Observational study of cool island in urban parks

Urban greening is one of the mitigations for urban warming, because the urban green parks has lower temperature than its surrounding town area. However, the degree of coolness (temperature difference) seems to vary by sites, maybe depending on the volume of greens and local climate. Therefore, accumulation of knowledge for various cases is needed.

This study summarizes the field observations on green cool island which had been taken for more than 20 years in Tokyo. The main fields of our study were urban parks of 0.5 km². We focus on the cool island at night.

The cool islands were formed in the urban parks in Tokyo, where parks were 1 – 3 ^oC cooler than the surrounding town area. The accumulation of cold air in the park reached 50 m above ground. The accumulated air drained away into the town area and traveled 120 m far out from the park. The cold air was 10 m deep even in the town area. Analysis for the travel speed of the cold air revealed that the cold air flow was the gravity currents which was driven by the air density difference between the park and town.

The mechanism of cold air generation in the green parks is discussed by the heat budget analysis in the park. We measured the full terms of heat budget in the park which includes turbulent and advection heat flux. It is revealed that the park cools at night by radiative cooling and its 80% cooling energy was used to cool the surrounding town area (20% for cooling the park itself). That town cooling energy was 39 Wm^-2 in average, and we need 1,300 unit of room air conditioner in 1 km² if we archive that cooling artificially.