Impact of urban greening and verticalization on the outdoor thermal comfort : a case study of Thimphu city, Bhutan

Abstract

Urban Heat Island (UHI) continues to the be the most explored and intensively studied in the field of urban climate. It is linked predominantly with the growing impacts of urbanisation and associated impervious surfaces. With higher UHI intensity, the use of outdoor spaces deteriorates which can be addressed with various measures. In this context, this study aims at studying the impact of trees and increased building heights on the outdoor thermal comfort in Thimphu, capital city of Bhutan. This study uses the Land Surface Temperature as a prime variable in studying the Surface UHI intensity and identifying the hot areas in the dominant Local Climate Zone classes 5 (Open mid-rise) and 9(Sparsely built). ENVI-met was used for assessing the outdoor Air temperature (Ta), Mean radiant temperature (Tmrt) and the Physiological Equivalent Temperature (PET) during one of the hottest days in August and coldest day in January of 2019. The street trees were found to be effective in reducing the temperature and improving the thermal comfort in summer for LCZ 5 while taller buildings were more effective in LCZ 9. The two interventions were more effective in reducing the Tmrt and PET and did not show significant change in the Ta. Due to reduced transmissivity in winter from the deciduous trees, no significant changes in the temperature were resulted from the model simulation. Therefore, the taller buildings played the major role in changing the temperature in winter. Since winter is already cold, other interventions are recommended to be explored to increase the temperature to make the outdoor spaces pedestrian friendly such as using evergreen trees which increases the temperature depending on the density of trees.

For a growing city like Thimphu, taller buildings can have positive impact thermal comfort specially in summer but in combination with urban greening, significant results can be achieved in improving the outdoor thermal comfort. This study overall concludes street trees to be an effective measure in improving outdoor thermal comfort in summer and recommends preserving the existing urban green spaces, adding street trees to make city roads pedestrian friendly. This study also supports and compliments the Norzin-lam redevelopment project which aims at improving the city road by incorporating dedicated lane for pedestrian, planting street trees and reducing the traffic flow.