Analytical and numerical study of the 26 June 1998 orographic rainband observed in Western Kyushu, Japan

Abstract

On 26 June 1998 during a field experiment called X-BAIU-98, an orographic rainband, called the Nagasaki line in this study, was observed extending northeastward from the Nagasaki Peninsula in western Kyushu, Japan. The convective cells in this rainband, which were about 5 km in horizontal scale and 40 min in duration, propagated northeastward at a speed of about 10 m s-1. They were deep in height in a northeastward direction. Around the Nagasaki line, a moist convectively unstable atmosphere was observed in the lower layer together with environmental winds that included southerly winds near the surface and southwesterly jet at 3-4 km in height. Numerical simulations of the Nagasaki line were conducted using an operational Regional Spectral Model (RSM) of the Japan Meteorological Agency and a Nonhydrostatic Cloud Model (NHM) of the Meteorological Research Institute. While the RSM simulated only a weak precipitation area in western Kyushu, the NHM reproduced many characteristics of the observed Nagasaki line. Sensitivity experiments for topography, humidity and wind profiles showed that a moist convectively unstable atmosphere, mesoscale convergence, and winds having both a strong southwesterly jet at 3-4 km and a strong vertical wind shear in the lower troposphere are essential for the formation of the Nagasaki line. Although small and low, mountains on the Nagasaki Peninsula are capable of forming an organized precipitation band under such environmental fields.