Validation of altimetry data near coast in the German Bight

L. Fenoglio-Marc1, R. Weiβ2, M. Becker1

1) Institut fur Physikalische Geodasie, TU Darmstadt, Germany; 2) Bundesanstalt fur Gewasserkunde - BfG, Koblenz, Germany

Introduction

Altimetry data near coast are validated in the German Bight using a network of tide gaugeand GNSS stations maintained by the German Federal Institute of Hydrology (BfG), theFederal Agency for Cartography and Geodesy (BKG) and the Waterway and ShippingAdministration (WSV).

Data and Methodology

Waterlevels–More than 160 tide gauges (TG) are maintained by WSV, about 100 located on tidalrivers (e.g. Elbe, Weser and Ems) [4]. 1-minute data are available since 2000.

– 15-minute model data from a regional operational North Sea level model [1] of theFederal Maritime and Hydrographic Agency (BSH)

GNSS at tide gauges– 19 GNSS (permanent network of BfG) are directly installed at tide gauges. Thebenchmarks and GNSS-antennas are mounted on the same physical structure, i.e.the relative position of benchmarks and GNSS marker may be regarded as constant.

– 3 GNSS sites (BKG) are near tide gauges. The relative position of GNSS antenne andtide gauge datum is monitored by BfG and WSV, the sites are part of EUREF Per-manent GNSS Network (EPN) and of German Geodetic Reference Network (GREF).

Altimetry–Along-track altimetry data extracted from RADS Database [3], see Fig.1 for dataavailability (checked are the harware and dH status, and ocean/non-ocean flags).

–TOPEX (TX), Jason-1 (J1), Jason-2 (J2), pass: 137, 213 (ASC) and 18, 94 (DSC)

–ERS-2 (E2) and Envisat (N1), pass: 85 (ASC) and 474 (DSC)

Methodology– Five tide gauge stations are selected: Helgoland (HELG), Sylt (HOE2), LighthouseAlte Weser (LHAW), Frontlight Dwargstat (FLDW) and Busum (TGBU).

–The gauge datum heights were derived from GNSS measurements (2008-2010). Theprocessing is based on approximately 30 GNSS-sites (IGS and EPN sites) and 10 IGSFrame sites as fiducial stations. Results are computed in IGS05 and transformed intothe ITRF2005. The GNSS time series are too short to derive trends.

"

""

"

"!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!!

!

FLDW

LHAW

TGBUHELG

HOER

9°0'0"E8°0'0"E7°0'0"E6°0'0"E

55°0'0"N

54°0'0"N

0 5025Kilometer

213

Altimetry% of data availability

! 5 - 10

! 11 - 20

! 21 - 30

! 31 - 40

! 41 - 50

! 51 - 60

! 61 - 70

! 71 - 80

! 81 - 90

! 91 - 100

137

94

18

85

474

Fig. 1 Data availability from RADS

"

"

"

"

"

!!

!

!

!

FLDW

LHAW

TGBUHELG

HOER

8

69

71

84

5

9°0'0"E8°0'0"E7°0'0"E

55°0'0"N

54°0'0"N

0 5025Kilometer

85

! Points

" GNSS@TG

EU-A

US-B

US-A

213

94

18

474

48km

60km

16km

41km

Fig. 2 Selected altimetry points and tide gauges

Conclusions

•Approach allows absolute comparison of SSHell from GNSS-TG stations and altimetry

• Instantaneous SSH from TG and altimetry near island and continent (8 Km) show goodagreement (corr 0.9 and std 6-7 cm), data gaps near continental coast for Envisat

•Monthly SSHs from TG and altimetry near island (8 Km) agree, with J1-J2a (corr=0.6-0.7, std 11-17 cm), and N1 (corr=0.5, std 20 cm)

• Instantaneous altimetric and modelled SSHs agree with corr 0.8, std 16 cm in theGerman Bight, the agreement is higher in open sea (std 12-16 cm)

Instantaneous comparison of altimetry and tide gauge data

Ocean- and load-tide, inverse-barometer corrections are not applied to the altimetric sea surfaceheights (SSH). Environmental flags are not checked, wet-tropo ECMWF model correction is usedto retain coastal data. The ionophere correction (IRI2007) is adopted for consistency. Minute-tidegauge data are uncorrected. Interval considered is 2000-2010.

Helgoland, a open sea island at about 50 Km from the continent (Fig. 2)

The time-series of the nearest altimeter points (8-10 km for E2-N1 and J1-J2-TX) are completeand agree with the tide gauge with correlation (corr) 0.9 and standard deviation (std) 6-7 cm.Mean difference of SSHell (1-5 cm) is smaller that mean difference of SSHEGM08 (6-9 cm),due to geoid slope.

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 20113700

3800

3900

4000

4100

4200

Year

[cm

]

HELG TXA J1A J2A

Fig. 3 Istantaneous SSHell at altimetric point 8 (TX, J1 andJ2) and at tide gauge Helgoland

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 20113700

3800

3900

4000

4100

4200

Year

[cm

]

HELG E2A N1A

Fig. 4 Instantaneous SSHell at altimetric point 84 (E2, N1)and at tide gauge Helgoland

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011-20

0

20

40

60

Year

[cm

]

TXA J1A J2A

Fig. 5 Differences between instantaneous SSHell at altimetricpoint 8 and at tide gauge Helgoland (TG-ALT)

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011-20

0

20

40

60

Year

[cm

]

E2A N1A

Fig. 6 Differences between instantaneous SSHell at altimetricpoint 84 and at tide gauge Helgoland (TG-ALT)

Mission Correlation Distance Difference SSHell Difference SSHEGM08 number ofmean value std mean value std observations

[km] [cm] [cm] [cm] [cm] obs./max.TX-A 0.9791 8.44 10.22 22.84 14.42 22.57 90/96J1-A 0.9980 8.48 3.92 6.85 8.24 6.94 241/261J2-A 0.9983 8.72 2.42 6.24 7.23 5.92 82/85E2-A 0.9964 9.24 4.49 7.21 8.2 7.09 30/30N1-A 0.9983 10.17 0.96 7.36 5.7 7.36 76/76

Tab 1: Tide gauge and altimeter at Helgoland with altimeter points 8 (TX-J1-J2) and 84 (E2-N1).

LHAW and FLDW, two Light Houses at about 20-10 Km from coast (Fig. 2).

The time-series for the nearest N1 altimeter point (8-10 km) have gaps (50% of data in LHAWand 20% in FLDW). The agreement between altimeter and the tide gauge is good (corr=0.9,std 6-10 cm). The mean difference of the instantaneous SSHell (2-8 cm) is, like in HELG,smaller than the mean difference of the instantaneous SSHEGM08 (3-10 cm).

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 20113700

3800

3900

4000

4100

4200

Year

[cm

]

LHAW E2A N1A

Fig. 7 Instantaneous SSHell at altimetric point 71 (E2, N1)and at tide gauge Alte Weser

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 20113800

3900

4000

4100

4200

4300

Year

[cm

]

FLDW E2A N1A

Fig. 8 Instantaneous SSHell at altimetric point 69 (E2, N1)and at tide gauge Dwarsgat

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011-20

0

20

40

60

Year

[cm

]

E2A N1A

Fig. 9 Differences between instantaneous SSHell at altimetricpoint 71 (E2, N1) and at tide gauge Alte Weser (TG-ALT)

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011-20

0

20

40

60

Year[c

m]

E2A N1A

Fig. 10 Differences between instantaneous SSHell at altimetricpoint 69 (E2, N1) and at tide gauge Dwarsgat (TG-ALT)

Point/TG Mission Correlation Distance Difference SSHell Difference SSHEGM08 Number ofmean value std mean value std observations

[km] [cm] [cm] [cm] [cm] obs./max.71/LHAW E2-A 0.9753 8.93 10.93 20.94 8.71 20.40 28/2971/LHAW N1-A 0.9977 7.70 7.28 9.57 2.98 7.10 52/9469/FLDW E2-A 0.9426 4.15 6.95 22.32 9.61 21.90 12/2969/FLDW N1-A 0.9952 3.84 2.55 6.69 5.84 5.50 13/94

Tab 2: Tide gauge and altimeter at LHAW with altimeter points 71 and 69 (E2-N1).

References[1] S. Dick, E. Kleine, S. Mueller-Navarra, H. Klein, and H. Komo. The Operational Circulation Model of BSH (BSHcmod). Model description and validation. Institute of Ocean Sciences,

Berichte des BSH, Nr.29/2001, 2001).

[2] L. Fenoglio-Marc, C. Braitenberg, and L. Tunini. Sea level variability and trends in the Adriatic Sea in 1993-2008 from tide gauges and satellite altimetry. Physics and Chemistry of theEarth, 2011.

[3] M. Naeije, R. Scharroo, R. Doornbos, and E. Schrama. Global altimetry sea level service. report GO 52320, NIVR/DEOS, The Netherlands, The Netherlands, 2008.

[4] A. von Gyldenfeldt, Christoph J. Blasi, A. Sudau, and G. Liebsch. National Report of Germany. GLOSS-Report, pages 1–5, April 2009.

Monthly comparisons of altimetry and tide gauge data

Differently than in istantaneous comparison, ocean-, load-tide and inverse barometer cor-rections have been applied to altimetry. Monthly tide gauge measurements are conform tothe monthly averaged mean tide water (MMTW) derived from minute data. The intervalconsidered is 1992-2010.

Helgoland (Fig.1)

The correlation between MMTW and the monthly SSHs is lower then for instantaneousdata. The SSHs derived from TX-J1-J2 have higher correlation with TG (corr=0.6-0.7)then the SSHs derived from E2-N1 (corr=0.4-0.5). The std are larger than in [2].

1992 1994 1996 1998 2000 2002 2004 2006 2008 20103850

3875

3900

3925

3950

3975

4000

Year

[cm

]

HELG TXA J1A J2A

Fig. 11 Monthly SSHell at altimetric point 8 (TX, J1 andJ2) and MTMW at tide gauge Helgoland

1992 1994 1996 1998 2000 2002 2004 2006 2008 20103850

3875

3900

3925

3950

3975

4000

Year

[cm

]

HELG E2A N1A

Fig. 12 Monthly SSHell at altimetric point 84 (E2, N1)and MTMW at tide gauge Helgoland

1992 1994 1996 1998 2000 2002 2004 2006 2008 2010-60-40-20

0204060

Year

[cm

]

TXA J1A J2A

Fig. 13 Differences between instantaneous SSHell ataltimetric point 8 and MTMW at tide gauge Helgoland

1992 1994 1996 1998 2000 2002 2004 2006 2008 2010-60-40-20

0204060

Year

[cm

]

TXA J1A J2A

Fig. 14 Differences between instantaneous SSHell ataltimetric point 84 and MTMW at tide gauge Helgoland

Mission Correlation Distance Difference SSHell Difference SSHEGM08 Number ofmean value std mean value std observations

[km] [cm] [cm] [cm] [cm] obs./max.TX-A 0.6432 8.36 2.86 16.57 7.15 16.42 319/365J1-A 0.7039 8.28 0.86 11.89 5.11 12.03 246/261J2-A 0.6616 8.61 2.12 13.00 6.88 12.70 82/85E2-A 0.5394 10.16 -14.78 20.70 -9.93 20.57 154/160N1-A 0.4838 10.12 -13.69 19.68 -8.93 19.84 78/94

Tab 3: Tide gauge and altimeter at Helgoland with altimeter points 8 (TX-J1-J2) and 84 (E2-N1).

One altimeter location and four different tide gauges (Fig. 2)

Correlations and std are similar (0.7, 14 cm) for all stations. The SSHell differences(>100cm) are due to the geoid slope and to the distance between locations.

1992 1994 1996 1998 2000 2002 2004 2006 2008 20103850

3900

3950

4000

4050

Year

[cm

]

HELG HOER TGBU LHAW TXA/J1A/J2A

Fig. 15 Ellipsoidial sea surface heigths at different tidegauge sites

1992 1994 1996 1998 2000 2002 2004 2006 2008 2010-60

0

60

120

180

Year

[cm

]

HELG - ALT HOER - ALT TGBU - ALT LHAW - ALT

Fig. 16 Ellipsoidial differences between monthly altimetryand different tide gauge sites

Mission Correlation Distance Difference SSHell Difference SSHEGM08 Geoid Number ofmean value std mean value std TG-ALT observations

[km] [cm] [cm] [cm] [cm] [cm] obs./max.HELG 0.6878 16.24 1.32 14.37 5.01 14.34 -3.62 610/669HOER 0.7024 60.47 104.55 14.59 12.42 14.56 92.20 610/669TGBU 0.7025 47.74 53.60 14.30 12.86 14.28 40.80 610/669LHAW 0.7038 40.90 36.71 14.00 3.35 13.97 33.42 610/669

Tab 4: Tide gauge and altimeter at Helgoland, Sylt, Buesum and Alte Weser with altimeter points 5 (TX-J1-J2).

Modelled sea level and instantaneous altimetry

Simulated SSHs are routinely compared to tide gauges, which are assimilated in themodel. Instantaneous altimeter and model data are in agreement in the German Bight(corr 0.9, std 16) and in the North Sea (corr 0.7-0.9, std 12-18 cm) (Figs. 17, 18).

!2˚

!2˚

0˚

0˚

2˚

2˚

4˚

4˚

6˚

6˚

8˚

8˚

10˚

10˚

12˚

12˚

52˚ 52˚

54˚ 54˚

56˚ 56˚

58˚ 58˚

0.0

0.2

0.4

0.6

0.8

1.0

!2˚

!2˚

0˚

0˚

2˚

2˚

4˚

4˚

6˚

6˚

8˚

8˚

10˚

10˚

12˚

12˚

52˚ 52˚

54˚ 54˚

56˚ 56˚

58˚ 58˚

!2˚

!2˚

0˚

0˚

2˚

2˚

4˚

4˚

6˚

6˚

8˚

8˚

10˚

10˚

12˚

12˚

52˚ 52˚

54˚ 54˚

56˚ 56˚

58˚ 58˚

!2˚

!2˚

0˚

0˚

2˚

2˚

4˚

4˚

6˚

6˚

8˚

8˚

10˚

10˚

12˚

12˚

52˚ 52˚

54˚ 54˚

56˚ 56˚

58˚ 58˚

!2˚

!2˚

0˚

0˚

2˚

2˚

4˚

4˚

6˚

6˚

8˚

8˚

10˚

10˚

12˚

12˚

52˚ 52˚

54˚ 54˚

56˚ 56˚

58˚ 58˚

!2˚

!2˚

0˚

0˚

2˚

2˚

4˚

4˚

6˚

6˚

8˚

8˚

10˚

10˚

12˚

12˚

52˚ 52˚

54˚ 54˚

56˚ 56˚

58˚ 58˚

!2˚

!2˚

0˚

0˚

2˚

2˚

4˚

4˚

6˚

6˚

8˚

8˚

10˚

10˚

12˚

12˚

52˚ 52˚

54˚ 54˚

56˚ 56˚

58˚ 58˚

FINO3

FINO1HELG

TGBF

HOE2

LFDW

LHAW137

18

94

213

Fig. 17 Correlation of modelled and altimeter instantaneousSSHell at altimeter crossovers

!2˚

!2˚

0˚

0˚

2˚

2˚

4˚

4˚

6˚

6˚

8˚

8˚

10˚

10˚

12˚

12˚

52˚ 52˚

54˚ 54˚

56˚ 56˚

58˚ 58˚

8

10

12

14

16

18

20

!2˚

!2˚

0˚

0˚

2˚

2˚

4˚

4˚

6˚

6˚

8˚

8˚

10˚

10˚

12˚

12˚

52˚ 52˚

54˚ 54˚

56˚ 56˚

58˚ 58˚

!2˚

!2˚

0˚

0˚

2˚

2˚

4˚

4˚

6˚

6˚

8˚

8˚

10˚

10˚

12˚

12˚

52˚ 52˚

54˚ 54˚

56˚ 56˚

58˚ 58˚

!2˚

!2˚

0˚

0˚

2˚

2˚

4˚

4˚

6˚

6˚

8˚

8˚

10˚

10˚

12˚

12˚

52˚ 52˚

54˚ 54˚

56˚ 56˚

58˚ 58˚

!2˚

!2˚

0˚

0˚

2˚

2˚

4˚

4˚

6˚

6˚

8˚

8˚

10˚

10˚

12˚

12˚

52˚ 52˚

54˚ 54˚

56˚ 56˚

58˚ 58˚

!2˚

!2˚

0˚

0˚

2˚

2˚

4˚

4˚

6˚

6˚

8˚

8˚

10˚

10˚

12˚

12˚

52˚ 52˚

54˚ 54˚

56˚ 56˚

58˚ 58˚

!2˚

!2˚

0˚

0˚

2˚

2˚

4˚

4˚

6˚

6˚

8˚

8˚

10˚

10˚

12˚

12˚

52˚ 52˚

54˚ 54˚

56˚ 56˚

58˚ 58˚

FINO3

FINO1HELG

TGBF

HOE2

LFDW

LHAW137

18

94

213

Fig. 18 Standard deviation (cm) of modelled and altimeterinstantaneous SSHell at altimeter crossovers

Acknowledgements

We acknowledge ESA and JPL/CNES, AVISO and the RADS database for the altimeter data, WSV for tide gauge data, BfG and BKG forGNSS-Data and BSH for model data. The study is part of the Project COSELE founded by the Deutsche Forschungsgemeinschaft (DFG)5th Coastal Altimety Workshop 16 - 18 October San Diego U.S.A.

JP03 Global and regional sea-level change