Vertical structure of the cool island in a large urban park

Abstract

Greening of cities is one of the mitigation strategies for urban hot environments in summer, because green parks in cities have a lower temperature than the surrounding built environment in summer, called cool islands phenomena. This study took the micro-meteorological measurements in an urban park (Shinjyuku Gyoen, Tokyo Japan) and analyzes the vertical structure of a cool island at night. Vertical profiles of air temperature were measured simultaneously in the park and the nearby town area by the tethered balloons. In the park area, a cold air pool was present in the first 50 m from the ground, as opposed to neutral stratification in the town area. The cold air pool grew vertically at night. Meanwhile, surface air temperature difference between the park and the town increased up to 2 °C.The ground-based instruments recorded the park breeze which is cold airflow to town area for 21 nights over two summer months. In most cases, the park breeze had at least 16 m depth and traveled for more than 122 m from the park edge. It is hypothesized that the park breeze is the gravity current. The observations made in our study can aid evaluating worth of urban green.