Orographically Induced Flash Floods on the Northern Italian Coast

A cloud resolving numerical simulation of two massive flood occurring on the northern Italian coast are performed to better understand conditions leading to these events. The two events are the 26 September, 1992 Genoa flash flood and the 5-6 October, 1998 Friuli flood case. These Fall season events occur as the middle latitude westerlies become active and draw the deep well-mixed desert boundary layer northward from the Sahara over the Mediterranean with an approaching upper level trough. This forms an Elevated Mixed Layer (EML) above the surging flow that acts as a capping temperature inversion allowing low level instability to build through the process of air-sea interaction. The accumulated moisture and Convective Available Potential Energy (CAPE) is then released en masse as the moisture surge reaches the shoreline. There topographically induced lifting breaks the cap and releases the stored instability.

It is further shown that the Atlas mountains to the south, together with the Italian Appennino mountains act to channel the approaching surge. This to further enhance the effect by drawing the surge along warm coastal waters, especially north of Lybia. Numerical simulations of these events are performed using the University of Wisconsin Nonhydrostatic Modeling System. This model employs a variably stepped topography representation that is capable of resolving severe topography slopes. In addition, a moving nested rid is employed that can follow the active precipitation area with cloud resolving (2 km) resolution. Trajectory analysis and sensitivity experiments are employed to isolate important processes.