Validation of a set of input source terms based of altimeters data Jean-Michel Lef è vre, Lotfi Aouf, Fabrice Ardhuin and Pierre Queffeulou. WISE 2008, HELSINKI, FINLAND 1-5 June 2008

 Description of the experiments  Comparisons with altimeter data  Comparisons with buoys  Conclusion and Perspectives Motivation Overview of the Talk  Works have been done to improve wave dissipation parametrisation within the last few years (Alves and Banner 2002, Lefevre et al. 2004, Bidlot et al. 2005)  Now, model performances « very good » for Hs (typically 12-13%) for hindcasts, but inhomogeneous in space, and not for Tp  Global data sets available (JCOMM and Altimeters)

Global model, 0.5°x0.5°, ETOP05 or GEBCO Spectra: 30 x 24,fmax 0.55 for WAM; 32x24,fmax 0.7 for WW3 ECMWF wind analysis 2007 every 6h 0.5x0.5° ECWAM : WAM+ Janssen + BAJ, Adv 1 order, implicit, Sbot MFWAM: WAM +Janssen + BAJ, Adv 1 order, semi-implicit no islands filter, No Sbot WAM_MA: WAM + Makin (including negative input) +AB +Sbot, No islands filter WW3_BAJ: WW3 + Janssen (Bmax 1.25) +BAJ, adv 3order, implicit scheme) + Sbot + islands filter TEST350: WW3 + modified Janssen + negative input + Saturation + Sbot+islands filter Description of the experiments

Janssen input source term Drag coef is Sea-state dependant

Makin input source term The drag coef is Also Sea-state dependant, the theory is based on the direct coupling of waves of all scales to the wind accounts for stress due to the separation of the airflow (AFS) from short and dominant waves and also for the wave-induced stress (WOWC). The parameterization of the surface stress (sea drag) is based on this theory and it accounts for the wind speed, wave age and finite bottom dependencies of the surface stress This is an extension of Plant (1982) relation for fast moving waves :  R~1 for slowly moving waves, R~0 for fully dev. seas  R is negative for fast moving waves or waves travelling against wind

BAJ and BA Dissipation Integral spectral steepness Mean frequency and mean wave number defined to put more weight to the high frequency part  important for mixed sea and swells B(k) is a local saturation parameter, Br the threshold saturation parameter, for B(f) Br, dissipation rate function of B(k)/Br at the power 3 (p=6). According to Babanin and Westhuysen 2008,the formulation is not in agreement with experimental results and true Br should be 2/3 times lower than given in AB.

Hs bias Model-Alt BAJ MFWAM WW3-BAJ Courtesy J.R Bidlot ECWAM-HINDCAST Similar large patterns but some details are different Hs too high in Eastern Pacific

Wave Periods, Tp or Tz Tp too high in North-Eastern Pacific Tp ok in the Atlantic Similar for MF WAM

TEST332 TEST350 Hs bias Model-Alt TEST332 BAJ WW3-BAJ Significant improvment for Lat > 30°S Too high below (similar to ECWAM)

WW3_BAJ TEST350 Wave Periods, Tp or Tz Tp improved in North-Eastern Pacific

MFWAM WAM_MA Bias reduction in the in Eastern Pacific, but large negative bias appears in the tropics Hs bias Model-Alt

Tp improved in North-Eastern Pacific, with Hs But Tp and Tz too high elsewhere Wave Periods, Tp or Tz

Hs RMSE Model-Alt BAJ WAM-ES WW3-BAJ MFWAM Bias effect found in some places (East pacific) West part of basins better than East part, related to youger waves?)

WW3_BAJ TEST350 Hs RMSE Model-Alt Improvments in NRMSE almost everywhere, including in the West part of basins (also in the Med)  faster growth

WAM_MA MFWAM Hs RMSE Model-Alt Improvments in NRMSE dominated by large regional bias

Positive Bias for US West Coat, negative Bias for US East Coherent with altimeters

WW3_BAJ SWH

TEST350 SWH

Bias for US West and East Coast reduced

Global Performances according to altimeter data WW3_350 better than WW3_BAJ for low and moderate sea-sate (Hs < 8m, in term of bias But WW3_BAJ and WAM_MA better than WW3_350 for high sea-states (Hs > 8m), in term of bias (WW3_350 tis too low for large HS) WAM_MA only good above 10 m in term of NRMSE

ECMWF OPER ECMWF HINDCAST MFWAMWW3 BAJ WAM MA TEST 350 Tp (%) Tz (%) Hs buoys (%) Altimers (%) NRMSE (%), BUOYS LIST1 (38)

CONCLUSIONS  Altimers data essential to fill the gaps (buoys)  WW3_BAJ, ECWAM and MFWAM broadly similar but some details are different: to be investigated  BAJ « better » than MA (but MA tuned for the North Sea)  Best results for TEST350, with significant improvments for Hs and Tp (Tz), although Tp still too large in the Pacific Ocean. BAJ better for large Hs (Hs>12)  Makin + Ardhuin dissipation  Further tuning- impact of numerics?  Impact on the coast? Brittany, March 10