It is difficult to forecast hourly rainfall locally even using the latest meteorological models, although hourly rainfall averaged spatially to some extent can be used for calculating practical rainfall. This study conducts numerical experiments with triple nesting on the 2012 heavy rainfall event in northern Kyushu using the weather research and forecasting (WRF) model and examines the features of hourly rainfall averaged spatially. The dependence of rainfall is averaged spatially on a spatial averaging scale and clarified by comparing rainfall calculated by simulation using the WRF model with radar/AMeDAS precipitation analysis data. This study’s findings indicate the effective spatial averaging scale making relative error of calculated values to the observed ones minimum.

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 km2lasting 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.

学生員 修(工) 九州大学大学院 総合理工学府(〒816-8580 春日市春日公園6-1) 2 正会員 工博 九州大学大学院教授 総合理工学研究院(〒816-8580 春日市春日公園6-1)The data obtained in the summer seasons of 2003 and 2004 were analyzed to examine the formation process of the urban heat island. The heat island structures with concentric isothermal lines are formed over Fukuoka metropolitan area when the background wind is weak. When the wind is strong, they are deformed because of the advection and the mixing of heat. When the background wind is weak and the cloud amount in the nighttime is small, the radiation cooling occurs and the falling rate of temperature in the rural area becomes larger than that in the downtown area. Its difference forms an obvious heat island structure over the whole area of Fukuoka metropolitan area. The heat island intensities decrease with the increase of the background wind speed and the cloud amount. The larger the solar radiation is in the daytime, the stronger the heat island intensities become in the nighttime, because the heat stored in the down town area is released in the nighttime.

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Numerical sensitivity experiments are made to investigate better combination of physical schemes in a meteorological numerical model, i.e., The Weather Research and Forecasting (WRF) by comparing numerical results with radar-AMeDAS data. The numerical experiments are carried out in three nested computational domains to simulate the heavy rainfall event in northern Kyushu in July 2012. It is difficult to predict accurately precipitation at a local point even through the latest meteorological model. The present study also examines whether computational precipitation averaged within spatial scale can provide practical precipitation or not. The time lag of hourly precipitation and the relative error of total precipitation simulated by the WRF model vary depending on the scheme combination and the spatial scale for averaging. The WRF model with the better scheme combination reproduces approximately spatial distributions of 12-hours accumulated precipitation from the radar-AMeDAS data. The present results suggest that there is an effective spatial averaging scale for evaluating practical precipitation.

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