A simplified 3D urban unit representation for urban microclimate simulations: A case study in China’s ‘Hot Summer and Cold Winter’ climate zone

Urban and building energy simulation models are usually initiated with typical meteorological year weather data. However, the locations where these historical datasets were collected (usually airports) do not represent the local, site specific micro-climates that cities develop. An idealised “urban unit model” (250m radius) has been developed for use with simulation modelling as a method for adapting commonly available weather data files to the local micro-climate. The idealised “urban unit model” presented in this work is based on the main thermal and morphological characteristics of nine sites with residential / institutional (university) use in Hangzhou, China. The size of the urban unit was investigated in relation to the source area footprint of the air temperature signal at the centre of the unit, where air temperature and relative humidity measurements were collected. This newly introduced idealised “urban unit model” was implemented into micro-climatic simulations using a Computational Fluid Dynamics – Surface Energy Balance analysis tool (ENVI-met, Version 4). Following model validation, two scenarios were developed, one for assessing the impact of vegetated surface’s location on air temperature in relation to the air temperature measurement point (3.5m above ground) and a second one for analysing the effect of the vegetated surface area on the average urban unit (250m radius) air and surface temperature. The performance of the “urban unit model” was deemed satisfactory considering the assumptions integrated into the simulation initialisation process and the simulation itself. The performance evaluation indices were comparable to previously published work (RMSE: 1.3; MAPE: 3.1%). The results of Scenario 1 (spatial distribution of green space in the urban unit) showed that the location of the vegetated surface had only marginal impact on the air temperature at the middle grid cell of the urban unit. As would be expected, the air temperature was lower for the case where the vegetation was at the centre of the model. Scenario 2 (changing the green space percentage in the urban unit) revealed that large vegetated surface areas can reduce the day-time maximum air temperature. Specifically, the increase of green space resulted in an increase in the occurrence frequency of air temperatures towards the cooler end of the temperature distribution. This study concluded that in places with a humid subtropical climate such as Hangzhou, for successive hot, dry summer days, the reduction in soil water content can negate, to a large extent, the cooling benefits of added vegetation. This work presents a methodology that addresses the implications of urban morphology representation for micro-climate modelling. The method developed is appropriate for investigating the urban and building energy use and the implications for cities.