2004 Volume 77 Issue 6 Pages 403-420_1
Green spaces in urban areas as well as water surfaces have been expected to mitigate the urban heat island effect. In the hot and humid summers in Japan, their cooling effect is especially valuable as a natural resource for city planning. In this paper, we show the results of micro-climatologic observations performed in and around the large park Shinjuku Gyoen during summer. The park is 58.3ha, and located in one of the main business districts of Tokyo. It consists of a dense forest area, lawn area, and ponds. The topography in the park is almost completely flat but is a few meters lower around the ponds than their surroundings.
We observed the air temperature distribution in and around the park at 1-min intervals. We placed a total of 88 automatic temperature recorders along three measuring lines which crossed through the park. To record the cold air-seep phenomenon during calm conditions, we placed four three-dimensional ultrasonic anemometer-thermometers along the park boundary and at the center of the park. Their measuring height was about 1.5 m above ground, and sampling frequency was 10Hz.
The park was always cooler than the surrounding built-up area. On the other hand, there was a temperature difference within the park, i.e., the cool-island intensity was larger for the park forest during the daytime, but larger for the park lawn surface during the nighttime. In daytime windy conditions, a cold air mass from the green space chilled the leeward built-up area to about 250 m from the park boundary.
In clear calm midnight conditions, wind flowed out from the park to the surrounding area at all measuring points on the boundary. The wind direction changed and began to flow out obviously at about 22:00 LST and air temperature on the park boundary fell 1°C at the same time. After that, the decline in air temperature was not constant, and periodic oscillation was seen at some points. These results imply the accumulation of a cold air mass in the park and its gravitational outflow into the surrounding area. A significant air temperature drop in the adjacent built-up area was observed within the range of 80-90 m from the park boundary. This cold air seep is limited within a certain range through the night regardless of cool-island intensity.
Air temperature above the lawn surface was lower than that under tree crowns during the night. This temperature difference was diminished by intermittent cloud cover. Thus the radiative cooling on the lawn surface should be the source of cold air seeping out at the park boundary.
During the seeping out of cold air, the cool-island intensity increases, but sensible heat flux (at the park lawn surface) was almost zero. The cooling ability of parks is not directly related to cool-island intensity.
