The effect of complex orography on the development of a tornadic outbreak in the Po Valley

How can tornadic supercells be influenced by complex orography? In this study the authors address this issue by studying a tornado outbreak that affected the Po Valley in northern Italy on 19 September 2021. During the event seven tornadoes (four of them ranked as F2 according to the Fujita scale) developed between Lombardia and Emilia-Romagna regions in a few hours. Although tornadoes are not rare in Italy, so many tornadoes in such a short time is an unusual event. The event was studied exploiting observations and numerical simulations obtained with the convection permitting MOLOCH model. Observations showed that during the event there were two low-level boundaries in the Po Valley: a cold front coming from the Alps and a dry line generated by the downslope winds from the Apennines. These two boundaries created a triple point, like those observed during tornado outbreaks in the US Midwest, but on a smaller scale. Numerical simulations with 500 m grid spacing showed that a warm and moist air tongue from the Adriatic Sea played a fundamental role in generating the supercells. Moreover, by means of numerical experiments, it has been proved that the structure and location of the moist air tongue was sensitive to the Froude number of the south-westerly flow from the Apennines: the greater the Froude number, the further north and narrower was the tongue of air, with impacts on the development of supercells. Finally, the dry line played a key role in the generation of tornadoes. In fact, kinematic and windshear parameters were comparable to typical values observed in the US-tornado events only along a narrow path ahead the dry line.