Impacts of living roofs on urban climate in San Francisco

Abstract

The results from this study offer a micrometeorological profile of an extensive/intensive living roof in the Mediterranean climate of San Francisco California, specifically the roof's impact on the surface radiation budget and surface energy balance. Living roofs have long been touted for their ability to positively impact microclimate by reflecting solar radiation and cooling the atmosphere through the latent heat flux, thereby offsetting adverse effects of the urban heat island effect (UHI). This is the first study using the eddy covariance technique on a living roof, and was achievable due to the roof's large (one hectare) size and stringent (~50%) data rejection. The annual average albedo of the living roof was 0.20 with a seasonal monthly maximum of .22 and a minimum of 17.39. The annual ensemble average partitioning of energy balance terms indicated that latent and sensible heat fluxes were close to equal with an annual Bowen ratio of 0.96. On a diurnal temporal scale, the sensible heat began to surpass the latent heat in the midmorning, and on a seasonal timescale, sensible heat dominated the energy balance partitioning in the late summer and early spring, and was overtaken by the latent heat flux in the fall and winter. The latent heat flux produced an annual average cooling rate of 3.19 (MJ m^-2 dy^-1). Ground heat flux observations indicated that the substrate acted as insulation, with a small average diurnal maximum of 3 (W m^-2) of heat energy entering the building below. Energy balance closure as determined by linear regression showed that the turbulent fluxes underestimated available energy by 38% (R² = 0.92).