Western Mediterranean cyclones and heavy rain. Part I: Numerical experiment concerning the Piedmont flood case

Heavy rain events constitute a major operational problem in the Mediterrane an area. The present work tries to contribute to a better understanding of this problem by considering the possible indirect influence of surface depr essions in the triggering and location of the heavy rain. The idea is that any surface low contributes to the organisation of the low-level flow. This then affects the creation/intensification of a low-level feeding current o f warm and wet air that can sustain heavy rain if there is convective insta bility. Two approaches have been used: (a) a numerical and synoptic approac h, Part 1, and (b) a statistical approach, Part 2. In Part 1, the differenc es between a control run and a numerical experiment without the Iberian mou ntains for the 'MAP Piedmont flood event' are discussed. Among other Effect s, the Iberian orography seems to intensify a secondary low near Liguria as well as increase the mixing ratio convergence and the rainfall intensity, near it. A few other cases of heavy rain dye also briefly reviewed in order to identify the presence of a Mediterranean depression in the vicinity.