Numerical simulations of boundary-layer processes and urban-induced alterations in an Alpine valley

High-resolution simulations are performed with the Weather Research and Forecasting (WRF) model, coupled with an advanced urban parameterization scheme, to evaluate the modifications induced by the urban area of Trento on boundary-layer processes in the Alpine Adige Valley in a typical summer sunny day. Specific gridded datasets of urban morphology parameters and anthropogenic heat releases were created to provide high-resolution input information for the urban scheme. Comparison of model results against measurements from surface weather stations shows that the model simulates reasonably well the development of valley winds, as well as the complex interaction occurring north of Trento between the up-valley wind of the Adige Valley and a lake breeze flowing from a tributary valley. The urban heat island of the city is also well captured by the model, with stronger intensities at night and lower values during daytime. Comparisons with an idealized simulation, where all the urban land use grid points are replaced by cropland, suggest that the city inhibits the development of the ground-based thermal inversion at night and also affects valley winds, modifying both the typical down-valley wind, and the interaction between the up-valley wind of the Adige Valley and the lake breeze. Further sensitivity tests are performed to evaluate the impacts of the gridded datasets of urban morphology and anthropogenic heat releases, and to assess what are the benefits of using the advanced urban scheme against a simple bulk parameterization. Results highlight that even the moderate anthropogenic heat releases present in Trento appreciably affect nocturnal temperatures in the city, while gridded information on urban parameters is not essential in the present case, because urban morphology does not display a high spatial variability. However, it is shown that the choice of the urban scheme may have significant impacts on both temperature and wind fields.