Abstract
Most studies explored green-roof thermal effects on a few hot summer days based on short-term monitoring data. Few studies investigated the seasonal and diurnal patterns of thermal performance and associated weather effects. This research aims to address the following two questions: (1) how green-roof thermal performance varies with different season and time; and (2) to what extent can thermal performance be predicted by background weather parameters? A retrofitted extensive green roof was established on the top of a railway station in subtropical Hong Kong. Monitoring data covering a two-year period, one year before roof greening and one year after, were collected and analyzed. Results indicated notable seasonal and diurnal patterns of green-roof thermal performance. It exhibited cooling effects in spring, summer and fall, but warming effects in winter. The cooling effects were more pronounced in summer than spring and fall, on sunny days than rainy and cloudy days, and in nighttime than daytime. Air temperature, relative humidity, wind speed, solar radiation, and soil moisture could explain 83.6%–86% of the thermal effects’ variation. The multiple-regression models based on the five weather variables established in this study provide an uncomplicated and direct approach to predict the thermal performance of similar extensive green roofs in subtropical areas.
Highlights
The combined effects of global warming and urban heat island (UHI) effects have caused a large amount of energy use and greenhouse gas (GHG) emissions, as well as thermal discomfort in urban areas [1].Designing cities in a climate-conscious way contributes to UHI mitigation and urban sustainability, and offers cities opportunities to address global issues at the local level [2]
This study aims to answer the following two questions that have not been adequately addressed: (1) How green-roof thermal effects vary with seasons across a year, and with time across a day, in humid subtropical areas? (2) To what extent can thermal effects be predicted by background weather parameters? The research objectives are twofold: (1) evaluate the daytime and nocturnal modification of main weather parameters by green roofs for different season-cum-weather conditions; and (2) investigate the effects of key weather parameters on green-roof thermal performance based on multiple-regression models
The “wet island” effects of green spaces characterized by the increase of ambient humidity above the vegetation canopy are generally concurrent with their “cool-island” effects, both owing to the evapotranspiration process
Summary
The combined effects of global warming and urban heat island (UHI) effects have caused a large amount of energy use and greenhouse gas (GHG) emissions, as well as thermal discomfort in urban areas [1]. When applied on a city scale, green roofs may reduce the ambient temperature between 0.3 °C–3 °C [36] These studies, have largely focused on green-roof thermal effects in summer daytime, while ignoring their seasonal and diurnal variability and underlying weather effects. It has been widely reported that the UHI effect display notable seasonal and diurnal patterns with varying background weather parameters, such as radiation, humidity, temperature, and wind speed [37]. These weather factors exert significant influence on the heat and water dynamics of green-roof systems [7,19]. (1) evaluate the daytime and nocturnal modification of main weather parameters by green roofs for different season-cum-weather conditions; and (2) investigate the effects of key weather parameters on green-roof thermal performance based on multiple-regression models
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