Volume 90 (2012) Issue 5 Pages 737-753
A heavy rainfall event occurred along the Ibuki-Suzuka Mountains situated to the west of the Nobi Plain, Japan on September 2-3, 2008. This event was caused by a stationary precipitation band that formed along the mountains with a north-south alignment. This study examines the maintenance mechanism of the precipitation band and describes the characteristics of high rainfall intensity in the band. For this purpose, data from the Japan Meteorological Agency (JMA) radar, JMA wind-profiler radar, and a dual-Doppler analysis from two X-band polarimetric Doppler radars are used. The band stagnated for 13 hours from 12 Japan Standard Time (JST; 9 hours ahead of UTC) on September 2. Its length and width were approximately 100 and 20 km, respectively.
Low-level warm and moist southeasterly winds with equivalent potential temperature greater than 355 K below 1 km impinged on the eastern slope of the mountains and continuously developed precipitation cells. These cells propagated northward by southerly winds above 1 km and contributed to the formation of the precipitation band. The maintenance of the precipitation band can be attributed to a persistent vertical wind shear; that is, the low-level southeasterly and mid-level southerly winds.
The characteristics of high rainfall intensity in the precipitation band is examined by dual-Doppler analysis. Low-level southerly winds with high equivalent potential temperature converged over a microscale wedge-shaped valley that opens southward between the Ibuki-Suzuka Mountains and its branch, the Yoro Mountains aligned north-northwest to south-southeast. The existence of graupel particles near the melting level and a high amount of large raindrops below it, depicted by the polarimetric radar, are possible causes for the increased precipitation in the region.