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[AAS10-18] Occurrence timing of lightning jumps in convective precipitation in the Kanto region
Keywords:Convective thunderstorms, Extreme events, Japanese Lightning Detection Network (JLDN) data, Cumulonimbus
1. Introduction
In the Kanto region, severe local convective thunderstorms frequently occur in the afternoon in summer. These thunderstorms bring serious damage such as inside flooding by extreme precipitation and destruction by gusts and tornades. Prediction of these extreme events in advance is expected to reduce the disasters.
In recent years, increased lightning activity has been recognized as a sign to the occurrence of extreme events like heavy rains and tornadoes. The sudden increase in lightning activity is known as “lightning jumps (LJs).” Nishihashi et al. (2015) focused on the relationship between mesocyclones and LJs in the Kanto Plain. The results showed the gusts occurred 14 minutes after the LJ. However, Nishihashi et al. (2015) analyzed only one case; it is necessary to examine multiple cases to determine whether the timing of LJs is consistent. Furthermore, statistical analysis of the relationship between the timing of LJs and the developmental process of rain clouds is expected to improve the prediction accuracy of extreme events.
In this study, we analyzed 64 cases of convective precipitation that occurred in the Kanto region during August 2014-2023, focusing on the relationship between the timing of LJs and the developmental stage of cumulus clouds. This study used lightning data from the Japanese Lightning Detection Network (JLDN), provided by Franklin Japan Co., and precipitation intensity data from synthetic radar GPV.
2. Analysis and Results
For LJs detection, this study used the 2σ algorithm (Schultz et al. 2009) which is considered the highest detection probability and the lowest false alarm rate. LJs were detected in 26 out of 64 case days, and the number of LJs was 107. Gusts occurred 12 times within an hour after LJs detection. About 50% gusts in case days had LJs detect before the gusts occurred.
The lead time between LJs and gusts was immediately before the gust to 35 minutes in 11 of the 12 gusts. It is within the range of lead times assumed in previous studies, as Schultz et al. (2009) proposed a gust warning system for 45 minutes after LJs detection. This result confirms that in some cases, LJs in the Kanto region occur before the onset of gusts, as in previous studies.
We investigated how LJs relate to lightning activity and precipitation. The number of lightning strikes increased sharply from 10 minutes before LJ to the LJ time, and remained active for about 20 minutes after LJ time. The area of heavy rainfall (precipitation intensity >30 mm/h) reached a maximum at 20 minutes after LJ time. This relationship means that LJs occur prior to the development of the heavy rainfall area.
Based on the analysis, we discuss the meteorological significance of LJs, focusing on their relationship with the life cycle of thunderclouds. The life cycle of thunderclouds is said to have three stages: the developing stage, the mature stage, and the dissipating stage (Kitagawa 2001). As a result, LJs occurred before the peak of development of the heavy rainfall area. Furthermore, LJs occurred before the gusts. Gusts are known to occur in the early mature stage of cumulus clouds (Ishihara and Tabata 1995). LJs did not occur after the peak of the development of a strong rainfall area. This suggests that LJs occur at the timing of the beginning of the mature stage.
3. Summary
This study indicates that LJs occur before extreme events and the developmental peak of intense rainfall areas, suggesting that LJs are a precursor to the further thundercloud development. Therefore, the detection of LJs is considered an promising indicator for estimating thunderstorm activity during the developmental stage. Future study will explore the practical use of LJs in predicting extreme events.
In the Kanto region, severe local convective thunderstorms frequently occur in the afternoon in summer. These thunderstorms bring serious damage such as inside flooding by extreme precipitation and destruction by gusts and tornades. Prediction of these extreme events in advance is expected to reduce the disasters.
In recent years, increased lightning activity has been recognized as a sign to the occurrence of extreme events like heavy rains and tornadoes. The sudden increase in lightning activity is known as “lightning jumps (LJs).” Nishihashi et al. (2015) focused on the relationship between mesocyclones and LJs in the Kanto Plain. The results showed the gusts occurred 14 minutes after the LJ. However, Nishihashi et al. (2015) analyzed only one case; it is necessary to examine multiple cases to determine whether the timing of LJs is consistent. Furthermore, statistical analysis of the relationship between the timing of LJs and the developmental process of rain clouds is expected to improve the prediction accuracy of extreme events.
In this study, we analyzed 64 cases of convective precipitation that occurred in the Kanto region during August 2014-2023, focusing on the relationship between the timing of LJs and the developmental stage of cumulus clouds. This study used lightning data from the Japanese Lightning Detection Network (JLDN), provided by Franklin Japan Co., and precipitation intensity data from synthetic radar GPV.
2. Analysis and Results
For LJs detection, this study used the 2σ algorithm (Schultz et al. 2009) which is considered the highest detection probability and the lowest false alarm rate. LJs were detected in 26 out of 64 case days, and the number of LJs was 107. Gusts occurred 12 times within an hour after LJs detection. About 50% gusts in case days had LJs detect before the gusts occurred.
The lead time between LJs and gusts was immediately before the gust to 35 minutes in 11 of the 12 gusts. It is within the range of lead times assumed in previous studies, as Schultz et al. (2009) proposed a gust warning system for 45 minutes after LJs detection. This result confirms that in some cases, LJs in the Kanto region occur before the onset of gusts, as in previous studies.
We investigated how LJs relate to lightning activity and precipitation. The number of lightning strikes increased sharply from 10 minutes before LJ to the LJ time, and remained active for about 20 minutes after LJ time. The area of heavy rainfall (precipitation intensity >30 mm/h) reached a maximum at 20 minutes after LJ time. This relationship means that LJs occur prior to the development of the heavy rainfall area.
Based on the analysis, we discuss the meteorological significance of LJs, focusing on their relationship with the life cycle of thunderclouds. The life cycle of thunderclouds is said to have three stages: the developing stage, the mature stage, and the dissipating stage (Kitagawa 2001). As a result, LJs occurred before the peak of development of the heavy rainfall area. Furthermore, LJs occurred before the gusts. Gusts are known to occur in the early mature stage of cumulus clouds (Ishihara and Tabata 1995). LJs did not occur after the peak of the development of a strong rainfall area. This suggests that LJs occur at the timing of the beginning of the mature stage.
3. Summary
This study indicates that LJs occur before extreme events and the developmental peak of intense rainfall areas, suggesting that LJs are a precursor to the further thundercloud development. Therefore, the detection of LJs is considered an promising indicator for estimating thunderstorm activity during the developmental stage. Future study will explore the practical use of LJs in predicting extreme events.