Lawn and irrigation cooling from ground longwave radiation reduction: Understanding the climate-driven variability in cooling performance

Lawn is an important ecological strategy for cooling, while providing space for recreation and activities. However, research on the cooling benefits of lawns is limited, particularly regarding the role of irrigation. This study addresses these gaps by assessing the cooling performance of lawns in Changzhou, China, and quantifying the influence of irrigation on microclimatic parameters. The results show that lawns exhibited significantly lower mean pedestrian-level PET than asphalt surfaces (-1.16 degrees C, p < 0.001), with the cooling effect being more pronounced at night (-1.99 degrees C, p < 0.001) due to reduced upward longwave radiation (L up arrow) at the ground level. A significant PET reduction was detected within 37 min after irrigation in the morning (1.74 degrees C, p < 0.001), while the cooling effect of irrigation in the afternoon was not statistically significant. Further analysis of microclimatic parameters revealed that pedestrian-level thermal comfort improvement was mainly through reducing ground-level L up arrow rather than reducing pedestrian-level air temperature (T-a). Strong solar radiation in the afternoon weakened the contribution of L up arrow to PET reduction. These findings underscore the pivotal role of ground-level L up arrow reduction in lawn and irrigation cooling, and the variability in cooling performance under different climate conditions. The study provides insights into lawn design and management to regulate thermal environments in cities.