Meteorological Characteristics of Local Heavy Rainfall in the Fukuoka Plain

Heavy local rainfall has been increasingly observed in urban Fukuoka on fine summer afternoons in recent years. Such rainfall tends to occur suddenly on calm afternoons and is considered to be caused by local wind conditions influenced by local topography rather than by weather fronts or typhoons. This local rainfall is considered to be caused by a mechanism different from similar rainfalls occurring on fine Kanto plain afternoons. We set up 14 rain gauges in urban Fukuoka in this study to clarify and confirm actual local rainfall conditions there. Maximum local rain is about 64 km 2 lasting 10 to 30 minutes. The maximum 10-minute rainfall was 13.8 mm. The average surface air temperature on days with local rainfall differs 2°–3°C from that on fine days. Upper atmosphere humidity distribution differs greatly between fine days and those with heavy local rain. Accordingly, heavy local rain is more likely to occur if surface air temperature and humidity in upper atmosphere rise above a certain level. Some difference is seen between days of heavy local rainfall and fine day in terms of the K index ( KI ), a measure of atmospheric stability. We confirmed that the atmospheric state becomes more unstable on days with heavy local rainfall than on fine days. Heavy local rainfall often begins in either the eastern or western inland Fukuoka plain and moves toward the coast. That is, based on numerical simulation using the meteorological mesoscale weather research and forecasting (WRF) model, wind blowing opposite to the sea wind blows in the upper atmosphere, moving cumulonimbus clouds causing heavy local rainfall toward the coast. We also confirmed that heavy local rainfall tends to occur in eastern inland areas with wind from the west, but tends to occur in western areas with wind from the east. We therefore assumed that heavy local rainfall in urban Fukuoka was triggered by updrafts generated when wind struck the inland Fukuoka plain mountain system.