 01 May, 2026 Submission checks completed at journal 30 Apr, 2026 First submitted to journal 29 Apr, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9565181","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":638060330,"identity":"a9057519-9cfd-4138-82b8-05cab2f0587d","order_by":0,"name":"Hana Hirano","email":"","orcid":"","institution":"Kyoto University","correspondingAuthor":false,"prefix":"","firstName":"Hana","middleName":"","lastName":"Hirano","suffix":""},{"id":638060331,"identity":"74555780-92a8-459d-83e7-da2062e445d2","order_by":1,"name":"Kosuke Ito","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABFklEQVRIie2RsWrDMBBATxg0mWbVEvwLChm6NfRP7gh4ygcYWlyDQV0MWW0S/A3JkllBEC9OvyHe+wEtXaq4LUnAMR1L0QOdhE5Ph04ADscfhNvBEhuEnQ8Y4XlSdyv8pHgSa/y6pU+BM4ULpi6Ubm6CVLM8iql4TkA0ZUhzu+P5cBcA23eW4ZwjW9WGFr4GSZsZFYqjVaajBF6wW/Ela5SmUiAgbSJa7fzbDx88e7yWPUpMZXAATctWkbbKU7+yVh4thG0DJbMfxfQoIW7z2oyLjFLAXTguVIhsKauRuvKWIDXbJoviYV6Z6v3tcTqce0bDa/QQDER3x458J9rvgfs2gjx+U33NuGRyWg6y3ykOh8Px3/kEmfZcoCK6M9YAAAAASUVORK5CYII=","orcid":"","institution":"Kyoto University","correspondingAuthor":true,"prefix":"","firstName":"Kosuke","middleName":"","lastName":"Ito","suffix":""}],"badges":[],"createdAt":"2026-04-29 11:28:30","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9565181/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9565181/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":109205171,"identity":"6fe7bf52-82bb-4baf-8e65-196364a98261","added_by":"auto","created_at":"2026-05-13 15:03:38","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":3294145,"visible":true,"origin":"","legend":"\u003cp\u003eDistributions of the number of CG lightning for 8 years (2016 to 2023) per day in April to August observed by LIDEN (color; frequency). The red square in panel (b) indicates the location of the target region, spanning 22°N –31°N and 122°E –131°E.\u003c/p\u003e","description":"","filename":"11.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9565181/v1/93f3b370187a89ec2e2aa29b.jpg"},{"id":109205189,"identity":"b1cc51eb-11e5-4704-9481-82288804557f","added_by":"auto","created_at":"2026-05-13 15:03:42","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":792379,"visible":true,"origin":"","legend":"\u003cp\u003eSea surface pressure (color contour; hPa), the coastline (black line), and Equivalent blackbody temperature (monotone; K) at 12 UTC 23 April 2019.\u003c/p\u003e","description":"","filename":"12.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9565181/v1/deb2cd698da3e6d5b1ea15d9.jpg"},{"id":109150419,"identity":"01d4d921-81b4-4369-a124-37db48bb44b1","added_by":"auto","created_at":"2026-05-13 05:28:25","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1656075,"visible":true,"origin":"","legend":"\u003cp\u003eDaily mean equivalent potential temperature (color: K) and horizontal winds (the arrows; unit 10 m/s) on 23 April 2019. (a) shows equivalent potential temperature and wind at 950 hPa and (b) shows at 500 hPa. Red boxes on these figures show the target region.\u003c/p\u003e","description":"","filename":"13.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9565181/v1/e521efc86cb3159cc0cf7472.jpg"},{"id":109205346,"identity":"99e02751-aa35-474f-90c7-6d795b6aa19f","added_by":"auto","created_at":"2026-05-13 15:04:21","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1445388,"visible":true,"origin":"","legend":"\u003cp\u003eComposites of geopotential height (color; m) and horizontal winds (the arrows) at 300 hPa (a, b; unit 40 m s\u003csup\u003e-1\u003c/sup\u003e), at 500 hPa (c, d; unit 20 m s\u003csup\u003e-1\u003c/sup\u003e) and at 950 hPa (e, f; unit 10 m s\u003csup\u003e-1\u003c/sup\u003e). The left-hand side figures show the results of L1000 case, the other side figures show the results of the NL case. Black boxes in these figures indicate a target region.\u003c/p\u003e","description":"","filename":"14.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9565181/v1/be81e81786ce0b07ffbdb5af.jpg"},{"id":109205131,"identity":"5154c147-33ae-4300-b392-7edd32f7acc5","added_by":"auto","created_at":"2026-05-13 15:03:28","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1282609,"visible":true,"origin":"","legend":"\u003cp\u003eCross-sectional view of potential temperature (a, b) and equivalent potential temperature (c, d) (contour; K) at 125. The arrows indicate meridional and vertical winds (vertical unit: 0.03 m s\u003csup\u003e-1\u003c/sup\u003e, meridional unit: 6.0 m s\u003csup\u003e-1\u003c/sup\u003e), red and orange contours indicate 90% and 80% relative humidity, respectively. (a) and (c) are the results of the L1000 case, while (b) and (d) are those of the NL case.\u003c/p\u003e","description":"","filename":"15.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9565181/v1/76281b6ce74966240bf1be8a.jpg"},{"id":109150413,"identity":"5cf6b2cb-ea7d-43ee-9395-2edf250a14be","added_by":"auto","created_at":"2026-05-13 05:28:24","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":724072,"visible":true,"origin":"","legend":"\u003cp\u003e(a) shows the composite analysis of the CAPE (color; J kg\u003csup\u003e-1\u003c/sup\u003e) of L1000 case. (b) shows that of NL case. CAPE was calculated using parcels lifted from 1000 hPa. The thin black solid line in this figure indicates coastline around Japan. The thick black solid line indicates the target region.\u003c/p\u003e","description":"","filename":"16.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9565181/v1/6ee3fa23cbfcd4b36fa26070.jpg"},{"id":109150417,"identity":"fc4f0665-9285-4d18-958a-1f26adcbdbcb","added_by":"auto","created_at":"2026-05-13 05:28:24","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1657662,"visible":true,"origin":"","legend":"\u003cp\u003eThese figures show SST, SST Laplacian, and sea surface pressure Laplacian in April, June, and August. (a), (b), and (c) show SST (color; ) in April, June and August, respectively. (d), (e), and (f) show SST Laplacian (color; 10\u003csup\u003e-5\u003c/sup\u003e K m\u003csup\u003e-2\u003c/sup\u003e) and (g), (h), and (i) show Laplacian of sea surface pressure (color; 10\u003csup\u003e-4\u003c/sup\u003e Pa m\u003csup\u003e-2\u003c/sup\u003e) in April, June, and August, respectively. To avoid regional effects of typhoons and tropical cyclones, these analyses exclude data from areas affected by such storms within the region, as indicated by black boxes in these figures. In addition, to remove noises, the sea surface pressure Laplacian is smoothed by \u0026nbsp;grids.\u003c/p\u003e","description":"","filename":"17.jpg","url":"https://assets-eu.researchsquare.com/files/rs-9565181/v1/433d7a0d4b544f3577522960.jpg"},{"id":109206668,"identity":"80f0864c-cdd7-4629-a7c2-6d1d5657dc5d","added_by":"auto","created_at":"2026-05-13 15:15:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":11037583,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9565181/v1/d5e19dca-f487-4fd3-8d8e-91677e2bd93d.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Environmental Conditions Associated with Active Lightning Events in April around the Nansei Islands","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eLightning activities suggest active convection, severe weather and abundant ice nuclei. They sometimes bring disasters such as aircraft damage and the loss of human lives. Toward better understanding of global atmospheric electrical circuits, preventing disasters and utilizing the lightning information for weather prediction (Slocum et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), it is important to know their distributions and physical backgrounds.\u003c/p\u003e \u003cp\u003eLightning around Japan has been investigated by several researchers so far, most of which focus on the lightning in summer or winter. For example, ground heating caused by strong solar radiation is a primary factor contributing to active convection with lightning over land in summer (Fujibe \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e1988\u003c/span\u003e; Ishii et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Lightning strikes occur even in winter along the coast of the Sea of Japan (Hayashi and Marui \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Ishii et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). On the other hand, several studies have pointed out the large number of lightning strikes in the vicinity of the Nansei Islands. Iwasaki (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) showed that the annual-mean cloud-to-ground flash (CG) strike distribution exhibits the high frequency over the Kuroshio current along its path around the Nansei Islands and in the east coast of Japan. Kato et al. (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2025\u003c/span\u003e) also showed that lightning occurrence corresponds to the position of the Kuroshio, and that the number of lightning strikes increases in spring around the Nansei Islands. Hayashi and Marui (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) showed that Nansei Islands are the regions characterized by the largest number of lightning days around Japan in spring (March-May). However, as far as the authors\u0026rsquo; knowledge, the environmental conditions that bring enhanced lightning activity around the Nansei Islands in spring have not been investigated in detail.\u003c/p\u003e \u003cp\u003eIn this study, we begin with a description of the monthly distribution of lightning strikes following the method section. To our surprise, very active lightning events were detected in April, which is much earlier than the Baiu season and day-time heating in summer. Thus, the main topic of this paper is to reveal the atmospheric and oceanic background conditions leading to the April lightning bursts (ALBs) around the Nansei Islands. Finally, we summarize the proposed mechanism with some discussion.\u003c/p\u003e"},{"header":"2 Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Lightning data\u003c/h2\u003e \u003cp\u003eLightning data were observed by lightning detection network system (LIDEN). This system consists of 30 stations equipped with detection sensors, and each station approximately 200 km apart and is operated by the Japan Meteorological Agency (JMA). We used longitudes, latitudes (lightning location) and the kind of discharge observed by LIDEN. LIDEN measures CG flash and cloud-to-cloud flash by low frequency (LF) waves. In this study, we focused on CG lightning same as in Ishii et al. (\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). LIDEN data are available from 2006 to 2023, but we used LIDEN data from 2016 to 2023 because observation instruments in western Japan were replaced around 2015 (Drs. Shugo Hayashi and Satoshi Yoshida, personal communication on 29 December 2020). We grouped the data into bins with the intervals of 0.5\u0026deg; in longitude and latitude.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Atmospheric and oceanic data\u003c/h2\u003e \u003cp\u003eTo investigate the daily-mean atmospheric condition, we employed the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis of the global climate (ERA5; Hersbach et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). As for sea surface temperature (SST) data, we employed the 1/4\u0026deg; daily optimum interpolation sea surface temperature (OISST), which is a long-term Climate Data Record developed by National Oceanic and Atmospheric Administration (NOAA) (Huang et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). In this study, OISST data is used from 2016 to 2023.\u003c/p\u003e \u003cp\u003eThe target region around the Nansei Islands is defined as \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:22^\\circ\\:\\text{N}\\)\u003c/span\u003e\u003c/span\u003e to \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:31^\\circ\\:\\text{N}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:122^\\circ\\:\\text{E}\\)\u003c/span\u003e\u003c/span\u003e to \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:131^\\circ\\:\\text{E}\\)\u003c/span\u003e\u003c/span\u003e. To eliminate the effect of typhoons on the atmosphere and ocean, analyses are not conducted when a typhoon is within 800 km of the target region based on the JMA best-track data. To compare the environmental condition of many lightning strikes with that of no lightning strikes in April, we used daily mean environmental data for composite analysis. The criterion for a day with a large number of lightning strikes was defined as 1,000 or more CG lightning strikes. For presentation purposes, these cases are referred to as L1000 and the days with no lightning strikes are referred to as NL. The threshold of 1,000 lightning strikes was selected to ensure comparable sample sizes between the two groups (58 cases in L1000 and 87 cases in NL). In this study, we do not show the composite-mean atmospheric state of events with 1 to 999 lightning strikes. However, the composite-mean of environmental conditions is intermediate between L1000 and NL (figure not shown).\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Feature of lightning strikes in April in the Nansei Islands\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the distributions of number of CG lightning around Japan for 8 years (2016 to 2023) per day from March to August observed by LIDEN. The first notable feature is that high-frequency CG lightning area is located around the Nansei Islands, as indicated by the red box in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb (22\u0026deg;N\u0026ndash;31\u0026deg;N, 122\u0026deg;E\u0026ndash;131\u0026deg;E), and it expands from April as June approaches and shifts to mainland Japan in July and August. One question is why lightning activity was so active in the adjacent seas of the Nansei Islands as early as April. High frequency lightning from May to August can be explained by active convection by the Baiu front and daytime heating in summer. However, the high density of CG strikes around Nansei Islands in April has not been explained so far.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFurthermore, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows that high-frequency lightning area in April is more specially concentrated and localized compared to that in June and July. When each CG day is ranked by the number of lightning strikes from 2016 to 2023, 17 days out of the top 100 days occurred in April. This number is comparable to that in June\u0026ndash;August and substantially larger than in May (5 days). In addition, more than 30,000 strikes per day occurred twice in April during this period, which ranks within the top 5%. Thus, ALBs are particularly significant, and their environmental condition needs to be investigated.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Case study\u003c/h2\u003e \u003cp\u003eTo exemplify the specific environmental conditions during periods of ALBs, the environment condition of 23 April 2019 that had 11,193 lightning strikes was chosen. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows brightness temperature from Himawari-8/9 (Bessho et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Low brightness temperature area is triangular and spreads from 28\u0026deg;N, 127\u0026deg;E to northeast. Well-known carrot-shaped clouds accompanied with heavy rainfall and lightning activity at its tip. We made sure the heavy rainfall (\u0026gt;\u0026thinsp;100 mm/day) for this event based on Global Satellite Mapping of Precipitation dataset (figure not shown). Carrot-shaped clouds tend to persist for up to 10 hours (Itoh et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1992\u003c/span\u003e) and are appear when low-level wind is perpendicular to mid-level wind and air with high equivalent potential temperature (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\theta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e) flows into lower levels (Itoh et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Seko et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, cloud-top temperature of the carrot-shaped clouds is much lower than \u0026minus;\u0026thinsp;20\u0026deg;C, that satisfies the temperature requirement for charge separation mechanism (Takahashi \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e1978\u003c/span\u003e). Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows a daily-mean \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\theta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e and wind at 950 hPa and 500 hPa on the day. Bolton (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e1980\u003c/span\u003e)\u0026rsquo;s formula was used to calculate \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\theta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e. Target region was characterized by high-\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\theta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e air, with southwesterly winds at 950 hPa, and westerly winds at 500 hPa. The vertical wind shear and the inflow of high-\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\theta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e air into lower levels are conditions favorable for the development of a back-and side-building (BSB) type mesoscale convective system, in which new convective cells develop on the upstream side of the mid-level flow. Carrot-shaped clouds are thought to form in association with this type of convective system (Seko and Nakamura \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). The Nansei islands were under southerly-wind condition in the lower troposphere because they were at the western edge of a high-pressure system migrating to the east. It was underneath the southern portion of midlatitude westerly jet. In other words, the migrating high-pressure system provides vertical wind shear and transports the high-\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\theta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e air from the south in its western edge. Our subjective analysis with Himawari-8/9 indicated that carrot-shaped clouds were observed in about two-thirds of L1000.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Composite analysis\u003c/h2\u003e \u003cp\u003eIn this section, composite analyses of L1000 and NL are compared to reveal features of the environmental conditions. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the composite-mean of geopotential height and horizontal wind vector in L1000 and NL groups at 300 hPa, 500 hPa, and 950 hPa. At 300 hPa and 500 hPa, westerly wind blows around the Nansei Islands both in L1000 and NL. One noticeable difference between L1000 and NL is that a relatively deep trough was seen in the northeast of the Nansei Islands in NL than in L1000.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAt 950hPa, south-westerly wind is seen around the Nansei islands in L1000. Checking all 58 cases in L1000, we found that the southerly or southwesterly wind is robust around the Nansei Islands, and they are associated with southerly in the west of a migrating high-pressure system in most cases. Vertical wind shear is one of conditions of the BSB-type mesoscale convective system (Seko and Nakamura \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). In contrast, an anticyclone was typically over the Nansei islands area at 950 hPa and horizontal divergence was observed near the surface in NL.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows a potential temperature (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\theta\\:\\)\u003c/span\u003e\u003c/span\u003e) and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\theta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e along \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:125^\\circ\\:\\text{E}\\:\\)\u003c/span\u003e\u003c/span\u003ein both groups overplotted by the meridional and vertical components of wind. The aspect ratio of a wind vector is set to the same as the aspect ratio of a plot domain. Red and orange contours show the relative humidity of 90% and 80%, respectively. In L1000, wind vectors show the southerly and gentle upward motion from 15\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:^\\circ\\:\\)\u003c/span\u003e\u003c/span\u003eN to 30\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:^\\circ\\:\\)\u003c/span\u003e\u003c/span\u003eN below 900 hPa (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, c). Considering the gentle positive gradient of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\theta\\:\\)\u003c/span\u003e\u003c/span\u003e with increasing latitude in the baroclinic region, the southerly wind naturally undergoes upward motion and thus the air is humidified. Around 25\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:^\\circ\\:\\)\u003c/span\u003e\u003c/span\u003eN\u0026ndash;30\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:^\\circ\\:\\)\u003c/span\u003e\u003c/span\u003eN, the strong upright upward motion is nearly along the line with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\theta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e and crosses the line with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\theta\\:\\)\u003c/span\u003e\u003c/span\u003e. It suggests that the humid air mass moves along keeping certain \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\theta\\:\\)\u003c/span\u003e\u003c/span\u003e until condensation and then it exhibits strong upward motion along with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\theta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e after condensation. This profile underlies the active convection associated with ALBs in L1000. On the other hand, the composite mean profile shows that a flow vector aligned from 40\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:^\\circ\\:\\)\u003c/span\u003e\u003c/span\u003eN\u0026ndash;50\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:^\\circ\\:\\)\u003c/span\u003e\u003c/span\u003eN in the upper troposphere to 30\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:^\\circ\\:\\)\u003c/span\u003e\u003c/span\u003eN in the lower troposphere nearly along the line with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\theta\\:\\)\u003c/span\u003e\u003c/span\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb, d). It implies that the descending flow originated from the upper-level convergence to the west of an upper-level trough as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, d suppresses active convection around the Nansei Islands.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eConvective available potential energy (CAPE) is an important metric for revealing potential of convection. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows CAPE in both L1000 and NL. In L1000, CAPE is larger near the southwest of the target region. The composite-mean of CAPE was approximately 600 J kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e off the coast of east Taiwan. CAPE around the Nansei Islands is as small as 100 J kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. As such, high-CAPE area in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea did not correspond to high-CG-lightning areas ranging from southwest to northeast of the target region in April (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). This is probably because in the southwestern portion outside of the target domain, the CAPE is not activated for strong upward motion until the air condensates and reaches the level of free convection (see Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Also, active convections make atmospheric stability closer to the neutral condition.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSo far, we have shown atmospheric environmental conditions relevant to the ALBs. Another perspective might come from the oceanic environment. In particular, the Kuroshio current is one of the major currents in world oceans that flow clockwise from around the north of equator into south of Japan through the Philippine Sea and the East China sea in North Pacific (Steele et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). The Kuroshio carries warm water from low-latitude ocean. Minobe et al. (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) shows that the Gulf Stream affects the marine boundary layer on it and that causes convection of entire troposphere over the Gulf Stream by pressure adjustment mechanism that leads to surface wind convergence. Minobe et al. (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) shows that surface wind convergence over the Gulf Stream along its path corresponds SST Laplacian using a combination of operational weather analyses, satellite observations and an atmospheric general circulation model. Sasaki et al. (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) shows that a rain band in the East China sea in June is intensified on the Kuroshio because of the same mechanisms of Minobe et al. (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2008\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eFollowing Minobe et al. (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), Sasaki et al. (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2012\u003c/span\u003e), we calculated a contribution from SST Laplacian related to Kuroshio. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows sea surface temperature (SST), SST Laplacian, and sea surface pressure Laplacian in April, June, and August. As consistent with Minobe et al. (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), SST Laplacian well corresponds to sea surface pressure Laplacian, particularly over Kuroshio in April and June. Sea surface pressure Laplacian is expected to be related to divergence through the following formula (Minobe et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2008\u003c/span\u003e):\u003c/p\u003e \u003cp\u003e \u003cdiv id=\"Equ1\" class=\"Equation\"\u003e \u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:-\\left(\\:\\frac{\\partial\\:u}{\\partial\\:x}+\\frac{\\partial\\:v}{\\partial\\:y}\\right){\\rho\\:}_{0}=\\left(\\:\\frac{{\\partial\\:}^{2}P}{\\partial\\:{x}^{2}}+\\frac{{\\partial\\:}^{2}P}{\\partial\\:{y}^{2}}\\right)\\left(\\frac{\\epsilon\\:}{{\\epsilon\\:}^{2}+{f}_{0}^{2}}\\right)\\:$$\u003c/div\u003e \u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:u,\\:v\\)\u003c/span\u003e\u003c/span\u003e is zonal and meridional wind, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\rho\\:}_{0}\\)\u003c/span\u003e\u003c/span\u003e is the density of the marine boundary layer. \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:P\\)\u003c/span\u003e\u003c/span\u003e is sea-surface pressure, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\epsilon\\:\\equiv\\:\\frac{{C}_{D}\\left|{V}_{c}\\right|}{H}\\)\u003c/span\u003e\u003c/span\u003e where \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{C}_{D}\\)\u003c/span\u003e\u003c/span\u003e is a coefficient of friction, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{V}_{c}\\)\u003c/span\u003e\u003c/span\u003e is the scale of typical lower wind, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:H\\)\u003c/span\u003e\u003c/span\u003e is a depth of boundary layer, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{f}_{0}\\)\u003c/span\u003e\u003c/span\u003e is Coriolis parameter. There is a sharp variation of SST between the Kuroshio and adjacent regions in April as warm boundary current intervenes the cold northern ocean, and therefore SST Laplacian in April is much clearer than all the other months shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. Sea surface pressure Laplacian magnitude associated with the Kuroshio is up to 1.0 hPa in April. From the Eq.\u0026nbsp;(\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), since sea surface pressure Laplacian is in proportion of the opposite sign of wind divergence, sea surface wind convergence on the Kuroshio in April is strongest of all the other months (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eg). It is because the SST Laplacian on the Kuroshio is getting unclear as SST rises in summer (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed, e, f). Horizontal distributions of SST Laplacian and wind convergence over the Kuroshio along its path are similar to those of Minobe et al. (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) and Sasaki et al. (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2012\u003c/span\u003e), who indicated that convection can be driven by pressure adjustment mechanism over the Kuroshio. It is notable that the distribution of sea surface pressure Laplacian well corresponds to the distribution of ALBs (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). It suggests that the oceanic condition also plays an important role on active convection in this region. In the same analysis of Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, SST Laplacian and sea surface pressure Laplacian analysis have been conducted in L1000 and NL separately. However, the results were almost the same value in April (figures not shown). Therefore, the pressure adjustment mechanism can help the active convections continually in April, although above-mentioned atmospheric conditions are also prerequisite.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Proposed mechanisms\u003c/h2\u003e \u003cp\u003eHere we summarize the atmospheric and oceanic environmental conditions when ALBs were observed. Around the Nansei Islands, the combination of mid-level westerlies and low-level southwesterly or southerly winds results in vertical wind shear. In most cases, the Nansei Islands are located along the western periphery of a synoptic-scale high-pressure system that transports high-\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\theta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e air into the lower troposphere, thereby enhancing atmospheric instability over the region. In addition, there is a meridional gradient of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\theta\\:\\)\u003c/span\u003e\u003c/span\u003e value to the south of the Nansei Islands that makes the southerly wind have upward components. Under those environments, the pressure adjustment mechanism (Minobe et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) over the Kuroshio further helps the active convections with lightning.\u003c/p\u003e \u003cp\u003eIn contrast, the lower-tropospheric air around the Nansei Island is linked to a downward motion originating from the upper-level convergence in the west of a trough along the nearly constant \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\theta\\:\\)\u003c/span\u003e\u003c/span\u003e value in NL. Therefore, the horizontal divergence is around the Nansei islands at lower troposphere. As a result, convections with lightning are hardly observed, implying that the pressure adjustment mechanism over the Kuroshio does not beat the large-scale suppression.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this study, we first show that very active lightning strikes have been frequently observed around the Nansei Islands in April, which we call ALBs. Then, we proceed to clarify the atmospheric and oceaninc environmental conditions for those ALBs around the Nansei Islands with composite analysis of the environments of L1000 and NL case between 2016 to 2023 in April. The composite analysis of the L1000 case revealed westerly flow at the mid- and upper levels and southwesterly inflow of high-\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\theta\\:}_{e}\\)\u003c/span\u003e\u003c/span\u003e air at low levels, resulting in an unstable atmospheric environment. Furthermore, the mechanisms of convergence of a marine boundary layer over the Kuroshio triggers the convection over the Kuroshio and occurring high frequency lightning around it in April. Further work is needed to check the distribution of seasonal march, polarity and peak current of lightning, aerosols and radar-based observations. Also, we need to clarify the importance of each component by using numerical models. Finally, Virts et al. (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) reported that the lightning is active over Gulf Stream. The similarities and differences should be checked.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv class=\"DefinitionList\"\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eALB\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eApril Lightning Burst\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eBSB\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eBack-and-side building\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eL1000\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eCases with daily lightning strikes\u0026thinsp;\u0026gt;\u0026thinsp;1000 d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eNL\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eCases with no daily lightning strikes\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eCG\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eCloud-to-ground flash\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eLIDEN\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eLightning detection network system\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eJMA\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eJapan Meteorological Agency\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eLF\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eLow frequency\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eECMWF\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eEuropean center for medium-range weather forecasts\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eSST\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eSea surface temperature\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eOISST\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eOptimum interpolation sea surface temperature\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eNOAA\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eNational oceanic and atmospheric administration\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eCAPE\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eConvective available potential energy\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThis work was supported by JSPS KAKENHI Grant Numbers JP24H02226 and JP25K01073.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eHH analyzed the lightning and reanalysis data and was a major contributor in writing the manuscript. KI created figures and suggested the relevant mechanisms. All authors read and approved of the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eWe are grateful to Dr. Shugo Hayashi, Dr. Satoru Yoshida, and Dr. Takuya Kawabata for providing LIDEN data and giving us a lot of advice. We also thank Dr. Hiroyuki Yamada for discussions and giving valuable comments. Finally, we would like to express our sincere gratitude to Dr. Soichiro Hirano for encouragement throughout this study.\u003c/p\u003e\u003ch2\u003eAvailability of data and materials\u003c/h2\u003e \u003cp\u003eLIDEN data was provided by Meteorological Research Institute. It is also available from Japan Meteorological Business Support Center (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.jmbsc.or.jp/jp/online/n-online0.html\u003c/span\u003e\u003cspan address=\"https://www.jmbsc.or.jp/jp/online/n-online0.html\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e). The ERA5 and OISST datasets generated and/or analyzed during the current study are respectively available in the Climate Data Store repository, \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.ecmwf.int/en/forecasts/dataset/ecmwf-reanalysis-v5\u003c/span\u003e\u003cspan address=\"https://www.ecmwf.int/en/forecasts/dataset/ecmwf-reanalysis-v5\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e and National Centers for Environmental Information, \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://www.ncei.noaa.gov/products/optimum-interpolation-sst\u003c/span\u003e\u003cspan address=\"https://www.ncei.noaa.gov/products/optimum-interpolation-sst\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e. Equivalent blackbody temperature (produced from Himawari data) was provided by Japan Aerospace Exploration Agency (JAXA), \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://earth.jaxa.jp/en/data/2529/index.html\u003c/span\u003e\u003cspan address=\"https://earth.jaxa.jp/en/data/2529/index.html\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBessho K et al (2016) An introduction to Himawari-8/9\u0026mdash;Japan\u0026rsquo;s new-generation geostationary meteorological satellites. 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J Atmos Sci 72:2657\u0026ndash;2665. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1175/JAS-D-14-0233.1\u003c/span\u003e\u003cspan address=\"10.1175/JAS-D-14-0233.1\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"geoscience-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"gosl","sideBox":"Learn more about [Geoscience Letters](https://geoscienceletters.springeropen.com/)","snPcode":"40562","submissionUrl":"https://submission.springernature.com/new-submission/40562/3","title":"Geoscience Letters","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Lightning, the Nansei Islands, carrot-shaped clouds, Kuroshio, pressure adjustment mechanism, April","lastPublishedDoi":"10.21203/rs.3.rs-9565181/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9565181/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eA lot of cloud-to-ground lightning strikes sometimes occur around the Nansei Islands in April, while those April lightning bursts (ALBs) have not been investigated. This work aims at elucidating the environment contributing to ALBs through a case study and composite analysis. Two-thirds of the ALBs cases are characterized by a carrot-shaped clouds, indicating a mesoscale convective system known as the back-and-side building. The composite analysis was conducted for cases with daily lightning strikes\u0026thinsp;\u0026gt;\u0026thinsp;1000 d\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (L1000) and with no daily lightning strikes (NL). In both cases, westerly wind blows in the middle-troposphere. In L1000, the Nansei Islands were typically located to the west of a synoptic-scale migratory high at the surface and southerly or south-westerly wind advected the air of high equivalent potential temperature that makes the atmospheric condition more unstable to the Nansei Islands. Low-level south-to-southerly and middle-level westerly consisted of the vertical wind shear favorable for the carrot-shaped clouds. Geographically, the ALBs distribution well corresponded to the local-warm Kuroshio current, implying that the pressure adjustment mechanism over the Kuroshio current helps trigger the ALBs through the convergence. As in L1000, the pressure adjustment mechanism was active in NL; however, synoptic-scale downward motion and higher stability likely prevented the active convection.\u003c/p\u003e","manuscriptTitle":"Environmental Conditions Associated with Active Lightning Events in April around the Nansei Islands","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-13 05:28:06","doi":"10.21203/rs.3.rs-9565181/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"205115114217700828378013226635282978991","date":"2026-05-08T11:48:31+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-05-04T09:13:47+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-05-01T05:27:38+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-04-30T10:02:04+00:00","index":"","fulltext":""},{"type":"submitted","content":"Geoscience Letters","date":"2026-04-29T11:19:11+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"geoscience-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"gosl","sideBox":"Learn more about [Geoscience Letters](https://geoscienceletters.springeropen.com/)","snPcode":"40562","submissionUrl":"https://submission.springernature.com/new-submission/40562/3","title":"Geoscience Letters","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"BMC/SO AJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"14f4c22a-0da6-4618-8caa-080a436b6bb3","owner":[],"postedDate":"May 13th, 2026","published":true,"recentEditorialEvents":[{"type":"reviewerAgreed","content":"205115114217700828378013226635282978991","date":"2026-05-08T11:48:31+00:00","index":8,"fulltext":""},{"type":"reviewersInvited","content":"5","date":"2026-05-04T09:13:47+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-05-01T05:27:38+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-04-30T10:02:04+00:00","index":"","fulltext":""},{"type":"submitted","content":"Geoscience Letters","date":"2026-04-29T11:19:11+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-13T05:28:06+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-13 05:28:06","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9565181","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9565181","identity":"rs-9565181","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}