Numerical simulations of banded orographic convection during the Vaia storm over the eastern Italian Alps

Elongated and quasi-stationary convective rainbands triggered by small-scale orography and capable of producing heavy precipitation are often observed over the Italian Alps. Such features occurred in the final and most intense phase of the Vaia storm over the eastern Italian Alps, in the evening of 29 October 2018. South-east/north-west oriented bands, driven by the strong Sirocco wind, caused floods and landslides in several locations. In the present work, the thermodynamic conditions favorable for their formation and the triggering by small-scale topographic features are investigated through semi-idealized numerical simulations with the Weather Research and Forecasting (WRF) model. Simulations are initialized using radio-sounding data measured at Udine-Rivolto at 18:00 UTC, 29 October; the small-scale energy needed to develop convection is provided prescribing background thermal fluctuations embedded in the low-level flow. The first tests using a simplified smooth topography highlight that rainbands develop even without the triggering effect of small-scale topographic features. The simulated convection tends to organize in non-stationary bands which spread precipitation evenly over the mountain. The presence of the orographic uplift and strong winds result fundamental for the release and organization of the instability embedded in the flow. A sensitivity analysis with simulations at 1, 0.5 and 0.2 km grid spacing highlights that model results are independent of the model resolution. The influence of stability, wind intensity and wind shear on the development of rainbands is also investigated, using different idealized sounding profiles. Moreover, the role of orographic details in their structure and formation is studied using different levels of topography idealization, to better understand the mechanisms that occurred during the Vaia storm and to provide a deeper insight for future events.