The effects of greening a parking lot as a heat mitigation strategy on outdoor thermal stress using fixed and mobile measurements: case-study project “tertiary forest”

Abstract

An asphalt-concrete parking lot was converted to a small urban forest with 72 trees and permeable pavement as an urban heat island countermeasure. Microclimate monitoring was performed pre- and post-renovation using fixed and mobile measurements. We propose a protocol to estimate robust statistical effects over 24 h constructed with several days of observation, and to compare the spatial distribution of heat stress pre- and post-conversion regardless of differences in weather conditions. Detailed effects on solar and infrared radiation, microclimate and pedestrian heat stress are assessed and tested for statistical significance. Great improvements of pedestrian heat stress are reported, especially in daytime, following the creation of shade which significantly reduces the radiative load of the area. UTCI-equivalent temperature is reduced up to 6.2 °C (3.1 °C on a 24h-average). Mobile measurements were used to characterize the spatial disparity of heat stress. Strong thermal discomfort is exhibited under direct insolation. At night, positive effects on pedestrian heat stress are slightly counterbalanced by an increase in infrared trapping due to the reduction of the sky view factor beneath the trees while wind speed is slowed. Recommendations are formulated for urban planners.