Development, validation, and demonstration of the VTUF-3D v1.0 urban micro-climate model to support assessments of urban vegetation influences on human thermal comfort
Authors: Nice Kerry 
Published: July 2018

With urban areas facing future longer duration heat-waves and temperature extremes from climate change and growing urban development, adaptation strategies are needed. Examining the role that increased tree cover and water availability can have on human thermal comfort (HTC) in urban areas as part of these strategies has been done using observations, but further work requires a modelling tool suited for this task. Sufficient model resolution is needed to resolve variables used to calculate HTC, as well as the ability to model the physiological processes of vegetation and their interaction with water and with the rest of the urban environment. The lack of such a model has been identified as a research gap in the urban climate area and has impaired the ability to fully examine the use of urban greenery and water for improved human thermal comfort. A new model, VTUF-3D (Vegetated Temperatures Of Urban Facets), addresses this gap by embedding the functionality of the MAESPA tree process model (Duursma &amp; Medlyn 2012), that can model individual trees, vegetation, and soil components, within the TUF-3D (Krayenhoff &amp; Voogt 2007) urban micro-climate model. An innovative tiling approach, allows the new model to account for important vegetative physiological processes and shading effects using configurable templates to allow representation of any type of vegetation or water sensitive design feature. The high resolution of VTUF-3D is sufficient to examine the processes that drive human thermal comfort (HTC). This allows detailed calculations of surface temperatures, mean radiant temperature (Tmrt), and a HTC index, the universal thermal climate index (UTCI), across an urban canyon. An extensive validation process, using three different observation data sets to validate a number of different and key aspects of the VTUF-3D model, has shown it performs well and is suitable for use to examine critical questions relating to the role of vegetation and water in the urban environment. A demonstration of the model using modelling scenarios of varying canopy cover shows that average peak daytime HTC improvements of 1◦C UTCI (and 2.3◦C UTCI) are possible in doubling (and quadrupling) existing street cover canopies, with localised effects under canopy cover approaching 5◦C UTCI. These scenarios also show the value of the existing canopy cover, as reductions and elimination of existing cover can create reductions in HTC of 2◦C UTCI. In addition, reductions in average air temperature (Ta) across urban canyons can differ by 1◦C between streets with differing canopy cover. After the development, validation, and demonstration of this new model, it is now possible to conduct further analysis to quantify the impact each individual tree can have on temperatures in urban canyons. Further, the model can help inform the optimal arrangement and quantity of trees to maximise temperature moderation effects and be used to generate best practices guidelines for urban greening.