Cold-air Seeping-out Phenomena in an Urban Green Space

Abstract

The mitigation effects of green spaces in urban areas are investigated with several microclimatic observations in Tokyo. On a clear calm night, flow-out wind directions from green spaces to surrounding areas were discerned at measuring points along the boundary with a sharp temperature fall of a few degrees. These imply the accumulation of a cold air mass in the park and its gravitational flow-out into the surrounding area. Under this condition, a significant air temperature drop in an adjacent built-up area was observed within a range of 80-90 m from the boundary in the case of Shinjuku-Gyoen Park. This limit was more or less fixed throughout the night, regardless of cool-island intensity. During the seeping out of cold air, cool-island intensity increases, but sensible heat flux was almost zero. The cooling ability of parks is not directly related to cool-island intensity. Such cold-air seeping-out phenomena are observed not only in large parks but also at a green-full apartment complex, a small green slope and even a stepped roof garden.