WP2400: Sea State Bias

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1 Sea Level CCI Selection Meeting WP2400: Sea State Bias Ngan Tran, Jean-François Legeais (CLS)

WP2400: SSB Approach developed in collaboration with D. Vandemark (UNH) and B. Chapron (IFREMER).

Initially for Jason-1 within CNES and SLOOP projects, then applied to Envisat data. Tm : mean wave period from a numerical wave model (WAVEWATCH-III, v3.

SWH WW-III predictions provided on a global 1 x1 grid at a 6 hourly time step, and forced with synoptic winds from ECMWF.

2 WP2400: SSB Approach developed in collaboration with D. Vandemark (UNH) and B. Chapron (IFREMER). Development of 3D SSB (SWH, Ualt, Tm) to better model SSB behavior with improved description of the sea state. Initially for Jason-1 within CNES and SLOOP projects, then applied to Envisat data. Tm : mean wave period from a numerical wave model (WAVEWATCH-III, v3.14b released in 2009, [Tolman, 2002] used operationally at NOAA/NCEP). SWH WW-III predictions provided on a global 1 x1 grid at a 6 hourly time step, and forced with synoptic winds from ECMWF. Runs performed at the UNH ( available). 3D SSB model computed with direct method based on SLA [Vandemark et al, 2002] from 2-year dataset ( for Jason-1 and for Envisat). Ualt method Envisat Jason-1 2D SSB crossover DSSH ( 10-day) collinear DSSH (10-day) 3D SSB direct SLA direct SLA Tm Sea Level CCI Selection Meeting 02 th - 04 th May 2012

3 WP2400: SSB What has been done? In the frame of studies outside to CCI project Development of Envisat operational 2D SSB within the Phase-E ESL activities [Labroue, 2005; 2007]. The 2005 version has been implemented to generate products since cycle 041. The 2007 model has been used for the reprocessing exercise and for production since cycle 086. Differences between v2005 and v2007: consideration of a new IF mask, USO drift correction, orbit error term. Development of Envisat 3D SSB within the Phase-E ESL activities [Tran, 2010]. Development of Jason-1 3D SSB within the SALP activities [Tran, 2011]. In the frame of the CCI project Comparison of Envisat 3D SSB version with 2D ones based on same diagnoses than for other altimetric corrections Production of RRDP reports

4 WP2400: SSB The following RRDP have been performed : Ref. correction Studied correction Missions 1 2D SSB (v2005) 3D SSB Envisat 2 2D SSB (v2007) 3D SSB Envisat RRDP are on line on the SLCCI ftp: 1. RRDP_WP2400_SSB_3D_vs_2D2005_ pdf 2. RRDP_WP2400_SSB_3D_vs_2D2007_ pdf Validation report is available : ftp://ftp.esa-sealevelcci.org/data/validationreports/slcci-validationreport_wp2400_ssb.docx

WP2400: SSB Climate Applications Envisat [October 2002- December 2009] Temporal Scales Round Robin Data Package (RRDP) 3D vs 2D_2005 3D vs 2D_2007 Global Mean Sea Level Regional Mean Sea Level Inter

5 WP2400: SSB Climate Applications Envisat [October December 2009] Temporal Scales Round Robin Data Package (RRDP) 3D vs 2D_2005 3D vs 2D_2007 Global Mean Sea Level Regional Mean Sea Level Inter annual signals (> 1 year) Annual and semiannual Signals + Annual and semiannual Signals Overview results for Envisat : Impact of the 3D SSB is significant on global and regional mean sea level trends and for mesoscale applications when compared with results from 2D versions. Mesoscale Signals < 2 months + + Specific regional area : Coastal areas + + Signals < 2 months

6 WP2400: SSB Climate Applications Envisat [October December 2009] Temporal Scales Round Robin Data Package (RRDP) 3D vs 2D_2005 3D vs 2D_2007 Impact on Envisat Global MSL trend is significant (-0.2 mm. yr -1 / 2D versions) Global Mean Sea Level Inter annual signals (> 1 year) Annual and semiannual Signals + Altimetric mission D SSB D SSB 3D SSB Envisat 0.69 mm/yr 0.69 mm/yr 0.49 mm/yr Regional Mean Sea Level Annual and semiannual Signals Altimetric mission GDR_C 2D SSB 3D SSB Mesoscale Signals < 2 months + + Jason mm/yr 2.25 mm/yr Specific regional area : Coastal areas + + Signals < 2 months

7 WP2400: SSB SLA with SSB_3D : 2.25 mm/yr SLA with SSB_2D_dir : 2.53 mm/yr SLA with SSB_2D_dir : 2.56 mm/yr SLA with SSB_2D_col : 2.58 mm/yr changes mainly due to use of Tm and not due to differences in model development

WP2400: SSB Climate Applications Envisat [October 2002- December 2009] Temporal Scales Round Robin

Inter annual signals (> 1 year) Annual and semiannual Signals + Regional Mean Sea Level Annual and

8 WP2400: SSB Climate Applications Envisat [October December 2009] Temporal Scales Round Robin Data Package (RRDP) 3D vs 2D_2005 3D vs 2D_2007 Impact on regional MSL trends Global Mean Sea Level Inter annual signals (> 1 year) Annual and semiannual Signals + Regional Mean Sea Level Annual and semiannual Signals Mesoscale Signals < 2 months + + Specific regional area : Coastal areas + + Signals < 2 months

9 WP2400: SSB changes mainly due to use of Tm and not due to differences in model development

WP2400: SSB Strong impact of the 3D SSB on mesoscale signals:

Applications Envisat [October 2002- December 2009] Temporal

Annual and semiannual Signals + -2 Regional Mean Sea Level

10 WP2400: SSB Strong impact of the 3D SSB on mesoscale signals: reduction of the SLA variance (referenced to the MSS) Climate Applications Envisat [October December 2009] Temporal Scales Round Robin Data Package (RRDP) 3D vs 2D_2005 3D vs 2D_ cm² Global Mean Sea Level Inter annual signals (> 1 year) Annual and semiannual Signals + -2 Regional Mean Sea Level Annual and semiannual Signals cm² Mesoscale Signals < 2 months + + Specific regional area : Coastal areas + + Signals < 2 months -2

11 WP2400: SSB Similar features for Envisat and Jason-1. Mostly same regions of high oceanic variability are impacted (reduction in blue). Some issues in high latitudes for Envisat to investigate.

WP2400: SSB Climate Applications Envisat [October 2002- December 2009] Temporal Scales Round Robin Data Package (RRDP) 3D vs 2D_2005 3D vs 2D_2007-0.

12 WP2400: SSB Climate Applications Envisat [October December 2009] Temporal Scales Round Robin Data Package (RRDP) 3D vs 2D_2005 3D vs 2D_ cm Impact on the coastal MSL trend Global Mean Sea Level Regional Mean Sea Level Inter annual signals (> 1 year) Annual and semiannual Signals + Annual and semiannual Signals -1.4 cm 0 km Coastal Distance 100 km Mesoscale Signals < 2 months + + Specific regional area : Coastal areas + + Signals < 2 months 3 mm 7 mm 5 mm Sea Level CCI Selection Meeting 02 th - 04 th May km 100 km

13 Sea State Bias RECOMMENDATIONS

Sea State Bias As expected, these analyses have confirmed that the 2007 2D version of the non parametric SSB correction is better than the currently used 2005 version (results not shown).

The spatial distribution of the SLA variance differences between the 3D SSB and the currently used 2005 2D SSB correction shows a strong variance reduction (>5 cm 2 ) located in regions of high ocean

14 Sea State Bias As expected, these analyses have confirmed that the D version of the non parametric SSB correction is better than the currently used 2005 version (results not shown). Concerning the new 3D SSB correction, the impact is strong for all the climate applications with changes close to 0.2 mm/yr on the global MSL and larger than 1 mm/yr regionally. The spatial distribution of the SLA variance differences between the 3D SSB and the currently used D SSB correction shows a strong variance reduction (>5 cm 2 ) located in regions of high ocean variability. Currently none diagnoses in RRDP are able to determine if this SLA variance reduction is an improvement in SSH computation or if it is an artifact for instance due to the changes in covariance terms between the SLA and SSB corrections. Recommendation: Although the use of the 3D SSB correction in the MSL computation provides interesting changes concerning some climate applications, we suggest to analyze the impact of this algorithm further before recommending its use for climate studies. In the meanwhile, for Envisat we recommend to use the 2007 version of the 2D SSB correction as a cautious position if the CCI SSH computation remains on the same standards as used for this validation.

15 Sea State Bias OPEN ISSUES

16 Sea State Bias Issue 1 : To reprocess Envisat 2D SSB after the CCI algorithm selection? Empirical correction, last to be determined to be efficient because it depends on the choice made for the other standards used to compute SSH. Changes on SWH and wind speed will also have an impact. SSB_2D_2007 vs SSB_2D_2005 CCI dataset (GDR-A-B-C Orbits products + CLS updates with GDR-C Orbits and V2.1 standards except: new SWH (new σp), new Radiometer processing, new L1b PTR) Same SSB_2D_2007 model Reprocessed dataset (different swh, so different SSB estimates and not same SSH computation)

17 Sea State Bias Issue 2 : What SSB should we use for ERS missions? status of REAPER SSB models If CCI selection of standards for ERS SSH computation is different from REAPER one, new SSB solutions should be computed

18 Sea State Bias Issue 3 : Impact of instrumental drift on SSB and therefore on global MSL at the end of TOPEX-A period From 1996 to 1999, drifts on SWH and Sigma-0 are observed :+30 cm for SWH and 0.6 db for Sigma-0 Due to theses anomalies, the SSB variations changed the global MSL evolution during this period: this impacts the observation of inter-annual signal on this period (Niño 98) Reprocessing of TOPEX data is necessary to correct such features and to reduce its effects

19 Sea State Bias Issue 4: How to evaluate the quality of SSB solutions for climate applications? Definition of new diagnoses, which ones? Issue 5: Constant bias between SSB models is it a real issue? Models are determined to within a constant due to the constraint SSB (swh=0, u=0) = 0 due to lack of knowledge on absolute calibration of SSB values Change of this constraint? Choice of a reference mission, a (swh, u) pair and a fixed value for SSB To accept to have positive values of SSB sometimes

20 Sea State Bias Issue 6: source of Tm for 3D SSB

21 Sea State Bias Issue 7: Other issues YOU want to raise?

Sea state bias correction in coastal waters. D. Vandemark, S. LaBroue, R. Scharroo, V. Zlotnicki, H. Feng, N. Tran, B. Chapron, H.

Sea state bias correction in coastal waters D. Vandemark, S. LaBroue, R. Scharroo, V. Zlotnicki, H. Feng, N. Tran, B. Chapron, H. Tolman 5-7 Feb. 2008 Coastal Altimetry Workshop 1 Overview of group consensus