Micrometeorological observations of carbon, water vapor and heat exchanges on the California Academy of Sciences’ living roof using eddy covariance

Abstract:

Living roofs have very different surface energy, water and carbon budgets than conventional roofs. Since roofs cover approximately one third of the planimetric surface area of cities, they are a significant driver of the urban boundary layer. Living roofs have been thought to be beneficial for reducing the urban heat island through increased latent heat exchange, uptake of atmospheric carbon dioxide and storage in soil and plant matter, building energy conservation through soil heat storage and latent heat fluxes and reduction in runoff. Here we present evidence of some of these through ongoing observations of surface energy, water and carbon budget estimates for the extensive living roof of the California Academy of Sciences building in Golden Gate Park, San Francisco, California. Micrometeorological measurements including the eddy covariance approach are used to estimate CO₂, water vapor and both ground and atmospheric heat fluxes. The California Academy’s roof encompasses an area of 18,000 m². Vegetation surveys were conducted in the spring; beach strawberry (Fragaria chiloensis) and California bentgrass (Agrostis) were found to dominate the project footprint out of the 26 species observed. Eddy covariance measurements are made about one meter above the 10-20 cm tall vegetation on the downwind side of the building. Approximately 50% of data are rejected due to less than 80% of the flux source area being contained in the roof or due to low friction velocity. Nevertheless, we are able to develop robust diurnal ensemble fluxes, and will present data from a nine month period. During summer, the roof acted as a carbon sink of approximately 1.5 gC m^-2 d^-1. Turbulent heat fluxes were dominated by sensible heat flux with a mean Bowen ratio of approximately 1.5 and daily evapotranspiration rates of about 1.8 mm d^-1. The role of seasonality and meteorology on surface microclimate characteristics will also be discussed.