Modelling extensive green roof CO2 exchanges in the TEB urban canopy model

Abstract. Green roofs are promoted to provide ecosystem services and to mitigate climate change in urban areas. This is largely due to their supposed benefits for biodiversity, rainwater management, evaporative cooling, and carbon sequestration. One scientific challenge is quantifying the various contributions of green roofs using reliable methods. In this context, the green roof module already running in the Town Energy Balance urban canopy model for water and energy exchanges was improved by implementing the CO2 fluxes and the carbon sequestration potential. This parametrisation combines the ISBA (Interaction Between Soil Biosphere and Atmosphere) photosynthesis, biomass and soil respiration module with the green roof module in order to quantify the net CO2 amount emitted or fixed by the green roof over a time period. The parametrisation was fully achieved by using data of an extensive Sedum non irrigated green roof located at the Berlin BER airport in Germany from 2016 to 2020. The five years of measurements were used to do a sensitivity analysis of the photosynthesis module parameters in order to classify the parameters according to their influence, followed by a calibration over the most important parameters and evaluation. Results show a good agreement of the simulated leaf area index and CO2 fluxes with in situ observations, with good diurnal, seasonal and inter-annual variability, even if the model tends to be overly responsive on the day to day variability. The model reproduces well the Net Ecosystem Exchange which provides a reliable estimation of the annual carbon sequestration. Those results are encouraging in quantifying the potential of carbon sequestration of green roofs and open up the possibility of applying the new parametrisation on a city-wide scale to evaluate green roof scenarios.