The terrain roughness in the 23 wards of Tokyo cannot be easily evaluated because of a variety of structures with different dimensions. The authors have characterized the city from the viewpoint of terrain roughness, using the numerical data of dimensions of all structures in the 23 wards. The area of the 23 wards was divided into about 2,600 regions with a 500m mesh, and the terrain roughness of each region was analyzed. The results indicate that the regions are classified into the following three groups; that is, (i) regions in which the ratio Rf≥4 of the building area to the mesh area for the buildings with four stories or over is less than 20%, (ii) regions in which 20%≤ Rf≥4 <40%, and (iii) regions in which Rf≥4≥40%. In addition to the conventional homogeneous terrain roughness, resulting from low-rise buildings, the influence of buildings with four or more stories should be taken into consideration in the evaluation of the wind speed profile for design purpose.
In Japan, strong winds generally appear when an atmospheric depression was caused by Typhoon or extratropical storm. In this paper the air density is shown as a function of the atmospheric pressure and temperature. The air density distributions near ground in severe storms have been clarified using meteorological data during recent ten years which have been observed by the Japan Meteorological Agency. The recommendation of the air density for the structural design on buildings and structures is obtained as ρ0 = 1.18(kg/m3); V10≥ 25m/s, ρ0 = 1.45-0.008V10(kg/m3); 10 ≤ V10< 25m/s, where ρ0 means the air density at sea level and V10 means the wind velocity at 10m high above the ground. It has also been clarified the air density dose not almost change by humidity or some rain drops.