Evaluation of the outdoor thermal environment for three typical urban forms in Nanjing, China

Urban form has been proved to be an important factor that significantly influences the microclimate as well as the outdoor thermal comfort at the pedestrian level in urban spaces. Various kinds of urban forms arise from rapid urbanization, accompanying different thermal environments in urban spaces. However, some severe urban climate issues, such as the urban heat island phenomenon, emerge in dense urban areas frequently, especially in large cities with hot and humid climates. In this study, three different archetypes, including the freestanding, scattered, and enclosed types, were selected as the representatives of typical urban forms in the central area of Nanjing city. The outdoor microclimates of these urban forms under typical summer conditions were simulated for calculating the thermal comfort index. The effects of urban form characteristics on the thermal environment were compared and analysed. The results show that the maximum differences in average air temperature, mean radiant temperature, and PET index between these three typical plots could achieve 5.4 degrees C, 24.2 degrees C, and 12.4 degrees C, respectively. Furthermore, the scattered urban form was evaluated to be holding the best outdoor thermal environment in comparison to the others based on a holistic and comprehensive view. Finally, some basic design suggestions based on the results and relevant discussions were recommended for urban design in the cities with similar climate conditions.