Urban Roughness Estimation Based on Digital Building Models for Urban Wind and Thermal Condition Estimation—Application of the SkyHelios Model

Roughness length is a critical parameter for estimation of wind conditions, and it is therefore also relevant for the estimation of human thermal conditions in urban areas. The high density of buildings in urban areas causes large changes in land coverage, thereby increasing surface roughness. This influence atmospheric flow and also leads to a reduction in urban air ventilation, thus increasing the risk of human thermal stress. In this study, a digital building model of Tainan city was used to calculate roughness length using an approach based on Voronoi cells by applying the microclimate model, SkyHelios. The model was also used to estimate the wind conditions, including the wind speed and wind direction. For estimation of the thermal conditions, this study obtained meteorological data for air temperature, relative humidity, globe temperature, wind speed, and wind direction on two specific days (31 July 2015 and 21 January 2016). To quantify the thermal stress, the physiologically equivalent temperature (PET) was used to represent the thermal conditions. The wind conditions results obtained from the model indicate that even microscale conditions with vortices and corner flow can be represented with high precision and resolution. The thermal conditions results demonstrate that different created environments and microclimate conditions affect the thermal environment. The difference in PET can be up to 3 ◦C. This study confirmed that comparison of microclimate thermal conditions based on measurements and obtained from modeling using SkyHelios are in sufficient agreement and can be used in urban planning in the future.