Can a city intensify a local severe storm?

It is well known that urban areas influence the atmospheric boundary layer, e.g., increasing the temperature with respect to thesurrounding rural areas (the “urban heat island”, UHI), and perturbing the wind field. Some studies pointed out that cities can also modifythe rainfall patterns, in particular deep moist convection is more frequently triggered over and downwind urban areas. While these topicsare well addressed in the literature, the effect of cities on the most extreme convective events, such as hailstorms, downbursts ortornadoes, is poorly studied. Can a city intensify a local severe storm? This is a relevant question, since the exposure and vulnerability tothe severe storm risk is higher in cities than in the surrounding rural areas. Our analysis started with a case study, that occurred on 24-25 July 2023, when multiple severe storms caused extreme hailstorms anddownbursts over Northern Italy. The most affected areas were the plain of the Friuli Venezia Giulia region and the urban area of Milan. Forthis latter area we tried to investigate if the urban land use may had intensified the severe storm. The analysis of surface weather stationsshowed that during the night between 24 and 25 July 2023, when the most severe storm occurred, the UHI was not present over Milan. Onthe other hand, analysing radar images, severe storms seemed to intensify crossing the urban area. A hypothesis is that the change ofroughness due to the presence of the city, intensified the vertical motions. To address this hypothesis, numerical simulations with theWeather Research and Forecasting (WRF) model were performed. Those were carried out using the BEP-BEM parametrization schemefor urban areas (WRF-URBAN), and they were compared to observations to validate the model. First results from a real case studysuggest that the city of Milan may had an effect on the severe storm development: we plan to perform some idealized simulations toconsolidate our findings.