Probabilistic sea level forecast for Venice: ensemble system and "dressed" deterministic prediction

Riccardo Mel, Piero Lionello, Luigi D'Alpaos, Luca Carniello

Thursday 2 july 2015

9:30 - 9:45h at Oceania (level 0)

Themes: (T) Flood risk management and adaptation, (ST) Early warning systems

Parallel session: 10L. Floodrisk - Early warning systems

Sea level (SL) forecast for the city of Venice is of paramount importance for the management and maintenance of this historical city and for operating the movable barriers that are presently being built for its protection. In this paper an Ensemble Prediction System (EPS, based on an ensemble of 50+1 simulations) for operational forecasting of storm surge in the northern Adriatic Sea is presented and applied to three months period in the year 2010, during which an exceptional sequence of storm surges occurred. Results are compared to observations at tide gauges distributed along the Croatian and Italian coast of the Adriatic Sea. Unfortunately, performing multiple simulations through tidal deterministic models requires a considerable, if not prohibitive, computational cost. In this paper, a simple and efficient tool to estimate the uncertainty affecting storm surge prediction in the northern Adriatic Sea is presented, allowing to accurately estimate the sea level variance without performing extensive calculations. This is possible because uncertainty in sea level forecast is principally caused by the uncertainties which affect the forcing meteorological fields, the predicted value of sea level and how it increases with the forecast time range. The variance of high storm surge events, computed with a shallow water model forced by EPS input fields, has been replicated considering all this factors. It is shown that the variance estimation is acceptably realistic.