Data and methods
Wind speed and wave height measurements from ERS-1, ERS-2, and TOPEX are analyzed over the 1991 - 2000 time period. The data are the official products of the space agencies:
- TOPEX-POSEIDON Merged GDR distributed on CD-ROM by AVISO-CNES
- ESA Ocean PRoducts processed and distributed by CERSAT, for ERS altimeters
- ESA WNF products processed and distributed by CERSAT, for ERS scatterometers
Wind speed and SWH statistics are computed over successive 10-day time periods, for the global ocean limited by the 66° North - 66° South latitude range of the TOPEX orbit ground track. Data are selected according to the various data quality flags given in the products.
On average over 10 days the number of altimeter measurements (1 s along track samples) is about 420000 for ERS-1 and ERS-2 and about 490000 for TOPEX. Data are discarded when less than 300000. For ERS scatterometers the 10-day average number of cell measurements (50 km by 50 km) is about 1 700 000. Data are discarded when less than 1 450 000.
Significant wave height
Significant differences exist between the three altimeter SWH estimates. A mean bias of 0.6 m is observed between ERS-1 and TOPEX ( II blue). The level change occuring in March 1995 (A on the graph) corresponds to a processing software change (for ERS-2 compatibility).
The bias between ERS-2 and TOPEX is about 0.2 m in 1995 and 1996. It then increases, after February 1997 (B on the graph), up to 0.5 m on February 1999 (C). This has been clearly identified as an electronics drift of the TOPEX altimeter [1-2]. The TOPEX spare side-B of the altimeter was then switched on, on February 1999, with the consequence of a sharp 30 cm change in SWH estimate (C).
Time series of 10-day averages (I) and standard deviations (III) of SWH from ERS-1 (blue), ERS-2 (red) and TOPEX (black) altimeters. Differences between SWH averages are shown in panel II
Seasonal variations do not appear in the mean value time series, but are observed in III for the SWH standard deviation, with a maximum of variability during the southern hemisphere winter. SWH standard deviation is larger for ERS-2 than for ERS-1 (about 5 to 10 cm), and is also larger for TOPEX than for ERS.
TOPEX-POSEIDON and ERS-2 SWH were compared more precisely. Measurements of the two satellites were selected when within a one hour time window at ground track crossing points. To smooth the wave variability within this time window, the measurements were further averaged along track, 50 km each side of the crossing point. The mean value of the SWH differences between the altimeters was estimated over each TOPEX cycle (about 10 days). The method developed previously  is applied here taking into account the most recent data.
Mean values of SWH differences between altimeter co-locatedmeasurements, as a function of TOPEX cycle number
A constant bias about 0.20 m is observed on the difference between ERS-2 and POSEIDON (blue line).
The TOPEX SWH drift is obvious from the red stars, till February 1999. The standard deviation of differences over a cycle is about 10 - 15 cm. It can be also observed that there is some trend on the values of differences between Ku and C-band TOPEX SWH estimates (black squares), and also that the noise over successive cycles is increasing.
To correct the TOPEX SWH drift, an exponential curve was fitted to the data (continuous red line) and used to establish the correction relationship (1) as a function of the TOPEX cycle number. This correction was applied to TOPEX SWH for cycles 98 to 235, and a 7 cm mean bias was substracted to TOPEX SWH for cycle numbers larger than 235.
ERS-1 data was corrected using previous buoy comparison analysis , according to (2), for data before March 1995 (previous version of OPR ERS-1 processing), and according to (3) after March 1995 .ERS-2 was corrected using (4), from .
Time series of 10-day averages (top) of corrected SWH from ERS-1 (blue),ERS-2 (red) and TOPEX (black) altimeters.Differences between SWH averages (bottom)
The above corrections were applied to 1 second altimeter data, and 10-day statistics computed. Results are shown in the opposite figure. There are still some trends on differences, but the amplitudes are reduced to less than about 10 cm.
The 10-day mean wind speed fluctuates between 7 m/s and 8.5 m/s. On panel I the ERS-1 altimeter wind speed (blue) exhibits a large trend, almost 1 m/s, from mid-1991 to mid-1996. This is also shown on the difference (II) and furthermore, as for SWH, the change in OPR processing (for ERS-2 compatibility) induces a shift in the ERS-1 wind speed (A in panel II).
Concerning TOPEX, it seems that there is some increase of the difference relative to ERS-2, that could be associated to the electronics drift (between B and C), as for SWH. ERS-2 also increases suddently near the end of the time serie, about 1 m/s at the beginning of year 2000.
The standard deviation (III) is seasonally modulated, curiously less for ERS-1 (blue), between about 3.6 m/s and 4 m/s for ERS-2 and TOPEX. The maximum of variability is observed during the southern hemisphere winter.
Time series of 10-day averages (I) and standard deviations (III) of surface wind speed from ERS-1 (blue), ERS-2 (red) and TOPEX (black). Differences are shown in panel II.
The trend of ERS-1 wind speed is more precisely shown here. In the top panel 10 day mean values from ERS-1 and TOPEX altimeters are plotted together with the ERS-1 scatterometer mean wind speed. Linear regression lines are fitted to the altimeter data. No trend is observed on TOPEX, for this time period. The linear fit to ERS-1 data indicates an increase about 0.7 m/s over 5 years (mid 1991 to mid 1996). This is a high value that could affect climate studies. The bottom panel shows the ERS-1 sigma0 drift, about 0.21 dB over 5 years.
Comparison between altimeters & scatterometers
The ERS-1 altimeter wind speed drift is observed relative to the scatterometer (blue curves). The correlation between mean wind speed from ERS-2 altimeter and scatterometers is high (green and red), and differences are low, untill the significant change of the altimeter wind speed level in January 2000 - consequence of a drop in sigma0 on January 16, 2000, and of gyroscope changes in February and March 2000. Preliminary estimates to correct sigma0 are proposed in a recent note , but are not tested here.
10 day averaged wind speed from ERS scatterometers,ERS altimeters (top) and TOPEX altimeter (bottom).
In bottom panel, wind speeds from TOPEX altimeter (black) and ERS scatterometers (green and blue) are also well correlated, till end of 1996 when the TOPEX wind speed increases relative to the scatterometer, over the time period B-C, corresponding to the TOPEX electronics drift.
 G. S. Hayne and D. W.Hancock, "Observations from long-term performance monitoring of the TOPEX radar altimeter'', presented at the Joint TOPEX/POSEIDON and Jason-1 SWT meeting, 13-15 October 1998, Keystone, Colorado.
 Minutes of the Second Joint TOPEX POSEIDON and Jason-1 Science Working Team Meeting, October 25 - 27, St Raphael, edited by Y. Mesnard, TP-2JO-CR-1425-CNES, France 1999.
 P. Queffeulou, "Long-term comparison of ERS, TOPEX and POSEIDON altimeter wind and wave measurements'', Proc. of the ninth International Offshore and Polar Engineering Conference, ISOPE-99, May 30 - June 4, 1999, Brest, France, Vol. III,114-120, 1999.
 P. Queffeulou, A. Bentamy, Y. Quilfen and J. Tournadre, 1994, "Validation of ERS-1 and TOPEX POSEIDON altimeter wind and wave measurements'', Document de Travail DRO-OS 94-08, December 1994, IFREMER, BP 70, 29280 Plouzané, France.
 P. Queffeulou and A. Bentamy, "Cross validation of wind and waves from the altimeters of ERS-1,ERS-2 TOPEX and from ADEOS NSCAT'', proc. of the CEOS Wind and Wave Validation Workshop, 3-5 June 1997, pp 175-180, ESA WPP-147.
 J. Dorandeu, F. Mertz and J. Stum, "Note on ERS-2 sigma0 vatiations since January 2000", CLS/DOS/NT/00.286, CLS 8-10 rue Hermes - Parc Technologique du canal, 32526 Ramonville cedex, France, August 2000
Written by : Pierre Queffeulou "Département d'Océanographie Spatiale"/IFREMER
Copyright © Ifremer 2000. The ERS data is courtesy of the European Space Agency. The QuikSCAT data is courtesy of the Jet Propulsion Laboratory, NASA. The TOPEX data is courtesy of CLS/Argos.