Ecological energetics of tropical intensive green roof

Few green roof studies cover intensive and tropical types and specific canopy microclimate. We examined the ecological energetics of a sky woodland in humid-tropical Hong Kong. Environmental sensors monitored the microclimatic and soil parameters for 14 months. Key biophysical variables of transpiration, wind, light, and through-canopy energy flux are modeled to investigate seasonal and weather effects. The woodland forms a cloistered subcanopy environment with rather stable microclimate. Transpiration and latent heat loss are enhanced by solar radiation and low relative humidity, but less by wind. On sunny days, about 20% of incident solar radiation can reach the soil surface. The canopy reflected more near-infrared radiation (NIR) than photosynthetically active radiation (PAR), highlighting a hitherto neglected passive-cooling mechanism. The highest transpiration rate occurs in autumn rather than summer due to dry-mild weather. The woodland canopy could reduce 300 W m−2 energy flux into the substrate. The canopy warmed by solar energy transmits heat to subcanopy air. Latent and sensible heat loss in the subcanopy domain is suppressed, thus dampening the passive-cooling effect. The capability of the tropical intensive green roof to reduce temperature is relatively inefficient comparing with temperate region counterparts. The findings could inform design and choice of green roofs.

• We evaluated the ecological energetics of a tropical intensive green roof by empirical and modeling studies.
• Transpiration and latent heat loss are enhanced by solar radiation and low relative humidity, but less by wind.
• The canopy reflected more near-infrared radiation than photosynthetically active radiation, a neglected cooling mechanism.
• Woodland shading permits about 20% of solar radiation to reach the soil surface.
• The woodland can prevent about 300 W m−2 of energy flux from entering the building.