Cities in complex terrain represent regions of high population density often exposed to dangerous conditions (e.g. windstorms, poor air quality). The extreme spatial variability and wide range of processes affecting weather and climate in these locations are not well captured by traditional measurement or modelling techniques. However, thanks to recent technological advances it is now possible to conduct high resolution large-eddy simulations (LES) for real cities, allowing detailed 3D investigations of atmospheric turbulence.
Building-resolving LES will be used to study the atmosphere in and above the alpine city of Innsbruck. A key objective is to establish a suitable modelling strategy which represents the influence of both the local building structure and the surrounding orography. Following evaluation against a range of observations, the LES will be used to investigate several open questions concerning the change in turbulence characteristics with height within and above the urban canopy layer, the importance of non-measurable quantities (e.g. advection) in surface exchange, the representativeness of observational datasets, the effect of individual buildings on displacement height and the influence of the urban surface and surrounding terrain on the atmosphere.
Two sets of building-resolving LES will be conducted and carefully evaluated. The first will use a small domain confined to the flat valley floor around the Innsbruck Atmospheric Observatory urban site and will be used to study the influence of the urban surface on turbulent exchange. To more directly incorporate orographic effects the second set of simulations will nest this domain within a series of larger domains with the outer domain (1-km grid) covering the Alpine region. The Weather Research and Forecasting (WRF) model will be used for the outer domains and PALM for the inner domains.
This project will break new ground in high-resolution modelling of cities in complex terrain, enabling the effects of the urban surface and orography to be studied together for the first time. The rich observational dataset will allow much-needed detailed model evaluation (against eddy covariance, scintillometer, Doppler wind lidar and microwave radiometer) of WRF and PALM, including PALM’s newly developed urban components. The validated LES will be used as a virtual laboratory, providing fine-scale turbulence data that cannot be obtained from measurements and access to new information concerning the structure of the urban atmosphere, leading to new insights into exchange processes.
SCHiRM is funded by the the Austrian Science Fund (FWF) through the Elise-Richter programme (V 888-N). The 4-year project started in February 2022 and is led by Helen Ward at the Department of Atmospheric and Cryospheric Sciences, University of Innsbruck.