Thermal stress during heat waves in urban areas is recognized for its strong devastating effect on human health. A lot of effort to mitigate urban heat was put into planting more vegetation and removing sealed surfaces. However, many green roofs and green areas are not irrigated, which in turn leads – during eventually longer or more extreme future summer heat periods without precipitation – to drought stress of the plants. As a result, evapotranspiration from vegetation surfaces is reduced. Therefore, many non-irrigated green roofs may not reach the expected cooling effect.

During droughts, agricultural surroundings cannot provide full daytime cooling effects as well. Further, simulations show that the conversion of solar energy into electricity by photovoltaic panels may reduce the urban heat island since less energy is available for heating the air. Local energy production by PV also reduces the anthropogenic heat generated because of importing energy to the city. Taking this factor into account, the cooling effect of PV might be even higher.

The project aims to investigate open questions regarding urban overheating and to incorporate them into a holistic analysis. Questions that this project adresses are therefore the quantification of the cooling potential of green areas within and around Vienna via evapotranspiration, the future irrigation needs, further the cooling effect of PV panels and the quantification of potential anthropogenic heat reductions mainly by electrification of local energy production and transport.

Four measurement sites were set up on green roofs in different local climate zones of Vienna (compact-midrise rooftop, compact-midrise below rooftop, open mid-rise and large low-rise). For microclimatic measurements, automatic pot lysimeters were installed at these four different green roof sites over the city of Vienna with a setup for a shallow and a deep soil substrate (25 cm and 10 cm, respectively). Besides the local microclimate (especially wind, temperature, air humidity as well as soil wetness within the pots), the mini lysimeters measured quantitatively water loss through evapotranspiration by weighing the pots. The measurement period was starting in June 2022 and completed by end of 2023, including 2 years of different weather conditions.

This data was used to calibrate the FAO model according to Allen et al. 1998, which simulates the soil moisture for selected drought periods on green roofs. ARIS (Drought monitoring system) data were used to simulate the soil moisture of the surrounding agricultural areas of Vienna.

To estimate the anthropogenic heat flux of Vienna energy use data (oil, gas, electricity, and others) were compiled for different spatial and temporal scales to derive the fluxes stemming from Viennese traffic, services, private households, and industry. The anthropogenic heat data were adjusted for the defined simulation periods and developed as time series functions based on the standard load profiles. This was done for the three categories (low residential, high residential, industry). These scenarios included energy use in buildings and traffic.

Further, the properties and climate effects caused by using photovoltaics on roofs were investigated. Literature research and communication with stakeholders were used to update and refine the prospective changes of Viennese building parameters used in the different urban categories of models.

All collected data were used to initialize, run, and validate the coupled WRF-TEB model and simulate the atmospheric condition for present and future summer drought episodes to estimate the expected thermal strain on inhabitants in Vienna. The offline TEB model was used to calculate the indoor temperature, urban canyon microclimate and the microclimatic contribution to the urban climate and to compare scenarios.

Last Update: 09-04-2025