Multiscale urban climate model coupling mesoscale meteorological model with all-physics model at urban microscale

  • Strebel, Dominik (ETH Zurich)
  • Kubilay, Aytaç (ETH Zurich)
  • Carmeliet, Jan (ETH Zurich)
  • Derome, Dominique (Université de Sherbrooke)

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Due to climate change and its effect on the urban climate, specifically on the increasing frequency and magnitude of heat waves, and on the urban heat island effect, showing higher temperatures in cities compared to the surrounding rural environment, the modelling and numerical simulation of the urban climate is of high interest. Classical mesoscale meteorological models allow simulating the climate at a resolution of more than 10 km, where the city is not represented or, if so, in a very simplified manner. However, using a three-step down-nesting approach of ERA-5 boundary conditions and using an adequate urban surface or canopy parametrization, the urban climate can be simulated at a resolution of 250 m. The accuracy of the mesoscale meteorological model for urban climate prediction can be improved using machine learning approach based on available measured data from nearby urban weather observation stations. The hybrid mesoscale meteorological modelling approach can be used to study the variation of building cooling energy demands over cities during summer periods. As a next downscaling step, the mesoscale simulation results are used as boundary conditions for an all-physics urban climate model at microscale, where urban infrastructures like the buildings, urban materials and vegetation are explicitly modelled. Boundary conditions like wind velocity, wind direction and air temperature are applied at the lateral boundaries of the microscale domain using a blending layer with source/sink terms. The microscale urban climate model, developed by the authors is based on OpenFOAM, and includes CFD modelling of turbulent air flow due to wind and buoyancy effects, coupled heat and moisture transport in the air domain and in the porous domain of urban materials, shortwave and longwave radiative exchanges between urban surfaces and the sky. Consecutive steady RANS calculations predict the air flow, temperature and moisture content distribution in the air domain using surface temperatures and moisture content as predicted by the coupled dynamic heat and moisture transport model for the urban porous materials. Trees are modeled as porous media, including transpirative cooling based on the plant physiology. This model is used to study the design of different mitigation scenarios for urban overheating during heatwaves, first analyzing large scale scenarios, followed by local mitigation scenarios using vegetation, water spraying and night ventilation.