The effect of different tree configurations on heat stress in courtyards

Abstract

Among the major urban cooling strategies, urban morphological changes and increasing greenery coverage have proven effective in mitigating heat stress. One building type that has been used for centuries in hot and arid climates is the courtyard typology as it can create comfortable thermal conditions. However, little is known about the climate performance of different tree planting patterns in these courtyards. This study aimed to assess the effectiveness of different tree configurations in courtyards with a specific focus on the Amsterdam Metropolitan Area. First, a literature review of the existing studies on the effectiveness of tree planting patterns to reduce heat stress was conducted. Next, 34 historic courtyards in Amsterdam (hofjes) and their tree planting patterns were analysed both qualitatively and quantitatively. Through this analysis, prevalent tree planting patterns and general guidelines for tree planting patterns were derived. Based on the literature review and hofjes analysis, six different tree planting patterns have been established. These were: the Cornered pattern with trees in the south facing corner, the Perimeter pattern with a line of trees along the perimeter, the Dispersed patterns with evenly dispersed trees, the Double Row pattern with two rows of trees, the Two Sided Double Row pattern with two double rows of trees on either side of the courtyard, and the Clustered Double Row pattern with two rows of trees clustered in the centre. These tree configurations were modelled in a courtyard to determine their effect on the Physiological Equivalent Temperature (PET) levels using ENVI-met 5, a 3D model simulation software. The courtyard used in this study is located at the Urban Comfort Lab, which is an experimental testing facility in Hoofddorp, The Netherlands. Simulations were carried out for two hot days in 2022, July 17th to 19th, during which temperatures reached up to 35.4 °C. Results showed that during the morning and late afternoon, the the Double Row and Clustered Double Row patterns were most effective in reducing the share of maximum PET values experienced in the courtyard. In the afternoon, during the hottest part of the day, the Dispersed pattern was the most effective in reducing the share of extreme PET values in the courtyard, with PET reductions of up to 7 K. Despite this reduction, it is important to note that PET levels inside the courtyard remained above 41 °C for the majority of the day, indicating the persistence of extreme heat stress conditions. The different mean radiant temperature values of the patterns exerted the largest influence on variations in PET, as wind speed, relative humidity and air temperature showed little variations among patterns. The findings of this research can help architects, urban designers and municipalities in creating more liveable and climate resilient cities in the face of rising temperatures. Additionally, this research contributes to the limited existing literature on the effect of tree planting patterns on heat stress in courtyards and offers a methodology for analysing existing tree configurations in courtyards.