Effects of the urban land use on a severe convective windstorm

Urban land use may cause an intensification of convective rainfall over or downstream of cities. Little is known about the effect of urban land use on other damaging convective phenomena, such as convective windstorms. This is a relevant topic since cities are far more vulnerable and exposed than rural areas to weather-related risks. The question is well posed because severe local storms are influenced by the thermodynamic and kinematic properties of the atmospheric boundary layer that respond directly to the presence of a city. To explore the origin of the interaction between an urban land use and a convective windstorm, high-resolution numerical simulations are set up using the Weather Research and Forecasting model on a relevant case study that occurred on July 25, 2023, in Milan, northwestern Italy. An urban and a no-urban (i.e., with urban land use replaced by croplands) physics ensembles at 1 km grid spacing were generated, employing the Building Effect Parametrization-Building Energy Model, different microphysics and atmospheric boundary-layer schemes, and land use datasets. Simulations highlighted an alteration due to the urban land use with a northward shift of the storm in the "upstream" region. Over the city, wind gusts are reduced by about 13% and updrafts are intensified. Downstream of the city the storm is broken, with a delay of about 5 km compared with a no-urban scenario. These observations are accompanied by a decrease of surface equivalent potential temperature and a decrease of storm relative helicity over the city in the pre-storm environment. In conclusion, even an intense convective windstorm is significantly impacted by the urban land use: its trajectory, morphology, and intensity are modified by the city. These modifications seem to be mostly related to the effect of the buildings on the gust front rather than on the thermodynamic alteration induced by the urban land use in the pre-storm environment.