L’assimilation des spectres de Sentinel 1-A dans le modèle MFWAM : Conséquences sur le couplage avec l’océan L. Aouf (1), A. Dalphinet (1) et H. Giordani (2) (1) Météo-France, DIROP/MAR, Toulouse (2) CNRM/GAME, Météo-France, Toulouse Journées GMMC, 7-9 Juin 2016, Toulon

MOTIVATION Améliorer la prévision des propriétés directionnelles de la houle dans le modèle de vagues MFWAM : confirmer l’expérience de l’utilisation des spectres SAR d’Envisat. Implémentation de procédure de contrôle de qualité sur les produits SAR-L2 de Sentinel-1A : préparation pour l’opérationnel Evaluation de l’impact de l’assimilation des spectres SAR-L2 de S-1A (Wave mode 1 &2) dans le modèle MFWAM. Quelles conséquences pour le couplage avec un modèle d’océan (1-D couche de mélange)

Operational global wave forecasting system at Météo-France Two global wave models MFWAM (0.5°) forced by analyzed ARPEGE and ECMWF winds.  MFWAM uses the new source terms developed in Ardhuin et al (2010).  The model was upgraded since november 2014 with improvements from the work in Mywave project (final report 2014). In operations assimilation 6 hours: Jason-2 SARAL since 10 December 2013 Cryosat-2 since 23 April 2014 Snapshots of SWH on 19 October 2104 at 0:00 (UTC) JA2SRLCR-2

Control of MFWAM(Global-CEP) with altimeters 12-hour forecast of SWH Comparison with altimeters Jason-2 and SARAL Since the upgraded version of MFWAM the normalized Scatter index of SWH ~ < 10 % in the 12-hour forecast Thanks to upgraded version (Mywave )

Pliusieurs modes : StripMap (SM) Interferometric Wide Swath Extra-Wide swath (EW) Wave mode (two look directions 23° and 36.5° WV 1 & 2) Sentinel-1 carries a single C-band synthetic aperture radar instrument (C-SAR) operating at C-Band (centre frequency of GHz). A right- looking active phased array antenna providing fast scanning Level 2 products Ocean Wind field (OWI) Ocean Swell Spectra (OSW) Surface Radial Velocity (RVL)

Daily coverage of S-1A (WM-1 & 2) 23 April 2016 The SAR (L2) wave spectra of Sentinel- 1A Two wave modes of S-1A provides wave Spectra with a resolution of 60 frequencies from 0.04 to 0,23 Hz and 72 directions (by step of 5°) S-1A wave spectra along track spaced by 100 km

MFWAM wave spectra FG SAR L2 wave spectra Partitioning wave trains Cross assignment FG and SAR partitions Optimal interpolation OI Mean energy and wave numbers Reconstruction of analysed wave spectra Description of the assimilation of SAR wave spectra

The auto-correlation functions of the model MFWAM are computed by using difference between forecasted swell wave parameters for different ocean basins. Optimized correlation model for the assimilation Atlantic oceanPacifficTropics a Atlantic0,93 Pacific0,76 Indian0,64 Tropics0,66 Best fit of exp(d/ ) a d is the distance Correlation length of 250 km swell mean period

Description of test runs New processing has been implemented for the level 2 wave products of S-1A - period from 8 to 30 Avril 2016 The model MFWAM with grid size of 0.5° and the wave spectrum in 24 directions and 30 frequencies (starting 0.035Hz). The model MFWAM is driven by 6-hourly ECMWF analysed winds) Performed runs : - run with wave spectra from both WM 1 & 2 of Sentinel-1A - run with wave spectra from both WM 1 of S-1A - run with wave spectra from both WM 2 of S-1A - run with wave spectra (WM 1&2) of S-1A and altimeter data from Saral

Impact of the assimilation of sentinel-1A (WV 1 &2) in the forecast period Difference of wave parameters with and without assimilation of S1A Snapshots with a step of 6 hours in the period of forecast starting on 20 April 2016 at 0:00 UTC until 22 April at 0:00 Swell wave height Mean wave period

Validation of the assimilation of S1-A wave spectra Significant wave heights April 8 to 30, 2016 Comparison with SARAL and JA2 Bias = 0.02 SI = 11.2% RMSE = 11.2% Slope = 1.01 Intercept = Bias=-0.03 SI=11.8% RMSE=11.8% Slope=1.05 Intercept=-0.11 Scatter index is slightly improved by 5 % MFWAM-ASSI-S1A MFWAM-NOASSI Better slope

High Lat |  |> 50° Intermediate lat 20°<|  |<50° Tropics |  |<20° Comparison with Ja-2 and SARAL Performance of Scatter index of SWH (%) in different ocean basins Performance of scatter index of SWH (%) Performance of Scatter index of SWH (%) in different ocean basins Impact of the assimilation of S1-A spectra (WM 1 & 2) Period of April 2016 Improvements in all ocean basins (stronger impact in high latitudes)

Impact of the assimilation of S1-A spectra (WM 1 & 2) Period from Dec-Jan 2016 Validation with Jason-2 and SARAL Statistical analysis High Lat |  |> 50° Intermediate lat 20°<|  |<50° Tropics |  |<20° WV-1 slightly better in the trop and intermed

The impact of the assimilation in the forecast period September 2015 Validation with Jason-2 and Saral Normalized Scatter index of SWH (%) Black line : MFWAM without assimilation Blue line : MFWAM with S1A Red line : MFWAM with altimeters (CR2+SRL) and S1A

Validation with buoys wave data in pacific ocean Thanks to NDBC Hawaii-Hilo-SW Stratus Scatter index (%) Period of April 2016 Strong reduction (~39%) of SI for peak period at Hilo-SW buoy without assimilation with SAR (1&2) The model is good on SWH but not on Tp

The water side stress (breivik et al. 2014) computed by the wave model is :  oc =  a -  in -  ds, where  a is the air side stress,  in is the momentum flux absorbed by the waves and  ds is the momentum injected by breaking waves to the ocean Impact on coupling parameters Difference of stress (%) with and without SAR spectra Snapshots on 29 April 2016 at 0:00 (UTC) Example of ratio  oc /  a

Temperature V-current U-current Impact of SAR spectra on the 1-D mixed layer South-west Hawaii, Hilo 1-D ocean mixed layer (Giordani et al. 2005) Is forced by the stress provided by the model MFWAM (with and without SAR) during 22 days. 4 pronostic variables (U,V, Temp, Salinity) are analysed at Hilo-SW position The assimilation of SAR enhances the difference for the current and temperature in swell dominant sea state (Hilo-SW) Without assimilation with assimilation of SAR depth

 l’impact de l’assimilation des spectres SAR de S-1A est positif dans les périodes d’analyse et de prévision (comparaison avec saral et Jason-2)  Bientôt une mise opérationnel de l’assimilation des spectres de S-1A (dès que les données seront dispo sur le GTS)  La couche de mélange océanique est sensible à l’assimilation des spectres SAR. Les variations en courants et température ont été bien identifiées.Des travaux sont en cours pour valider cette signature et étendre les tests à un modèle d’océan 3D (voir poster 73 de S. Law-Chune et al.)  Des tests d’assimilation (et OSSE) avec plusieurs sources de spectres (S-1A et 1B) seront effectués pour préparer la mission CFOSAT. Conclusions et travaux futurs