Capturing Extreme Water Vapor and Instability With High‐Resolution GNSS Monitoring

ABSTRACT This study utilized data from a high‐resolution Global Navigation Satellite System (GNSS) network with a horizontal spacing of less than 10 km, including stations operated by private companies, to analyze the horizontal and vertical distributions of water vapor during a sustained heavy precipitation event in Japan. The network revealed concentrated regions of high precipitable water vapor (PWV > 70 mm) within a narrow area, accompanied by the development of a moist absolutely unstable layer (MAUL). PWV values approached 80 mm in some areas, and a tomographic analysis identified a deep moist layer associated with heavy rainfall and abundant moisture. These findings demonstrate that the high‐resolution GNSS network effectively captured the fine‐scale three‐dimensional water vapor structure associated with a heavy rainfall system composed of multiple deep convective clouds. The results suggest its potential to enhance water vapor monitoring and forecasting for extreme precipitation events.