Modelling CO2 flows from extensive green roofs within the TEB (town energy balance) urban canopy model

To mitigate climate change impacts in cities, nature-based solutions are broadly promoted due to their supposed benefits for biodiversity, rainwater management, evaporative cooling, and sequestration of carbon. Among existing solutions, green roofs show the advantage of tackling the lack of space available for greening in urban areas. But green roofs are still underdeveloped due to their cost and the lack of scientific knowledge around their potential, especially for carbon sequestration. Quantifying the various contributions of green roofs using reliable scientific approaches is a major challenge. Thus, it is essential to build a numerical model capable of simulating green roofs development and functioning at city scale in order to provide information to decision-makers with relevant indicators.

 

Here, the urban canopy model Town Energy Balance (TEB) with the module TEB-GREENROOF is used to model green roofs. The TEB-GREENROOF model, evaluated in previous study for heat and water transfers, is improved by activating the photosynthesis model ISBA-A-gs in order to represent the CO₂ exchanges of the vegetation implemented on the green roof. The modelling is informed by 6 years of continuous CO₂ flux data on a non-irrigated extensive green roof located in Berlin (Germany) in partnership with the Technische Universität Braunschweig. In order to evaluate and improve the thermal, hydrological and respiration characteristics of the ISBA-A-gs model on a green roof, an initial simulation is carried out by forcing the monthly evolution of the leaf area index (LAI) by LAI data estimated experimentally. The model is then applied with a dynamic calculation of LAI in order to enable it for simulations of roof greening scenarios on a city-wide scale under any climate with no information on the LAI.

 

Results show that the model is able to estimate the annual net ecosystem exchange of the Berlin green roof and to correctly reproduce the CO₂ fluxes for both diurnal cycles and annual variation under climate variability, with drier years showing less carbon sequestration.