Storm surge forecast at Météo-FranceStorm surge forecast at Météo-France

Pierre DanielPierre Daniel

Storm surge model

• Two versions of one model:

Overseas territories (for tropical cyclones storm surges)

Metropolitan coasts

Model installation

Model equations

• Shallow water equations• Finite differences on a fix grid mesh (Arakawa C)• At bottom: Chezy condition• Wind stress: Smith and Sandwell formulation• Coast: normal componant of current is set to zero• Open boundary: inverted barometer effect + radiation

condition

• Inputs: wind, atmospheric pressure, bathymetry

Atmospheric forcing• Modified Holland model: pressure profile and gradient

wind• Information needed: position, intensity, size• Provided by cyclone advisories

• Example:• FORECAST VALID 21000Z 20.7S 165.8E.• MAX SUSTAINED WINDS OF 80 KT.• RADIUS OF 50 KT WINDS 65NE 40SE 25SW 40NW• RADIUS OF 34 KT WINDS 85NE 60SE 40SW 60NW

Bathymetry

• Hand extracted from nautical charts• 30 islands, grid mesh from 150 m to 2 km.• Examples:

Model calibration• On selected cyclones from 1975 to 1990• Very few tide gauges: 6 for 30 islands

Largest surge: 250 cm in Guadeloupe (Hugo, 1989)

Model forecasts

• Largest surges: Luis, 1995• estimated: 2 m in St Martin (2 m forecast)

•measured: 40 cm in Guadeloupe (35 cm forecast)

What is the best way to use the model ?

Real-time forecast accuracy• When a hurricane is crossing an island, a

small error in the trajectory forecast gives a large error in the space distribution of the surge.

Pre-computed storm surge data base• More than 1000 model runs for each island.• A data base available for graphical display on a computer.

Storm surge data base

Coastline of France• Same equations and numerical scheme• Wind stress: Wu formulation• Tide: 16 waves + 13 tide gauges• Calibration on selected situations (1987-1992)• In operation since October, 1999.

• Under development: Improvement of the wind interpolation (spatial and temporal).

• Sensitivity to atmospheric forcing: ECMWF, ARPEGE, ALADIN.

Surges with ECMWF analyses (6 hours)

Surges at La Rochelle (ECMWF analyses – 6 hours)

Temporal frequency impact of atmospheric forcing at Le Verdon (ALADIN - December 27th, 1999)

Next steps

• Mediterranean sea

• North sea

• Wave surges in the lagoons

• Real time access to tide gauge data

• Data assimilation