Formation and Maintenance Processes of a Stationary Band-shaped Heavy Rainfall Observed in Niigata on 4 August 1998.

Abstract

On 4 August 1998, a torrential rainfall was observed in the Niigata area, in central Japan. The rainfall was induced by a quasi-stationary rainband aligned in the east-southeast direction from the west of Sado Island. The rainband lasted for several hours with the characteristic features of a back-building formation, in which new convective cells were successively generated in an upstream direction. The rainfall area and intensity simulated by a nonhydrostatic mesoscale model with a 5-km horizontal grid agreed well with the observation. The rainband with a back-building nature was reproduced successfully by a nonhydrostatic model with a 2-km grid. The simulation results showed that the heavy rainfall formed as a result of strong convective instability enhanced by low-level humid and middle-level dry inflows from the southwest into the weak convergence region associated with the Baiu frontal zone. Sensitivity experiments indicated that neither the terrain of Sado Island, nor the evaporative cooling of raindrops affected the formation of the rainband. Although moved to the downstream when the low-level wind convergence was not enhanced enough, the simulated rainband stagnated after the convergence became so strong that the whole low-level air could be lifted upward while crossing the convergence zone. Therefore, the quasi-stationary rainband was maintained by the convection-enhanced low-level wind convergence existing upstream of the rainband. Over the convergence zone associated with the quasi-stationary rainband, new convective cells successively formed and separated from the zone after having developed higher. This successive formation of new cells could maintain a back-building formation. Such a back-building formation was found at several points over the wind convergence zone. These meso-β-scale convective systems with a back-building nature produced the precipitation region with a length over 200 km.