Urban land use and complex topographic effects on high-impact weather events

The prediction and understanding of high-impact weather phenomena remain particularly challenging in areas characterized by complex topography, where observations are often insufficient, and numerical weather prediction models frequently struggle to accurately reproduce and predict such events. To illustrate these challenges, two remarkable case studies are presented: the sinking of the Bayesian yacht at the interface between the sea and the mountainous inland of Sicily on 19 August 2024, and the Bora windstorm that affected Trieste on 11 - 12 February 2012. Although the numerical models analyzed demonstrated generally good skill in simulating these episodes, some limitations emerge in capturing near-surface strong winds. The scarcity of observations emerged as a significant shortcoming for the interpretation of the meteorological processes involved and the assessment of model performance. Subsequently, the thesis investigates the effect of urban land use on local severe storms. Urban areas can be considered as elements of topographic complexity, associated with heightened exposure to weather-related hazards. A detailed analysis of a convective windstorm that occurred in Milan on 25 July 2023 reveals clear urban-induced modifications, showing that even a fast-moving and highly energetic convective storm can be significantly influenced by a large city. To overcome the intrinsic limitations of individual case studies, idealized numerical simulations of a storm interacting with a circular urban area are also performed. These experiments indicate a statistically significant weakening of the storm with increasing city size, primarily driven by the urban dry island. Nevertheless, in the simulation with the largest city, a downwind pressure minimum induced by the urban heat island triggers a new storm. Overall, this research advances our understanding of the complex interactions between urban environments and deep moist convection, clarifying the roles of buildings, vegetation, and urban surfaces in shaping storm dynamics, with the ultimate goal of improving early warning systems.