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3rd HyMeX workshop 1-4 June 2009 Heraklion (Gournes), Crete-Greece

Uncertainty of high-resolution forecasting of a flood event


Silvio Davolio (ISAC - CNR); Oxana Drofa, Andrea Buzzi

A mesoscale convective system (MCS) affected the area of the Venice Lagoon in the early morning of 26 September 2007, causing a local severe flood. Heavy precipitation in excess of 320 mm in less than 12 hours, of which more than 240 mm in only three hours, was observed.
The operational forecasts provided by the high-resolution, convection-resolving model MOLOCH, essentially captured the event. However, remarkable sensitivity in terms of QPF to the specification of the initial and boundary conditions, as well as to different model configurations, was detected.
An analysis of different aspects of the event, such as the relevant mechanisms leading to the flood and the main characteristics of the MCS, and an estimation of the predictability of the episode, are performed. Although low predictability is expected due to the convective nature of rainfall, the large scale flow in which the MCS development was embedded favoured a relatively small growth of the forecast error, resulting in a quite good accuracy of the precipitation forecasts.
Moreover, the analysis of the precipitation patterns and of other meteorological fields allowed to identify the relevant mechanisms responsible for the flood.