Role of orography in the distribution of precipitation extremes and in shaping the event predictability: the Vaia storm case study

The Vaia storm, which impacted northeastern Italy from October 27 to 29, 2018, brought extreme winds and heavy rainfall, with some areas receiving over 850 mm of rain within three days. This study has two main objectives: first, to evaluate the accuracy of convection-permitting models (Moloch and WRF) in simulating precipitation during the Vaia storm by comparing them to observational data; and second, to analyze the impact of orography on precipitation patterns, particularly focusing on how orographic complexity influenced the distribution of maximum rainfall. The results show that the interaction between the airflow and the Alpine barrier played a crucial role in shaping the distribution and intensity of precipitation. Windward slopes experienced increased rainfall due to orographic lifting, while leeward areas were impacted by strong downslope winds. A key finding of this study is the variation in precipitation estimates across different simulation setups, which highlights the importance of accurately representing topography in numerical weather prediction models. The differences observed between WRF and Moloch simulations, particularly when initialized with GFS or IFS data, emphasize the influence of initial conditions on forecast accuracy. Additionally, the comparison with observed precipitation data allowed for an evaluation of model performance, revealing potential biases that should be considered in future applications. In conclusion, this research underscores the significance of orographic effects in extreme weather events and emphasizes the need for high-resolution modeling to better understand and predict such phenomena.