Temporal and Spatial Variation of Urban Cloud in Various Coastal Cities in Japan

Formation of cloud and its thickness highly effects the intensity of downward shortwave radiation at the surface level (Thapa Chhetri et al. (2015)). The objective of the study is to see the temporal and spatial variation of urban cloud in various coastal cities in Japan, using satellite downward shortwave radiation dataset developed by Takenaka et. al (2011) .The diurnal variation of cloud was analysed by calculating the clear sky rate (CSR). Among the study area, Tokyo, Osaka and Fukuoka are the first, third and the seventh largest city in Japan, where as Hiroshima is the smaller city compared to Tokyo or Osaka. Various studies have shown the effect of Urban Heat Island (UHI) in these cities. Diurnal spatial analysis of the cloud with the help of CSR was conducted in the study areas between June, July and August. Around 14-17 days with 50%-80% cloud coverage were selected as the representative days for this study. The study shows that, Tokyo plane is almost covered with cloud with no appreciable effect of urbanization on the cloud up to the mid-day. In the late afternoon, formation of clear clouds line can be seen in the urbanized area. The result agrees well with those observed by Kanda et al. (2001), which suggest that the formation of cloud in that particular place is due to (i) the convergence of two sea breezes from Tokyo Bay and the Pacific Ocean, respectively, and (ii) additional heating due to an urban heat island.

In Osaka, the formation and distribution of cloud tends to be higher at the reasons around the edge of the urban area in the Osaka plane such as north and east part of Osaka. The pattern is similar with the result shown by Kitao et al. (2009) in terms of temperature, which suggests that the sea breeze from the Osaka bay considerably influences the temperature around the Osaka plane. The temperature tends to be higher at the regions around the edge of the urban area in the Osaka plane. Inoue et al. (2009) have suggested that the distribution of small clouds is sensitive to the thermal contrast.

The gorge at the southeast part of Fukuoka and the northern part of Hiroshima is covered by the cloud almost throughout the day. Kawamoto et al. (2015) and Matsuo et al (2014) have shown the similar pattern of temperature at the Fukuoka and Hiroshima gorge respectively. The results show that the temperature of both the gorges are higher compared to the other parts of the plane.

This study concludes that the formation and distribution of cloud is highly affected by the wind speed and its direction. Due to the active sea breeze in the coastal areas, the presence of cloud in the coastal area is low compared to the areas far from the coastal areas and the mountaintops. The effect of urbanization in all the coastal areas are not so prominent except Tokyo. However, the special features like, the formation of cloud lines above the urban area in Tokyo, the gorges in Fukuoka and Hiroshima are found in this study. With reference to other urban heat island studies in these study areas, it shows that the cloud formation is likely to be higher in areas having higher temperature. Additional study like numerical simulations to confirm and to know the actual mechanism of the special features are necessary. In this study, the dataset is used regardless of the wind intensity and direction. Since the formation and distribution of cloud also depends on wind property, further investigation into the effect of urbanization on the cloud using calm days' dataset, therefore, still required.

REFERENCE

• Thapa Chhetri, D.B., Moriwaki, R.2015: ‘Analysis of cloud properties in the Matsuyama plane using downward solar radiation dataset from geostationary satellite', ICUC9(Abstract),2015 • Takenaka, H., Nakajima, T. Y., ‘Higurashi,A., Higuchi, A., Takamura, T., Pinker, R. T., and Nakajima. T. 2011. Estimation of soalr radiation using a neural network based on radiative transfer', Journal of Geophysical Research, Vol.116, No.D8. • Kanda, M., and Inoue, Y., 2001: ‘Numerical Study on cloud line over an urban street in Tokyo', Boundary-Layer Meteorology 98, pp.251-273 • Kitao, N.,Moriyama, M., Nakajima, S.,Tanaka, T., Takebayashi, H. 2009: ‘The characters of Urban Heat Island based on the comparision of temperature and wind field between present land cover and potential natural land cover', The seventh International Conference on Urban Climate. • Inoue, T.,and Kimura, F., 2009: ‘Fair- weather cumulus clouds forming over urban areas around Tokyo',The seventh International Conference on Urban Climate • Kawamoto, Y.,2015:‘Effect of urbanization on the urban climate in the coastal city, Fukuoka-Kitakyushu meteropolitan area,Japan',(ICUC9 Abstract), • Matsuo, K., Tabaka, T.,2014:'Urban environmental climate maps for urban planning considering urban heat island mitigation in Hiroshima', Journal of Heat Island Institute International, Vol.9-2