Abstract
Continued from the previous paper, we carried out the field observations about the microclimate in and around the Ota River flowing through Hiroshima City. This time, both horizontal and vertical extents of thermal river-effect in warmer seasons were confirmed by the detailed moving observation along the crossed streets and by the balloon observation above the river and in downtown area simultaneously. The routes of measurement were selected from the viewpoints of a river width, a road width and a building density. Speaking about the river-effect as a cooling source, in case of see-breeze blowing approximately parallel to the river, cooler temperatures are wider spread as a density of buildings is lower and both streets and rivers are wider. And it is also affected by wind speed and direction. Namely, the effect of the river is large when the wind is strong and it includes the component of wind direction from the river to the city area. The bank along the river side obstructs the diffusion of cooler air-mass above the river. According to the comparison of temperature distributions, the bank 4.3 m high has the equivalent effect of 70 m distance from the river side. Regarding the vertical extent, the midday difference of air temperature between the river and the built-up area is discernible more than 80 m in summer. In particular, these differences are conspicuous up to 20 m, which corresponds to the average height of buildings in the area. In addition, the upper limit of this prominent temperature difference varies with wind velocity. That is, the depth of thermal boundary layer in the city area is reduced in case of windy conditions. From these results, we suggest that a well-ventilated city design should be considered to improve the unbearable hot summer environment in Japan.
