Abstract
Street spaces lined with trees in a central business district (CBD) are micro-climatologically interesting, from the two viewpoints of the urban canyon phenomenon and greenspaces. The purposes of this study are to clarify the distribution of air temperature in an urban canyon with thick street trees, and to understand the climatological role of trees in a CBD.
Observations were conducted in and around Jozenjidori Avenue in the CBD of Sendai city (Fig. 1). It is oriented in an almost east-west direction and surrounded by mid- or high-rise buildings on its north and south sides; Zelkova serrata are planted in four lines on both sidewalks and on the median strip of the street (Fig. 2).
The results obtained are as follows;
1. The temperature of the street space lined with thick trees is about 1.0°C lower than that of the CBD around the street (Fig. 3-b). The deviations of temperature are greater in the afternoon and evening on sunny days (Fig. 4).
2. The relative humidity of the green street canyon is higher than that of the CBD in accordance with the lower temperature (Fig. 3-c). However, the vapor pressure is not always higher (Fig. 3-d).
3. The vertical cross-sections of daytime air temperature in the urban canyon differs according to the season. In the foliation period from May to October, high temperatures are observed around the tree canopy which acts as a heat source and receives strong insolation on sunny days, and an inversion layer is developed under the canopy. In the defoliation period from November to April, high temperatures are observed on the north side of the street, where insolation strikes at about noon (Fig. 5).
4. The distribution of daytime air temperature in the urban canyon is deformed by the winds blowing across the canyon. The deformation somewhat differs according to the season. In the defoliation period, winds are blocked by buildings, and backflows are supposed to transport the warm and cool air near the ground. In the foliation period, winds blow less under the tree canopy, so more warm air is left there, and little deformation of the temperature distribution is observed (Fig. 6).
5. The degree of temperature difference between the inside of the canyon and the roofs of buildings shows no seasonal difference. The vertical difference of air temperature in the canyon is significant in the foliation period, because the tree canopy layer is supposed to be a heat source in the daytime and a heat sink at night on sunny days (Fig. 7).
