Spectral Parameters of GNSS Wet-Delay Fluctuations as Precursors of Baiu Rainbands Over Kyushu

Quasi-stationary rainbands cause most flood disasters of the Baiu season in western Japan, yet where they form remains difficult to forecast. Global Navigation Satellite System (GNSS) antennas measure the water vapor above them. The fluctuations of this signal also record the size and contrast of the moisture structures drifting overhead. Over the Baiu seasons of 2023 and 2025, 184 GEONET stations on Kyushu show that these structures grow larger and more contrasted in the hours before rain, at night as in the afternoon and at equal water vapor. Combined into a texture index, both changes add to radar extrapolation far from existing echoes. The six-hour change of this index locates imminent rain better than the water vapor convergence index used in Japan. On maps, areas of high index travel with rain and lie on the inflow side of stationary rainbands, where new cells form.