Numerical simulations of boundary-layer phenomena and urban-scale processes in the Alpine city of Trento

Simulations with the Weather Research and Forecasting (WRF) model coupled with an urban parameterisation scheme are performed to evaluate the alterations caused by the urban area of Trento on boundary-layer processes in the Alpine Adige Valley. Simulations, with 500-m grid spacing, focus on a typical summer sunny day, when both valley winds and the urban heat island are well developed. Specific gridded datasets of urban morphology parameters and anthropogenic heat flux releases were created to provide high resolution input information to the urban scheme. Validation of numerical results against measurements from surface weather stations shows that the model is able to simulate reasonably well the development of valley winds, as well as the complex interaction occurring north of Trento between the local up-valley wind of the Adige Valley and an incoming lake breeze from a tributary valley. The urban heat island of the city is also well captured by the model, with strong intensities at night and low values in the central hours of the day. It is found that the city inhibits the development of the ground-based thermal inversion at night, especially in the city centre, displaying a denser urban morphology and higher buildings. Furthermore comparisons with an idealised simulation, where all the urban and use grid points are replaced by cropland, suggest that the city also affects the development of valley winds, modifying both the typical down-valley wind in the early morning, and the interaction between the up-valley wind flowing in the Adige Valley and the lake breeze. Finally sensitivity tests are performed to analyse in detail the impact of the gridded datasets of urban morphology and anthropogenic heat flux releases on the near-surface temperature and wind field.