open session on operational oceanography -os4.1/cl4.11 egu2011-7488 use of ensemble forecast...

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 CavaleriISMAR-CNR, Venice, Italy


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.


Venice location

Venice lagoon

City of Venice


Venice flooding

Even a small storm surge (e.g. 30cm) with the tidal contribution can cause the flooding of the City.

Sea level Flooded surface

190 cm 100%140 cm 90%

130 cm 70%

120 cm 35%

110 cm 12%

100 cm 4%

Pavement lowerthan 90 cm


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)


ECMWF wind andpressure forecast Tide gauges

Models

Subjectiveforecast Alerting

population

Otherobservations

Operational storm surge forecast in Venice

ICPSM operational chain


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


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.


Method

50 + 1 Ensemble low res.forecasts

1 High res. deterministicforecast

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.


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.



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



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

Err

or

[m]

No relation

A posteriori

A priori


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.



run: 29 November 2008, 00UTC

Good estimation

Low time or spatial resolution?

22 hours seichesSeiches + forcing



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

Err

or

[m]


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;

open session on operational oceanography -os4.1/cl4.11 egu2011-7488 use of ensemble forecast...

Documents

operational oceanography

open session

operational forecast

ensemble forecast

icpsm slide

divergence slide

storm surge events

small storm surge