1. 1 ACTIMAR – 24, quai de la Douane – F 29200 Brest - +33 (0)298 44 24 51 – www.actimar.fr A SIMPLE OPERATIONAL MODEL FOR THE ANALYSIS AND FORECAST OF POLLUTANT AND OBJECT DRIFT Philippe Craneguy ACTIMAR (craneguy@actimar.fr) Sea Tech Week – Brest Technologies for Search, Assistance and Rescue 18-20 October 2004

2. 2 INTRODUCTION  Aims of SURPOL model:  A simple and reliable drift model for any marine area  On the basis of available oceanic models and weather forecast  Easy and fast to implement for applications  Applications:  Marine Environmental Protection oil pollution at sea and to the coast, …  Search and Rescue persons in water, life rafts, …  Safety of Navigation, Recovery containers, debris, …  Development of the model  Gathering existing tools Ocean model - Trajectory diagnostic - Drag forces  Background (oil drift prediction) PREVIMEL + ARIANE  Extension towards object drift prediction In test

3. 3  Ocean model  1D-vertical mixing (atmospheric fluxes, friction at the bottom)  Worlwide and easy to set up within 1 day for operational purpose  Weather Forecast Model (wind stress and speed, heat fluxes, …)  Remote data assimilation (SST)  In-situ data assimilation (XBT, buoys, …)  Reference: Gaspard (1990) PREVIMEL  Sea-surface horizontal velocity field (Ekman drift + wind entrainment)  Thermal structure of the mixed layer and the thermocline

4. 4 Weather Forecast Model Data Assimilation ModuleMixed Layer model FLORENCE 1D TurbidityBathymetry Barotropic tidal current Atmospheric parameters Ocean Surface T°C files (US NAVY) AVHRR imagery (NOAA) In-situ measurements Air-Sea Interface fluxes: - Solar + IR radiation - Heat fluxes - Evaporation / Precipitation - Mvt amount flux from wind stress on the surface Vertical 1D T° profile Driving Current Field from surface wind Ekman Current field 3D T° Analysis field Initial Conditions field Drift Current field 3D T° Forecast field Lagrangian path computing module ARIANE 2D Operational Digital system for analyzing and forecasting from J to J+3 the thermal and dynamics behavior of the ocean upper layer down to 400m depth  PREVIMEL

5. 5  Trajectory tool - ARIANE  Based on non-divergence of flow  Water particles follow volume-preserving streamlines  Used here in a 2D-version at sea-surface or subsurface  Reference: Blanke & Raynaud (1997) ARIANE  Lagrangian trajectories of sea-surface or subsurface particles derived from ocean model outputs

6. 6 Guiding principles ARIANE First developed for tracking water masses 3D movements (origin and fate) Based on the non-divergence of the flow Volume conservation in elementary boxes on a C-grid (Arakawa, 1972) Water particles follow volume-preserving streamlines Non-crossing of the coastline or ocean bottom Allows reverse trajectory calculations Allows numerous multiple trajectories in order to represent the spreading of water patches

7. 7 Application to the Prestige oil drift prediction PREVIMEL + ARIANE Drift calculated from Previmel results (Ekman + 4% of the wind) Drift observed during the same period Nov. 19  Dec. 23, 2002

8. 8 Application to the Prestige oil drift prediction PREVIMEL + ARIANE Multiple particles view (Nov. 19  Dec. 23, 2002)

9. 9  Object drift – still in development  Adding drag coefficients for the immersed and emerged parts  Search and rescue issue  Reference: Allen and Plourde (1999), Breivik (2004) OBJECT DRIFT  Trajectories of objects at sea-surface or subsurface

10. 10 Force balance on free floating object OBJECT DRIFT Wind (V atmos ) Oceanic current (V water ) Radiative waves M.dV object /dt = F atmos + F water + F wave F atmos = ½.ρ a.C a.S a.|V atmos -V object |.(V atmos -V object ) = wind drag force F water = ½.ρ w.C w.S w.|V water -V object |.(V water -V object ) = water drag force F wave = ½.ρ w.g.C iw.L.A 2 = wave radiation force Resulting displacement (V object ) With: ρ a, ρ w : density of air/water S a, S w : emerged/immersed surface of the object C a, C w : drag coefficient in the air/water C iw : incident wave reflection coefficient A: wave amplitude L: object length scale g: gravitational accelaration

11. 11 Analytical solution OBJECT DRIFT M.dV object /dt = F atmos + F water + F wave Assumptions and methodology Steady state assumed at each lapse time → dV object /dt = 0 F wave negligible for small objects (container, person in water, …) → F wave = 0 V atmos = wind speed given by Weather Forecast Model V water = drift current given by Previmel C a,C w : drag coefficients compiled by US Coast Guard (Allen and Plourde, 1999) Trajectories calculated by Ariane with the solution of ρ a.C a.S a.|V atmos -V object |.(V atmos -V object ) + ρ w.C w.S w.|V water -V object |.(V water -V object ) = 0  V object = V water + (V atmos -V water ).sqrt[(ρ a.C a.S a )/(ρ w.C w.S w )]

12. 12 Task sequencing (24H set-up) CONCLUSION V object = V water + (V atmos -V water ).sqrt[(ρ a.C a.S a )/(ρ w.C w.S w )] Weather Forecast Model (Worldwide) PREVIMEL (1D-mixing model) V water (drift) V atmos (wind) ARIANE 2D (lagrangian trajectories) OBJECT DRIFT