Simulating Radiative Transport for Vegetation in Complex Urban Environments with Green Infrastructure

Understanding the impact of green infrastructure projects on urban energy use and microclimate is critical to developing sustainable long-term urban planning strategies. Green infrastructure projects come in many forms including: the development of parks, alteration of building rooftops, and the use of novel asphalt and concrete materials for streets and parking lots. They all share the common goals of reducing energy usage, mitigating pollution emissions and improving the urban microclimate. Due to difficulty in simulating the large disparity in length scales covering these processes, little is know about their impact. Our efforts have focused on developing environmental modeling tools to generate knowledge about the place specific effects of urban planning strategies on sustainability. Our software, QUIC EnvSim, is a multipurpose, 3D building resolving environmental software that models heat transfer in urban areas. While this tool includes components to compute convective and conductive heat transfer, our presentation will focus on the computational framework and validation of the vegetative radiative transport component. Our novel approach efficiently calculates the complex interactions within the vegetative canopy using graphics hardware to accelerate a ray traced sampling approach for computing radiative transport.

Radiative transport plays a driving role in the exchange of energy and water in urban and forestry ecosystems. Because of its high level of complexity, fully modeling radiative exchange across an urban landscape with green infrastructure is often cost-prohibitive in terms of computational resources and time. A variety of approaches have been proposed to model radiative exchange and its impact on the energy and water budgets in urban and forestry systems. To date, these have all included some compromises in terms of physical complexity and computational cost ranging from models that thoroughly represent the physics with relatively small numbers of vegetation to models that encompass large domains using generalizations about vegetative homogeneity. We present a radiative transport model for trees that physically represents the characteristics of tree leaf structure and is linked to spatially explicit energy and water budgets.

Our tree model is integrated with the QUIC EnvSim tool and is efficient enough to afford city-scale problems resolving both trees and buildings within the domain. As such, the simulation tool is able to predict how radiative fluxes on nearby surfaces are augmented by surrounding vegetation. The model includes solar irradiation and longwave radiation exchange between objects within the domain. Trees are treated as participating media and augment radiation through scattering, absorption, and emission. The tree crowns are composed of multiple cell volumes, each with their own physical properties and leaf energy and water budgets. Radiative fluxes are calculated using ray-tracing algorithms, which are traditionally used in computer graphics to render scenes with complex visual light transport. The model exploits the inherent parallelization of the ray tracing algorithm to gain significant speed up on commodity graphics processing units (GPUs). This affords the efficient simulation of urban landscapes with vegetation using relatively inexpensive desktop computers. Our talk will focus on the details of the computational model and present validation results comparing the model's radiative and temperature components to empirical data obtained within trees.