Presentation Information
[O12-P79]Wind conditions of cumulonimbus clouds causing localized heavy rainfall in Tokyo.
*Toshio Kawabata1 (1. Tokyo metropolitan high school of science and technology)
Keywords:
Meteology,Cumulonimbus cloud
In recent years, urban disasters caused by localized heavy rainfall have been increasing in Tokyo. For example, on July 10, 2025, when a record-breaking short-term heavy rainfall event was reported, flooding occurred at major locations such as Shibuya Scramble Crossing and Ichigaya Station, significantly disrupting transportation. Motivated by this, I focused on cumulonimbus clouds—the type of cloud responsible for such damage—and became interested in understanding how they form and what types of wind conditions contribute to their development.
The objective of this study is to identify the wind directions that make cumulonimbus cloud formation more likely in Tokyo, thereby improving prediction techniques and reducing urban disaster risks. The hypothesis was that, across the entire Tokyo metropolitan area, easterly to southerly winds contribute to the formation of cumulonimbus clouds.
The research method involved extracting rainfall events from 2019 to 2025 in which precipitation of 10 mm or more per hour was observed (as there is no clear standard definition for localized heavy rainfall). Among these, cases were selected in which cumulonimbus clouds developed locally. The selection criteria were that no typhoons or frontal systems were present nearby and that the rain clouds originated within the Kanto region. The causative clouds were identified using satellite imagery, and wind direction data from AMeDAS stations one hour prior to cloud formation were analyzed to determine prevailing wind conditions. Using this method, 24 rainfall events in Tokyo caused by cumulonimbus clouds formed within the Kanto region were identified during the study period.
The results showed that the frequency of easterly to southerly winds (east, east-southeast, southeast, south-southeast, and south) was approximately 84% at the Tokyo observatory, 54% at Nerima, 71% at Edogawa Rinkai, 87% at Haneda, 68% at Fuchu, 62% at Hachioji, 54% at Ome, and 31% at Ogouchi. Additional noteworthy findings include that calm conditions accounted for 17% of observations at the Nerima station. At the Ogouchi station, northwesterly winds accounted for 17% and northerly winds for 9%, indicating a relatively high frequency of northerly winds.
From these results, it can be inferred that at the Tokyo, Edogawa Rinkai, and Haneda observatories, the high frequency of easterly to southerly winds suggests the inflow of air from the sea. However, in the western observation sites of Tokyo, the proportion of such winds was not particularly high, and other wind directions, including northeasterly and west-southwesterly winds, were observed. Therefore, while a tendency for easterly to southerly winds was identified in the eastern part of Tokyo’s 23 wards, the hypothesis that such wind patterns dominate across the entire Tokyo area was rejected.
For regions west of the 23 wards (such as Ogouchi and Ome), it is suggested that southerly winds are not a primary factor in cumulonimbus cloud formation, and that other factors may be responsible.
As for future prospects, since this study found that southerly winds alone do not fully explain cumulonimbus formation, further analysis of additional factors will be conducted to better identify the causes and contribute to improving the accuracy of cumulonimbus cloud prediction.
References:
Weathernews Pro; Japan Weather Association, Historical Weather Data and Satellite Imagery; Japan Weather Association, Historical Weather Data and AMeDAS
Acknowledgments:
I would like to express my sincere gratitude to Mr. Matsuoka of the Public Relations Office of the Japan Meteorological Agency for his support in reviewing the research methodology.
The objective of this study is to identify the wind directions that make cumulonimbus cloud formation more likely in Tokyo, thereby improving prediction techniques and reducing urban disaster risks. The hypothesis was that, across the entire Tokyo metropolitan area, easterly to southerly winds contribute to the formation of cumulonimbus clouds.
The research method involved extracting rainfall events from 2019 to 2025 in which precipitation of 10 mm or more per hour was observed (as there is no clear standard definition for localized heavy rainfall). Among these, cases were selected in which cumulonimbus clouds developed locally. The selection criteria were that no typhoons or frontal systems were present nearby and that the rain clouds originated within the Kanto region. The causative clouds were identified using satellite imagery, and wind direction data from AMeDAS stations one hour prior to cloud formation were analyzed to determine prevailing wind conditions. Using this method, 24 rainfall events in Tokyo caused by cumulonimbus clouds formed within the Kanto region were identified during the study period.
The results showed that the frequency of easterly to southerly winds (east, east-southeast, southeast, south-southeast, and south) was approximately 84% at the Tokyo observatory, 54% at Nerima, 71% at Edogawa Rinkai, 87% at Haneda, 68% at Fuchu, 62% at Hachioji, 54% at Ome, and 31% at Ogouchi. Additional noteworthy findings include that calm conditions accounted for 17% of observations at the Nerima station. At the Ogouchi station, northwesterly winds accounted for 17% and northerly winds for 9%, indicating a relatively high frequency of northerly winds.
From these results, it can be inferred that at the Tokyo, Edogawa Rinkai, and Haneda observatories, the high frequency of easterly to southerly winds suggests the inflow of air from the sea. However, in the western observation sites of Tokyo, the proportion of such winds was not particularly high, and other wind directions, including northeasterly and west-southwesterly winds, were observed. Therefore, while a tendency for easterly to southerly winds was identified in the eastern part of Tokyo’s 23 wards, the hypothesis that such wind patterns dominate across the entire Tokyo area was rejected.
For regions west of the 23 wards (such as Ogouchi and Ome), it is suggested that southerly winds are not a primary factor in cumulonimbus cloud formation, and that other factors may be responsible.
As for future prospects, since this study found that southerly winds alone do not fully explain cumulonimbus formation, further analysis of additional factors will be conducted to better identify the causes and contribute to improving the accuracy of cumulonimbus cloud prediction.
References:
Weathernews Pro; Japan Weather Association, Historical Weather Data and Satellite Imagery; Japan Weather Association, Historical Weather Data and AMeDAS
Acknowledgments:
I would like to express my sincere gratitude to Mr. Matsuoka of the Public Relations Office of the Japan Meteorological Agency for his support in reviewing the research methodology.