Human Thermal Comfort Modeling at the Urban Microscale—New Possibilities of the SkyHelios Model

To analyze the human thermal bioclimate in the urban environment several meteorological input parameters (air temperature, air humidity, wind conditions and radiation fluxes) and thermo-physiological information (metabolism and clothing) are required. All input parameters are comprised in the known thermal indices (PET, PT, UTCI, mPET, etc.), to assess human thermal comfort.

In urban areas, all input parameters are modified, due to different morphological characteristics and physical properties of the surfaces within a city. The urban environment, provoking the urban heat island effect and urban moisture excess, modifies air temperature and humidity. The magnitude of the air temperature is mostly determined by superimposing weather conditions and further modified by varying surface temperature (land surface temperature vs. leaf surface temperature; due to different radiative and heat-related properties) and anthropogenic heat emissions. Turbulent heat transfer by convection and advection affect the spatial distribution of air temperature. Humidity is mostly modified by transpiration of the urban green infrastructure as well as anthropogenic exposition of moisture. Turbulent mass transfer maintains the spatial dispersion and mixing of humidity in the urban atmosphere. These volatile and important factors modify the thermal comfort conditions. Urban planning and architectural measures in the micro scale can modify them.

The SkyHelios model can calculate thermal indices (PET, PT, UTCI, mPET, etc.) with respect to the physiological response of green infrastructure affecting air temperature and humidity in a spatial manner. The species specific, physiological processes of photosynthesis and transpiration are modelled to assess cooling by transpiration of leafs surfaces and release of humidity (Penman-Monteith). The physiological model is coupled to the atmospheric model for heat and mass transfer, utilizing 3D computational fluid dynamics algorithm (Multi-relaxation-time lattice-Boltzmann method with a Smagorinsky turbulence module, accounting for buoyancy effect) to calculate the spatial variability of air temperature and humidity within the urban atmosphere. This extension of SkyHelios also allows for dispersion and diffusion modelling of moisture, gasses and aerosols within the model domain. Besides that, anthropogenic processes affecting heat and moisture exposition are not implemented in the model. Various common spatial data formats can be used for input and output of the calculations according to the Geospatial Data Abstraction Library.

The model is tested with mobile measurements in Karlsruhe, Germany, showing the urban heat island effect and urban moisture excess. The impact of mono-species areas and mixed species areas on the cooling potential of urban trees is quantified.