Comparative microclimate and dewfall measurements at an urban green roof versus bitumen roof

Urban green roofs are discussed as a local climate adaptation measure to limit surface warming and increase evaporative cooling by vegetation in urban environments. A five month measurement campaign was conducted to observe surface and air temperatures as well as dewfall dynamics and amounts on an urban green and co-located bitumen roof. Measurements were performed in the period from August to December 2012. Surface temperatures indicated differences of up to 17.4 K, which lead to measurable air temperature differences (ΔTA) at a height of 0.5 m above roof level. During August afternoons (3 pm) the green roof air temperature (TA) drops below TA of the bitumen roof by up to 0.7 K on average. By using a linear regression based approach differences in sensible turbulent heat flux densities (QH) between green and bitumen roof of 200 W m−2 on a hot day with 30 °C and wind velocities of 2 m s−1 were estimated. During the measurement campaign a total of 60 and 52 dew events were observed on the bitumen roof and the green roof, respectively. At both urban sites the number of dewfall events was distinctly smaller compared to the rural site (94 events). Roof dewfall turned out to be a negligible source in the green roof water balance compared to precipitation amounts. Inhibited dewfall on roofs could be one important factor for the phenomenon of urban moisture excess since roofs represent a high fraction of urban surfaces. •Comparative measurements of microclimate properties above green and bitumen roof.•Estimation of dewfall dynamics and amounts on urban roofs.•Mean air temperature differences reach 0.7 K at 3 pm (0.5 m above roof).•Dewfall amounts on green roof are negligible in comparison to precipitation.