A field experiment was performed in 25 parks and school yards in Tsukuba city, Ibaraki Prefecture, for 6-9 August 2008 to examine city-scale urban heat island phenomenon on clear summer nights. In addition, uncertainty in urban-rural temperature difference associated with observation site selection is evaluated. The results show that a center of heat island with the highest temperature is found in downtown Tsukuba near the Tsukuba station. Secondary centers are found around the Kenkyu-gakuen station, as well as in the boundary between Tsukuba and Tsuchiura cities. Nocturnal temperatures observed at four rural sites are similar to each other, but temperatures in four sites in the city center varied by >1℃ depending on time. The uncertainty range in temperature difference between particular sites inside and outside of urban area mau be as large as 2.2℃

This study demonstrated a mitigation effect of thermal environment by vegetation in the Hikarigaoka housing complex, Nerima, Tokyo, in summer, using our observed surface air temperature, humidity and Wet Bulb Globe Temperature (WBGT). It was found that surface air temperature of the inside of the housing complex was lower than that of the outside both in daytime and nighttime, which suggested temperature was reduced by evapotranspiration of the vegetation in daytime. On the other hand, higher relative humidity in the inside of the housing complex was observed, compared with that in the outside. Particularly, relative humidity in the inside of the housing complex reached nearly saturated condition in nighttime. This result may suggest tree-planting leads to not only reduction of high temperature but also high relative humidity as an adverse effect in nighttime.

There are few studies that conducted urban climate projection using Regional Climate Model (RCM) with high horizontal resolution that is capable of resolving urban area. In this study, urban climate projection for the 2070’s August for the three major Japanese metropolitan areas was performed by the Weather Research and Forecasting (WRF) model with 3-km grid increment coupled with un Urban Canopy Model (UCM). Dynamical downscale approach is adopted to simulate 2070’s urban climate using MIROC3.2-medres with IPCC SRES A2 emission scenario as a downscale source. The observed characteristics of heat island at early morning is well simulated both in 2000s and 2070s on the three major metropolitan area. Surface air temperatures at 5 Japan Standard Time (JST) in 2070’s in Nerima (Tokyo), Nagoya, and Osaka are 27.5℃, 28.5℃, and 28.7℃, and are 3.1℃, 2.9℃, and 3.2℃ higher than that of 2000’s, respectively. As in observations, the simulated mid-day (14 JST) temperature is the highest in the inland areas of the three major cities both in 2000s and 2070s. The temperature at 14 JST in 2070’s in Nerima, Nagoya, and Osaka are 33.6℃, 34.9℃, and 34.9℃, and are 3.1℃, 3.4℃, and 3.0℃ higher than that of 2000’s, respectively.

The City of Osaka has several urban waste-to-energy incineration facilities that both contribute to, and are affected by, the urban heat island. Applying mist evaporative cooling to the forced-draft, air-cooled condensers at these plants can both reduce the exhaust air temperature and increase the amount of electricity produced. The condenser reaches over 70℃ and the exhaust is released at 30m height. Power output drops by over 1%/℃ as air temperatures rise in summer. A misting system was installed over 1/8 of the air intake at a 14.5MW plant. Water droplets with a Sauter mean diameter of 45 microns were sprayed at up to 2.88tons/hr with near-complete mist evaporation. The exhaust air temperature dropped an average of 1.0K. If implemented full-scale, it should yield exhaust temperature reductions of 4-5K. The cooling effect increased the power output by 1.2% for misting periods greater than 45 minutes, but uncertainty is high. Full scale use at all Osaka plants during the summer could increase power generation by 6MW, offsetting 1700 tons of CO2 per season.