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P. Poli et al.: SVP-BRST: genesis, design, and initial results
Ocean Sci., 15,199-214, 2019
www.ocean-sci.net/15/199/2019/
Table 3. Individual calibration data for SST sensors from three HRSST-2 buoys that were fortuitously recovered. The mean error is the
average difference, for several verification points, between the temperature reported by the sensor and the temperature of the calibration bath.
Lab. no. 1 indicates the initial calibration and verification that was made then. The last column, showing temporal drift (in Kyear -1 ), is
365.25 times the difference between the mean error assessed by lab. no. 2 (or 3) minus the mean error assessed by lab. no. 1, divided by the
number of days elapsed.
WMO id.
Lab. no.
Date
Mean error
Time interval since
lab no. 1 (days)
Temporal drift since
lab no. 1
4400871
1
02/10/2012
-0.010 K
0
-
2
23/09/2016
-0.063 K
1452
-0.013 Kyear -1
3
16/08/2017
-0.043 K
1779
—0.007 K year -1
4400608
1
16/10/2012
-0.006 K
0
-
2
23/09/2016
-0.055 K
1438
-0.012 Kyear -1
3
16/08/2017
-0.037 K
1765
—0.006 Kyear -1
6200552
1
01/09/2012
0.031 K
0
-
2
23/09/2016
-0.007 K
1483
—0.009 Kyear -1
3
16/08/2017
+0.014 K
1810
—0.003 Kyear -1
cept of uncertainty validation is presented in detail by Cor-
lett et al. (2014). Briefly, the standard deviation of the satel
lite/drifter differences is comprised of contributions from
the satellite and drifter measurements, as well as terms to
represent the spatial and temporal differences between the
two measurements. Having used models to adjust the drifter
measurement to be the same time and depth as the satellite
SST, Corlett et al. (2014) showed the standard deviation of
the satellite/drifter differences approximately reduces to two
terms: the satellite SST uncertainty and the drifter SST un
certainty as in Eq. (1).
^Satellite minus drifter — -/^Satellite + ^drifter ( ^ )
Figure 4 shows a comparison between 1 October 2016 and 30
June 2017 of satellite SST validation results for the dual-view
three-channel retrieval from SLSTR for two sets of drifters:
all drifters in Fig. 4a (15 551 matchups) and a subset of
HRSST-1 and HRSST-2 drifters in Fig. 4b (625 matchups).
In the figure, the green lines indicate the theoretical disper
sion of uncertainties using Eq. (1) and a value of 0.20 K for
drifter (an assumption between those of O’Carroll et al., 2008
and Lean and Saunders, 2013). The blue lines indicate the
calculated dispersion for each set of data and the red lines in
dicate the standard error. If the assumptions are correct, then
the dispersion of the blue lines should track the spread of the
green lines, which we see is the case in Fig. 4a (all drifters).
Where the dispersion does not match the expected spread, the
large standard errors imply a low number of satellite/drifter
differences in those bins. For the subset of HRSST drifters.
Fig. 4b shows that the dispersion underestimates the spread,
even for low standard error cases, meaning one assumption
is incorrect in this case.
With all other factors being equal, the distinction in the
drifter type between Fig. 4a and b suggests the drifter uncer-
(b) HRSST drifters
0.0 0.1 0.2 0.3 0.4 0.5 0.0 0.1 0.2 0.3 0.4 0.5
SST uncertainty (K) SST uncertainty (K)
(SLSTR 3-channel nighttime) (SLSTR 3-channel nighttime)
Figure 4. SLSTR SST uncertainty validation plot for (a) all drifters
and (b) a subset of HRSST-1 and HRSST-2 drifters, with uncer
tainty bins of 0.01 K. An uncertainty of 0.20 K is assumed for the
drifter SST.
(a) All drifters
Theoretical
SD
Median +/- SE
. _ - "
' ;
---
tainty assumed (0.20 K) is inappropriate for the HRSST sub
set. To verify this. Fig. 5 contains the same data as Fig. 4 but
with the theoretical dispersion (green lines) calculated for a
drifter uncertainty of 0.05 K. While the calculated dispersion
does not track any more the expected spread for all drifters
(Fig. 5a), the assumption of 0.05 K for the uncertainty of the
HRSST drifter data gives a much better fit (Fig. 5b). This
demonstrates the improved quality of HRSST drifter data for
satellite SST validation.
2.5 Influence of the drogue on drifter SST
measurements
This section investigates the effect of the sea anchor or
drogue on drifter SST measurements. By exerting its own
weight and by following currents centered at 15 m depth, the
drogue pulls the float downwards via the tether. This main
tains the float and its drogue aligned in the vertical, in wave
troughs. When the drogue is lost, the float has more free
dom to oscillate by roll and pitch, and the temperature probe
can sometimes be exposed to waters closer to the surface.