Deep Banded Orographic Convection over an Idealized Mountain Range: Influence of Upstream Atmospheric Conditions

Elongated and quasistationary deep convective rainbands 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 on the evening of 29 October 2018. Vaia was an extreme storm, causing floods, landslides, and extensive forest damage in several locations of the eastern Italian Alps. In the present work, the thermodynamic conditions favorable for the formation of the rainbands are investigated through semi-idealized numerical simulations performed with the Weather Research and Forecasting (WRF) Model. In particular, the influence of wind speed and direction, stability, and relative humidity on the development of rainbands is investigated using different idealized sounding profiles and an idealized smooth ridge. First, a sensitivity analysis with simulations with 1-, 0.5-, and 0.2-km grid spacing highlights that WRF is able to reproduce the development of deep banded convection over the idealized smooth ridge and that results are independent of the model resolution. Rainbands appear as horizontal roll-like circulations with updrafts reaching altitudes up to 6–7 km MSL and varying their position in time. Then, various sensitivity experiments show that band-shaped convection is favored in the presence of unidirectional vertical wind shear, especially with the alignment of wind shear and wind vectors. The presence of convective inhibition in the lower layers is fundamental for the development of rainbands over the ridge, inhibiting the formation of upstream cellular convection. Conversely, strongly moist unstable stratification or saturation in the higher layers disrupts the convective organization.