Some recent improvements of the assimilation of upgraded ASAR L2 wave spectra

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Svend Villadsen
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1 Some recent improvements of the assimilation of upgraded ASAR L2 wave spectra L. Aouf 1,2, J-M. Lefèvre 1, B. Chapron 3 and D.Hauser, 4 (1) Météo-France, Toulouse, France (3) IFREMER, Brest, France (4) CETP/CNRS/UVSQ, Vélizy, France SEASAR 08, ESA Workshop, Frascati, January 21-25, 2008

2 Outline Motivation Brief description of the system Validation of 1-year assimilation of ASAR L2 wave spectra Improvements of the assimilation scheme Specific case of study (La Réunion) Conclusion and future works

3 Motivation Envisat wave spectral information by ASAR is very useful to describe swell sea state and its prediction optimisation of the assimilation scheme Validation and assessment of the long term assimilation ASAR L2 (1-year) preparation of the assimilation system to operational use

4 The ASAR level 2 wave mode provides: Geophysical parameters Partial Wave spectrum Wind speed estimate +many others parameters related to the signal Courtesy of Boost Tech.

5 1-year aquisition of ASAR L2 wave mode products Sep Oct Nov Dec Jan Fev Mar Apr MayJun Jul Aug Sep Oct Nov Dec Jan Fev Mar Apr May Jun Jul Aug Rejected data in % Quality control for ASAR L2 ratio of signal to noise (3<r <30) normalized variance of image (1-1.6) wind speed (2-17 m/s)

6 How the assimilation of ASAR works? Selecting model first guess at grid point mi s2 Selecting ASAR observation affecting mi depending on distance of influence s1 mi d1 d3 d2 s3 Partitioning of first guess and ASAR wave spectra and computing mean paramaters of every partition Indexing partitions with different wave systems (swell, wind sea and mixed sea) and cross-assignment between partitions from the same wave system d1, d2 and d3<dmax Performing the OI procedure

7 Model and Assimilation set-up Test runs set-up - Wave model WAM-C4 (global 1x1 in lat-lon), wave spectrum in 25 frequencies ( Hz) and 36 directions - 6-hourly analysed ECMWF wind fields - Assimilation every 3-hour (3h time-window) - Correlation length 250 km and distance of influence 600 km - Assimilation period from 1 Sep 2005 to 1 September 2006

8 Validation of 1-year assimilation of ASAR L2 Runs description Assimilation of upgraded ASAR wave spectra over a 1 year with a wavelenght cut-off of 240 m corresponding to about 12.5s Reference run without assimilation Impact study comparison with the control run without assimilation validation with Jason-1 and GFO-NOAA altimeters, and NDBC buoys data (significant wave height and mean wave period)

GFO Using Jason-1 and GFO altimeters data as independent verification data set at crossovers points Jason-1 60 50 40 30 20 10 0 Nb crossovers: 10008 In centimeters Nb

9 GFO Using Jason-1 and GFO altimeters data as independent verification data set at crossovers points Jason Nb crossovers: In centimeters Nb crossovers: BIAS RMSE BIAS RMSE Reduction o RMSE of 9.5% ASSI-F8 NOASSI Reduction o RMSE of 7.4% 1-year of assimilation of ASAR

10 Impact at altimeters locations for specific ocean areas Upgraded ASAR wave spectra (cut=240 m) Assimilation index (%) for Significant wave height reduction of RMS errors ,5 1,7 Jason-1 10,5 3,8 GFO Intermediate latitude 20< Φ <50 recorded data: for Jason-1 and for GFO Intertropical region: Φ <20 recorded data: for Jason-1 and for GFO Intermed Tropics 1-year of assimilation of ASAR

11 comparison with NDBC Buoys significant wave height At buoys locations Recorded buoys data: , BIAS RMSE SI (%) ASSI-F8 NOASSI Significant reduction of RMS error (AI) by 9.4 % 1-year of assimilation of ASAR

12 comparison with NDBC Buoys mean period At buoys locations Recorded buoys data: ,5 1 0,5 0 0,46 0,29 BIAS ASSI-F8 1,33 1,43 RMSE NOASSI Significant reduction of RMS error (AI) by 7.1 % 17, ,8 16,6 16,4 16, ,8 16,3 17,1 SI (%)

13 Impact in August 2006 : comparison with NDBC Buoys WAMC4 WAMC4-ASAR-F8 Significant reduction of RMSE 22.8%

14 Example of impact in August 2006 On August 5, 2006 at 0:00 (UTC)

15 Use of different correlation for the mean parameters One of the key parameters in the assimilation scheme is the ratio between the first guess and the observation error. The weights used in the OI must be computed differently depending on the assimilated wave parameters The new correlation considers different values of the ratio related to the wave energy and the wave numer components (Aouf et al.2006) use of high correlation length in the tropics Assimilation run for 1-month period (May 2006)

16 Impact on mean wave period (1-month test) Old correlation New correlation On may 23, 2006 at 0:00

17 Improvement of the OI (Analysis period) Assimilation index (%) for Significant wave height ,4 8,7 GFO-NOAA 5,3 5,2 Jason-1 recorded data: for Jason-1 and for GFO NEW COR OLD COR Test for 1-month of assimilation of ASAR in May 2006

18 Improvement of the OI (Analysis period) for the tropical region Assimilation index (%) for Significant wave height ,2 16,6 Jason-1 16,7 14 GFO recorded data: for Jason-1 and for GFO New cor Old cor Test for 1-month of assimilation of ASAR L2 in May 2006

19 Impact of new correlation model in the forecast mode Validation with GFO-NOAA Validation with Jason-1 Test for 1-month of assimilation of ASAR L2 in May 2006

20 Case of the storm of La Réunion on May 12, 2007 H12 wave height For waves > 12 sec ASAR Peak wavelength

21 Impact of ASAR L2 in the Case of storm on May 12, 2007 (6.3m SWH measured at La Réunion) assimilation run performed from May 11, 2007 at 0:00 (UTC) until May 13, 2007 at 0:00 (UTC) Step of 3h From may at 18:00 (UTC) Swell wave height H12

22 Comparison at Envisat Ra2 orbit tracks Storm of May 2007 Reduction of RMS error of 13.7% Recorded data 559

23 More impact for ASAR L2 wave spectra At time T (2006 May 1, 0:00 UTC) Increasing the assimilation window and keeping the same assimilation time step At time t+3h At time T+6h

24 Concluding remarks and future works The new correlation has increased the impact of ASAR L2 data in the wave analysis and forecast. Positive impact for both significant wave height and mean period after 1-year assimilation (Jason-1, GFO and NDBC buoys) Use of ASAR L2 wave spectra improves high swells prediction : case of La Réunion Increasing the time window ASAR wave spectra for more than 6 hours in the assimilation enhances slightly the impact on SWH in the forecast. We are currently implementing the new system for the operational use (ECWAM+the assimilation of ASAR L2+altimeters)

25 Impact in the forecast mode with different ratio error On June 2, 2006 at 0:00 (UTC)

26 Impact on Significant wave height assimilation time window T+3h+6h 7,6 7,4 7,2 6,8 7 6,6 6,4 6,2 7,4 1 T+3h+6h 6,6 T Reduction of RMS error in %

27 Comparison at Envisat Ra2 orbit tracks Storm of May 2007 Recorded data 4329

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