TOPEX B Side Altimeter Wind Speed and Significant Wave Height Calibration (1999-2001)

This note describes the calibration of the TOPEX B altimeter significant wave height and ocean wind speed measurements against in situ buoy data.

TOPEX v3 Geophysical Data Records data were used, as supplied by AVISO.  The in-situ data were from US, Canadian and UK offshore buoys.  These data were available courtesy of the US National Data Buoy Centre (NDBC), Environment Canada (EC) and the UK Meteorological Office (UKMO).  Data from buoys operated by the latter two agencies were retrieved from the NDBC web site.

For this study we used significant wave height and wind speed data extracted directly from the 1 Hz TOPEX GDR (v3), covering the period March 1999 to July 2001.  Apart from quality checks (see below) no further processing of these records took place.

Hourly records of wind, wave and other surface meteorological data were retrieved from a selection of 39 open ocean data buoys.  These buoys included 24 US NDBC buoys (from the north western North Atlantic, north eastern and central North Pacific), 6 Canadian offshore buoys (also from the north western North Atlantic and north eastern North Pacific), and 9 UK Meteorological Office offshore buoys (from the north-eastern North Atlantic and North Sea).  More details are given in a calibration report for Geosat Follow On (http://www.satobsys.co.uk/Projects/CalVal)

Where appropriate the buoy winds were adjusted to 10m according to Dobson [1981].

Extraction details are given in the GFO report (see above)

Details of the orthogonal distance/principle component regression procedure used are given in the GFO report.  Results are given below.

Four sets of regressions were carried out, one comparing TOPEX B to the combined buoy data, and a further three comparing TOPEX B separately to each of the individual buoy data sets.  Results (Equations 1 and 2, and Tables 1 and 2) are given in terms of the indicated linear correction required for the TOPEX B altimeter data.

The orthogonal distance regression yielded:

95% conf. limits 1.0006 1.0229 0.0456 0.00580

R² = 0.9631, N=1200

Equation (1) and Figure 1 show that, when all buoy data are considered, the ODR line and line of perfect match (intercept of 0.0 and gradient of 1.0) are almost coincident.  However, on close inspection we can see that the TOPEX B altimeter very slightly underestimates Hs.  The calibration gradient is indeed significantly different from 1.0 (at the 95% level), albeit by only just over 1%. The intercept is not significantly different from 0.0.  These values indicate a remarkably good agreement between TOPEX B and buoy data.  This near perfect agreement together with a low residual root mean square of 0.2244 m, indicate that TOPEX B is providing a high quality significant wave height data set.

The co-located data range from < 0.5 to 7 m.

Figure 1. Co-located TOPEX B and buoy significant wave height data (03/99-07/01). NDBC (red), EC (blue) and UKMO (green).  The ODR fit is shown as a solid black line, the line of perfect match (gradient=1.0, intercept =0.0) as a dashed line.

Figure 2. Co-located Topex B and Buoy 10m wind speed data (03/99-07/01).  NDBC buoy data (red), Environment Canada buoy data (blue) and UK Met. Office Buoy data (green).  The ODR line is shown as a solid black line, the line of perfect match (i.e. gradient=1.0, intercept =0.0) is given by a dashed line.

The orthogonal distance regression yielded:

95% conf. limits 0.8443 0.8836  0.5611 0.8843

R² = 0.8516, N=1200

From the relative positions of the dashed line (perfect match between altimeter and buoy) and solid line (ODR line) on Figure 2 we can see that TOPEX B slightly overestimates low wind speeds and underestimates higher wind speeds (Ubuoy > 5.3 ms-1 ).  At 0.86, the gradient is significantly lower than 1.0 (at the 95% level), and the intercept is significantly higher than 0.0, at 0.72 ms-1.  The consequence is that for Ubuoy = 20 ms-1 this underestimate reaches 2 ms-1.  The residual root mean square of the fitted data is 1.2704 ms-1, which is less than that seen in analyses of data from other contemporary altimeters (GFO, ERS-2).  The range of wind speeds covered by the data is <0.5 ms^-1 to > 20 ms^-1.

The TOPEX B data were also regressed separately against the data supplied by the three different agencies.  The results of these regressions are summarised in Table 1 (significant wave height) and Table 2 (10m wind speed).

Table 1 shows that, when compared to NDBC data, the calibration gradient for TOPEX B Hs is higher (significantly different at the 95% level) than that given when TOPEX B is compared to the EC and UKMO data. The results from the UKMO and EC data sets are not significantly different from each other.  The NDBC intercept is also significantly lower from that of the UKMO data.

From Table 2 we see that the gradients from the regressions against the NDBC and EC buoy data sets are consistent with each other (i.e. within 95% confidence limits), as are those against the EC and UKMO data sets (but not UKMO and NDBC).  All regressions show gradients significantly lower than 1.0.

A comparison of TOPEX B data with co-located buoy data indicates that the TOPEX significant wave height and wind speed measurements are of high quality.

The ODR regressions indicate that the TOPEX B altimeter very slightly underestimates significant wave heights, by just over 1% (Equation 1).

The ODR regressions on wind speed data (Equation 2, Table 2) indicate a gradient significantly lower than 1.0 (0.86 when compared to the combined buoy data) and an intercept significantly higher than 0.0 (0.72 when compared to the combined buoy data).  The combined effect of this calibration is to raise lower wind speeds, and reduce higher TOPEX B winds.

The transition occurs at 5.3 ms-1.  A subsidiary, but interesting, point to note is the apparent discrepancy between the NDBC buoy measurements of wave heights, and those from EC and UKMO. This effect has also been noted in a calibration of GFO data.