1. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 Use of ensemble forecast meteorological fields to force a storm surge model Marco Bajo, Luciana Bertotti and Luigi Cavaleri ISMAR-CNR, Venice, Italy

2. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 Summary The problem: Venice flooding; The method: Ensemble forecast forcing; Case I: 4 November 1966 (the highest flood); Case II: 1 December 2008 (operational forcing); Conclusions.

3. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 Venice location Venice lagoon City of Venice

4. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 Venice flooding Even a small storm surge (e.g. 30cm) with the tidal contribution can cause the flooding of the City. Sea levelFlooded surface 190 cm100% 140 cm90% 130 cm70% 120 cm35% 110 cm12% 100 cm4% Pavement lower than 90 cm

5. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 Storm surge forecast in Venice: ICPSM Both statistical and dynamical models are used for the storm surge forecast Operational: 24/24 hours - 7/7 days After the 1966 event an Office for the sea level forecasting and warning has been created (ICPSM)

6. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 ECMWF wind and pressure forecast Tide gauges Models Subjective forecast Alerting population Other observations Operational storm surge forecast in Venice ICPSM operational chain

7. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 Hydrodynamic model SHYFEM (Shallow water HYdrodynamic Finite Elements Model) Model main features: - 2D shallow water equations; - Staggered finite elements scheme; - Semi-implicit time discretisation scheme; - Computational domain: Mediterranean Sea; - Open boundary in Gibraltar. Level set to 0, free normal fluxes. Forcing: - ECMWF wind and pressure forecasts. Open source code. Visit: www.ismar.cnr.it/shyfem And www.ismar.cnr.it/kassandra

8. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 The ECMWF ensemble meteorological forecast Uncertaintes in the initial state Phase space DET ENS Time - 50 + 1 ensemble members; - Perturbations of the initial conditions (singular vectors); - Model perturbations.

9. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 Method 50 + 1 Ensemble low res. forecasts 1 High res. deterministic forecast Simulation of 2 storm surge events: 4 November 1966 1 December 2008 Modeled surge is extracted near Venice and compared with observations (without the tidal signal) FORCING 50 + 1 Ensemble runs 1 Deterministic run SEA LEVEL FORC.

10. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 Case I: 4 November 1966 4 November 1966, Venice (from www.graziussi.com) The highest event ever measured (1.94 m, with a storm surge of 1.8 m); City flooded for a long time, more than 30 hours; No operational forecast available.

11. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 Case I: 4 November 1966 run: 2 November 1966, 00UTC Time shift due to different locations of the data Time variation of the ens. peaks 22 hours seiches Ens. divergence

12. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 Case I: 4 November 1966 Errors on the estimation of the maximum  IQR =0.11 m  ENS =0.51 m  C =0.58 m  DET = 0.49 m 5days 4days 3days 2days 1day Error [m] No relation A posteriori A priori

13. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 1 December 2008, Lido beach, Venice Case II: 1 December 2008 Highest event of the last 20 years; Storm surge peak had the same time of the tidal peak; The storm surge event was not isolated.

14. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 Case II: 1 December 2008 run: 29 November 2008, 00UTC Good estimation Low time or spatial resolution? 22 hours seiches Seiches + forcing

15. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 Case II: 1 December 2008 Errors on the estimation of the maximum  IQR =0.15 m  ENS =0.12 m  C =0.17 m  DET = 0.24 m 5days 4days 3days 2days 1day Error [m]

16. Open session on Operational Oceanography -OS4.1/CL4.11 EGU2011-7488 Conclusions - The ensemble median can give an accurate estimation of the maximum storm surge peak, more coherent between different runs; - Possible estimate of the forecast error considering the ensemble width (e.g., IQR, standard deviation); Possible improvements: -Increase the temporal and spatial resolution of the forcing;