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Table 2. Similar to Table 1 but for HRSST-2 SVP-BS buoys (each buoy was also fitted with a CT probe). 
WMO identifier 
Deployment basin 
HRSST sensor model and S/N 
Start date 
End date 
4100736 
North Atlantic 
Digital YSI46000 10014 
14/02/2012 
26/01/2013 
6200513 
North Atlantic 
Digital YSI46000 10011 
18/03/2012 
17/01/2013 
6200505 
North Atlantic 
Digital YSI 46000 10017 
25/03/2012 
10/04/2013 
6200501 
North Atlantic 
Digital YSI 46000 10019 
29/06/2012 
10/12/2012 
6100788 
Mediterranean Sea 
Digital YSI 46000 10020 
04/09/2012 
16/02/2013 
3100739 
North Atlantic 
Digital YSI 46000 10016 
30/11/2012 
06/07/2013 
3100740 
North Atlantic 
Digital YSI 46000 10044 
01/12/2012 
06/03/2013 
6100530 
Mediterranean Sea 
Digital YSI 46000 10013 
30/01/2013 
19/05/2013 
6100525 
North Atlantic 
Digital YSI 46000 10042 
22/02/2013 
16/08/2013 
6100524 
North Atlantic 
Digital YSI 46000 10049 
22/02/2013 
05/05/2013 
6200504 
North Atlantic 
Digital YSI 46000 10045 
24/05/2013 
27/11/2014 
1300899 
Tropical Atlantic 
Digital YSI 46000 10043 
26/05/2013 
10/12/2013 
6200509 
North Atlantic 
Digital YSI 46000 10062 
27/05/2013 
15/10/2013 
2300587 
Indian Ocean 
Digital YSI 46000 10071 
09/06/2013 
07/09/2013 
2300588 
Indian Ocean 
Digital YSI 46000 10053 
09/06/2013 
07/09/2013 
4100737 
North Atlantic 
Digital YSI 46000 10059 
06/12/2013 
10/03/2015 
4100800 
North Atlantic 
Digital YSI 46000 10058 
06/12/2013 
16/01/2015 
6200500 
North Atlantic 
Digital YSI 46000 10054 
12/06/2014 
18/02/2016 
6500511 
North Atlantic 
Digital YSI 46000 10056 
17/06/2014 
25/06/2014 
3100719 
Tropical Atlantic* 
Digital YSI 46000 10020 
11/04/2015 
20/06/2015 
during daytime (the hull sensor being located closer to the 
surface). The differences are smaller at night and when the 
Sun is more than 30° below the horizon. The large depar 
tures observed sometimes during daytime suggest that one 
or other of the two SST sensors may have been differentially 
affected by direct solar radiation, or by the buoy heating up 
the sensor through heat conduction. 
Unlike promising new developments with wave drifters 
(Centurioni et ah, 2016), the HRSST-2 drifters did not pro 
vide any information about sea state. In past SST studies, 
wind speed is generally used to describe sea-state mixing 
(e.g., Donlon et al., 2002; Morak-Bozzo et al., 2016). In 
this study, we also consider significant wave height. Infor 
mation about both parameters can be obtained by co-locating 
with the ERA5 reanalysis (Hersbach and Dee, 2016; C3S, 
2017). The ERA5 reanalysis data are interpolated in space 
from their original resolution (spectral truncation T639) to 
the buoy locations, using the nearest-in-time hourly reanaly 
sis map. Figure 2b and c show (respectively) that the large- 
magnitude SST difference mostly arise when the wind speed 
is up to moderate (under 8-10ms -1 ) and when the wave 
heights are up to moderate (under 2-3 m). The agreement 
between the sensors increases when there is more wave ac 
tivity, probably because of greater mixing. When such is the 
case, almost all SST differences are found in the range from 
—0.1 to 0.0 K. Sea-state mixing caused by waves cannot be 
controlled or mitigated by a platform as small as a 40 cm di 
ameter drifter. However, the role of the waves, probably via 
mixing, is suggested here to be quite important when using 
the SST data collected by drifting buoys. A knowledge of 
the local SST dynamics, as the buoy is following a pendu 
lum movement and senses the temperature surface at various 
depths within the top few meters of the ocean, would help 
better understand the distribution of SST that is measured, 
and how it corresponds to satellite measurements, or how it 
should be considered in the cal/val process. 
The differences between the probes can also be inspected 
as a function of mean solar local time (MSLT) for each buoy. 
For this, we only retain the buoys that reported at least for 
250 days, without issue. For the subsequent data analysis, 
we filter out 12 cases when differences are larger than 20 K 
(visible in Fig. 1), likely to be erroneous. Figure 3 shows 
that the mean differences feature a diurnal cycle, with the 
maximum positive differences around 12:00 MSFT. This is 
consistent with the depth difference of the two probes in the 
context of diurnal vertical stratification of the surface temper 
ature. Diurnal stratification tends to peak around 14 h (e.g., 
Reverdin et al., 2013; Morak-Bozzo et al., 2016), and temper 
ature stratification larger than 0.1 K within the upper 0.5 m 
would tend to occur only at the lowest wind speeds. How 
ever, this daily cycle in difference may also be partially ex 
plained by the hull sensor being heated by the surrounding 
buoy and/or by direct solar radiation (an effect which might 
tend to peak more around 12h MSFT). These latter effects 
are not related to the environment and should be avoided. 
2.3 Recovered buoys 
Three HRSST-2 buoys manufactured in 2012, deployed in 
2014, ran ashore in 2016 in Great Britain and Brittany. They 
were recovered and offered together a unique opportunity to