A conceptual model for the development of tornadoes in the complex orography of the Po Valley

Abstract

The Po Valley in northern Italy is a hot spot for tornadoes in Europe in spite of being surrounded by two mountain ridges: the Alps in the north and the Apennines in the southwest. The research focuses on the case study of 19 September 2021, when seven tornadoes (four of them rated as F2) developed in the Po Valley in a few hours. The event was analyzed using observations and numerical simulations with the convection-permitting MOLOCH model. Observations show that during the event in the Po Valley there were two surface boundaries, that created a triple point: an outflow boundary generated by convection triggered in the Alpine foothills, and a dryline generated by downslope winds from the Apennines, while warm and moist air was advected westward from the Adriatic Sea east (ahead) of the boundaries. Tornadoes developed about 20 km north-east of the triple point. Numerical simulations with 500 m grid-spacing suggest that supercell and dryline development in the Po Valley were sensitive to the elevation of the Apennines. Simulated vertical profiles show that the best combination of instability and wind shear for the development of tornadoes was attained within a narrow area located ahead of the dryline. A conceptual model for the development of tornadoes in the Po Valley is proposed, and the differences between tornado environments over a flat terrain and over a region with complex terrain are discussed.