Statistical Study of Band-Shaped Rainfall Systems, the Koshikijima and Nagasaki Lines, Observed around Kyushu Island, Japan

Abstract

The favorable environmental low-level wind condition under which band-shaped rainfall systems form around Kyushu Island, located in the western part of Japan, was statistically examined using upper-air sounding data at Kagoshima and meteorological radar data. The representative rainbands extending northeastward from the Koshikijima Islands (the Koshikijima line), and the Nagasaki Peninsula (the Nagasaki line), are discussed in this study. The favorable conditions for their formation and maintenance are that the southwesterly low-level wind field persists for longer than 12 hours, and that the wind speed at an 850 hPa level is between 5 m s⁻¹ and 25 m s⁻¹.
When the Koshikijima line stagnates for a long time to cause heavy rainfall, other disturbances are usually found around the Koshikijima Islands. Meanwhile, the Nagasaki line forms more frequently than the Koshikijima line, and it often stagnates for a long time without other disturbance.