HAPpy Hour Seminar : The effects of urban land use on severe storms development

Abstract: It is well known that cities can modify rainfall distributions, particularly by more frequently triggering deep moist convection over and downwind of urban areas. However, the effect of cities on the most extreme convective events, such as hailstorms, downbursts, or tornadoes, is poorly studied. This topic needs further investigation since exposure and vulnerability to severe storm risks are greater in cities than in the surrounding rural areas. What happens if a severe convective windstorm impacts a large city? How does urban land use influence a supercell?

To address the first answer, a preliminary analysis was carried out for a case study that occurred on July 25, 2023, when a nocturnal convective windstorm affected the city of Milan in northern Italy. Numerical simulations with the WRF model show a northward shift of the storm in the upwind region when the urban land use is considered. Over the city, wind gusts are reduced by about 13%, while updrafts are intensified. Downwind of the city, the storm is broken into three convective cells, with a delay of about 5 km compared to a no-urban scenario. The southernmost convective cell produces more abundant rainfall than in the no-urban scenario.

To generalize the study and address the second question, idealized simulations are being conducted using the WRF model. An idealized circular city, representing the average morphological characteristics of Milan, is placed in a flat domain with cropland as the predominant land use. Urban-atmosphere exchange processes are resolved using the Building Effect Parametrization (BEP) and the Building Energy Model (BEM). A supercell is initiated in the domain in the afternoon using a thermal bubble and is then advected by the mean flow toward the city. To enhance the analysis, 10 ensemble members are generated for each experiment using SKEBS. The impact of different city sizes is assessed by examining how key diagnostic variables of the supercell change upwind, over, and downwind of the city, with statistical significance also evaluated. The study will soon be expanded to explore additional morphological properties of the city, such as building height.