Geographical research on the relationship between the occurrence of short-duration heavy rainfall and the spatial structure of surface property in the Tokyo metropolitan area

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In this research, relationships between the occurrence of summertime short-duration heavy rainfall and the urban surface structure in the Tokyo metropolitan area were analyzed in terms of climatology and urban morphology. The results can be summarized as follows :1. The heavy rainfall area in the northern-western and southern area of central Tokyo tends to develop in its neighborhood during a short period, which indicates the characteristic feature of heavy rainfall in the urban area of Tokyo.2. Local-scale differences in the occurrence frequency of heavy rainfall are distinct in the Tokyo ward area. The frequent heavy rainfall areas are found in the western and northern part of the Tokyo ward area. Heavy rainfall in former and latter areas tends to occur corresponding to easterly and southerly wind, respectively. These areas are located in the leeward of high-rise buildings such as Shinjuku and Ikebukuro. It is suggested that convergence of surface wind due to large surface roughness enhances the convective activity in the urban area.3. The calculation method of sky view factor (SVF) by using digital surface model (DSM) was introduced, and the calculation of SVF over the Tokyo ward area was conducted. As the spatial representativeness of SVF at a given point is scarce, scale-up of SVF is required for the discussion on the relationship between urban heat island and SVF. The typical SVFs in the Tokyo ward area are estimated from SVF histogram of 1km×1km areas.4. The aerodynamic parameters in the Tokyo ward area were computed by using DSM, and horizontal/vertical wind velocity distribution was evaluated. Large ascending wind appears Shinjuku-Meguro, Ikebukuro, etc., and the areas of ascending wind vary with wind directions. It is concluded that the local-scale differences in the occurrence frequency of heavy rainfall is strongly related to the surface roughness which high-rise buildings forms.