Storm Surge Ensemble Prediction for the City of Venice

Abstract

Abstract Sea level (SL) forecast for the city of Venice, Italy, 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 simulations) for operational forecasting of storm surge in the northern Adriatic Sea is presented and applied to 10 relatively high storm surge events that occurred in the year 2010. It is shown that storm surge peaks correspond to the maxima of uncertainty (as described by the spread of the EPS members), which increases linearly with the forecast range. Further, the uncertainty in storm surge level is shown to be linked to the uncertainty of the forcing meteorological fields. The quasi-linear dynamics of the storm surges plays a minor role in the evolution of uncertainty, except it produces its oscillation with a period associated with that of the 11-h seiche of the basin. The error of the ensemble mean forecast (EMF) is correlated with the EPS spread. For these cases, the EMF accuracy is very close to that of the high-resolution deterministic forecast (DF) and is more robust than the DF (meaning that its error is consistently smaller than the error of the DF, as the lead time of the forecast varies).