Contribution of trees and grasslands to the mitigation of human heat stress in a residential district of Freiburg, Southwest Germany

The potential of urban green coverage to mitigate human heat stress is studied using the ENVI met model V4. The simulation domain is a residential district in Freiburg, a mid-size city in Southwest Germany. It is characterised by residential buildings and street canyons with asphalt surfaces, grasslands and broad-leaved trees. The ENVI-met model was validated against human-biometeorological measurements and demonstrated good performance when simulating the urban thermal environment in terms of air temperature (Ta) and human heat stress in terms of mean radiant temperature (Tmrt) and physiologically equivalent temperature (PET). Simulations were performed for the heat wave day of 4 August 2003, which is a typical scenario for future summer weather in Central Europe as projected by climate models. Four scenarios with different types of green coverage were simulated. The results enable quantification of the daytime and nocturnal contributions of trees and grasslands, respectively, to the mitigation of human heat stress on different spatial scales. Averaged over 10-16 CET, trees on grasslands lead to a mitigation effect up to 2.7 K for Ta, 39.1 K for Tmrt and 17.4 K for PET. In comparison, the effect of grasslands can be up to 3.4 K for Ta, 7.5 K for Tmrt and 4.9 K for PET. Based on the findings, design implications are also provided from the perspective of urban human-biometeorology.