Thermal performance and microclimate modulation: A year-long, multi-site study of operational green roofs in Barcelona under severe water scarcity

This study provides a novel, high-resolution assessment of green roof hygrothermal performance in a Western Mediterranean city during a severe, year-long drought. While previous research has broadly demonstrated cooling effects, this work advances the field by offering a uniquely detailed, minute-scale comparison of air temperature, surface temperature, relative humidity, and wind speed across four diverse, long-term operational green and grey roof pairs in Barcelona. Measurements covering an exceptional drought, an extreme summer, and a mild winter allow a direct evaluation of system resilience under water-limited conditions. The resulting dataset, with a 1-minute temporal resolution sustained over a full year, represents one of the most comprehensive empirical resources for Mediterranean climates to date. A rigorous statistical framework correcting for autocorrelation and heteroskedasticity ensures the robustness of the results. Green roofs exhibited up to 18 ∘ C lower surface temperatures in summer and maintained 1–2 ∘ C cooler near-roof air than conventional roofs, while showing enhanced nocturnal thermal inertia in winter. Reduced daytime temperature dispersion under strong solar radiation is consistent with the hypothesis that vegetation moderates local air turbulence, a rarely quantified mechanism relevant to urban microclimate modelling. Overall, these results confirm that even under prolonged drought, extensive green roofs can meaningfully buffer temperature extremes, offering solid empirical support for climate-resilient urban design and green infrastructure planning.