Modelling the impact of increased street tree cover on mean radiant temperature across Vancouver's local climate zones

Extensive impervious surface cover, anthropogenic heat emissions, and lack of vegetation contribute to the formation of distinct urban microclimates where higher air and surface temperature as well as lack of shade intensify outdoor heat exposure and thermal discomfort for humans. Modifications to the thermal environment via urban design can be used to mitigate this effect. In this study, the potential for increased street tree coverage to reduce mean radiant temperature (T-mrt)(1) across six different local climate zones (LCZs)(2) in Vancouver, Canada, was examined using the Solar and LongWave Environmental Irradiance Geometry (SOLWEIG)(3) model. The radiant cooling effect of increased street tree coverage during the hottest day on record for Vancouver (July 29, 2009) was quantified by spatiotemporal changes to T-mrt. SOLWEIG was evaluated successfully prior to implementation of a street tree cover increase equivalent to 1% of plan area in each of six Vancouver LCZs investigated. Results indicate 3.2-6.3 degrees C reduction in spatially-averaged daytime (9:00 - 18:00) T-mrt and 3.3-7.1 degrees C reduction during the hottest period of day, 11:00-17:00. During the hottest period of day, the largest spatially-averaged T-mrt reduction (7.1 degrees C) was modelled in a low-rise residential area. Modelling suggested that a pedestrian standing directly under a tree canopy would experience T-mrt reductions of 15.5-17.3 degrees C in all LCZs. Also, under current conditions with no increase in tree cover, the compact high-rise and the large low-rise areas are shown to be the most and least comfortable environments regarding human thermal exposure with spatially-averaged T-mrt of 41.9 degrees C and 47.9 degrees C, respectively. We conclude that increases to Vancouver's street tree cover by 1% of plan area can substantially reduce T-mrt during extreme hot weather. The results of this study show that the cooling potential of added street trees is greater in lower density residential neighborhoods with 1-2 storey buildings compared to higher density neighborhoods occupied by high-rise or mid-rise buildings.