100 Lu et al: Modeling the Shading Effect of Vancouver’s Urban Tree Canopy accuracy of their input prior to executing the simula- tion. In simulating irradiance, users generate test points on surfaces of interest, such as the roof of a building, defined as a grid with an adjustable density assigned to the surface and parameters specific to the Radiance engine that define the physical intersection of simulated photons, such as the number of bounces allowed for a beam of light, or the sampling density of the beams per time step (Roudsari and Pak 2013). For the surfaces in this study, a density of 1 sample point per 0.5 m2 was assigned and simulation param- eters were set to more than accurate according to Radiance documentation, with the exception of ambi- ent bounces (number of times a ray of light bounces from surface to surface before dissipating). While Honeybee provides an interactive mode for the use of Radiance, it comes with drawbacks regard- ing computation resources. This can lead to lengthy sim- ulation times in large simulation sets and, specifically in the case of this study, if using processing hardware with low RAM (< 32 GB), failed simulations were experienced. To compensate for lengthy processing times, ambient bounces were set to 1 to reduce over- all simulation time without compromising the study aim of analyzing the impact canopy cover has on direct solar radiation. Ambient bounces control the amount of times a ray that has been cast is allowed to bounce across surfaces in the model. A higher value leads to more bounces and more ambient light resolu- tion but does not impact measured radiation as a result of the initial canopy cover. A sample of key radiance parameters is shown in Table 1. The engine used hourly solar data, referenced from an EnergyPlus Weather (EPW) file for Vancouver’s Table 1. Radiance parameters were set to the “High” quality in Honeybee, with the exception of ambient bounces. Key parameters are shown here. Radiance parameter Ambient accuracy Ambient bounces Ambient divisions Direct certainty Specular threshold Limit weight Ambient super-samples Ambient resolution Limit reflection Value 0.1 2 4,096 0.75 0.15 0.005 4,096 128 8 climate (EnergyPlus 2016). This file was used to gen- erate a cumulative sky to simulate hourly solar irradi- ation within the model (Robinson and Stone 2004). EPW files are constructed representative annual weather sets that typically describe a location’s climatologi- cally typical weather parameters (National Renewable Energy Laboratory 2020). The simulation utilizes the location of the sun for every hour of the chosen anal- ysis period to cast light rays onto the test surfaces and queries the EPW’s solar irradiance values to deter- mine how much solar radiation the exterior surfaces may receive given any shading objects that obstruct their view of the sky type, grid size, grid distance off surface, and legend (low and high bound) (Aksamija 2018). After setting up the input parameters, the user can run the simulation from Grasshopper. Following the simulation run, users can visualize and customize the results in several different ways with Honeybee’s components (Roudsari and Pak 2013). For example, results can be displayed with intuitive colors on the surfaces (e.g., street, roof, and walls of the 3D model). Although hourly output could be generated for the entire year, the scope of this project considered a peak heat and an unobstructed direct sunlight week (mid- July). Surrounding buildings and context such as tree canopies were considered during this analysis as a baseline case of shading. Input parameters were kept consistent between the baseline cases (only building and context shading) and the experiment cases (including the urban canopy as a shading element). The scope of this project focuses on canopy shading, therefore shading cast by buildings on other buildings and street surfaces is not included when comparing solar irradiance reductions. RESULTS Canopy Cover Density As expected, neighborhoods showed considerable variations in terms of canopy cover (total canopy area/total neighborhood area) and total number of trees (Figure 4). A total of 245,645 trees were pro- cessed with an average canopy cover density of 53% (standard deviation of 22%) for the entire City of Vancouver. Well-treed neighborhoods such as Shaughnessy, Dunbar-Southlands, and Kerrisdale had over 20% can- opy cover and over 20,000 trees, while neighborhoods such as Downtown, Strathcona, and Sunset had less than 10% canopy cover with about 5,000 trees. There ©2022 International Society of Arboriculture
March 2022
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