Investigations of the Park Cool Island Effect of Golden Gate Park, San Francisco

Abstract

The cooling effect of urban green spaces, termed the "park cool island" (PCI) is touted for energy savings and as a mitigation for heat waves, which are expected to increase in frequency and severity as a result of climate change. The variability and complexity of the Golden Gate Park cool island in San Francisco, CA is explored using bicycle transects of near-surface temperature profiles from six fine-wire thermocouples mounted between 0.15 and 2.5 m above the surface. Transects were measured over different surfaces including city streets at varying distance downwind of the park as well as short grassy meadows and a grove of coastal redwood trees within the park. Long-term meteorological station data inside and nearby the park provide regional context. Surface temperature profiles varied significantly from super-adiabatic lapse rates over street surfaces to isothermal conditions in the redwood grove. A significant daytime PCI effect with mean 1.1°C in the grassy meadow and 1.8°C in the redwood grove was evident, which increased to 2.5°C in the meadow and 3.5°C in the grove under heat wave conditions. The vertical temperature gradient was greatest within 0.15 m of the surface, while the horizontal gradient between park and urban surroundings dissipated within 500 m of the park. Temperature characteristics within the park varied significantly with surface cover and patterns of sun and shade. PCI intensity was related to overall temperature and absolute humidity within the park. Results suggest that the cooling effect of Golden Gate Park, while persistent, is not a significant source of cooling outside the park. However, vertical profile measurements are a promising method that could provide a baseline for assessing the impact of planned urban greening interventions such as green roofs, street trees, and permeable pavement on the near-surface thermal environment in cities.