P.  Poli  et  al.:  SVP-BRST:  genesis,  design,  and  initial  results

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www.ocean-sci.net/15/199/2019/

Ocean  Sci.,  15,199-214,  2019

These  requirements  were  adopted  on  a  number  buoys
deployed  by  the  Economic  Interest  Group  (EIG)  EUMET-NET
  Operational  Service  for  surface  marine  observations
(E-SURFMAR)  and  European  partners.  This  brought  about
four  major  technical  improvements,  as  compared  to  standard
practices  at  the  time.
First,  the  location  accuracy  was  increased,  thanks  to  GPS
instead  of  Argos  for  estimating  position,  and  several  buoys
adopted  Iridium  instead  of  Argos  for  the  transmission,  to  ensure ­
  regular  hourly  data  reports.  Second,  the  temperature  was
reported  and  transmitted  to  shore  at  a  resolution  of  0.01  K.
These  technical  improvements  are  collectively  known  as
“HRSST-1”.  While  only  few  buoys  adhered  to  the  HRSST-1
  requirement  in  2009,  it  has  now  become  the  standard,  at
the  time  of  writing,  for  almost  all  drifters  deployed  globally.
From  there,  a  third  requirement  appeared,  namely  the  adoption ­
  of  a  new  Binary  Universal  Form  for  the  Representation
of  meteorological  data  (BUFR)  template  in  2015,  to  encode
the  SST  data  at  the  resolution  of  0.01  K,  and  transmit  to  operational ­
  data  users  via  the  World  Meteorological  Organization
(WMO)  Global  Telecommunications  System  (GTS),  without ­
  loss  of  information.  That  template  became  operational
at  most  data-originating  centers  by  the  end  of  2016;  before
that,  many  data  transmitted  on  the  GTS  were  sent  at  reduced
SST  resolution  of  0.1  K.  At  the  time  of  writing,  all  these  three
improvements  are  standard  for  most  operational  drifters.
The  fourth  technical  improvement  was  for  each  buoy  to
use  an  individually  calibrated  temperature  probe,  instead  of
one  picked  from  a  batch  calibration,  in  order  to  guarantee
the  more  stringent  total  uncertainty  requirement  of  0.05  K,  as
well  as  traceability  to  national  standards.  This  requirement
(on  top  of  previous  ones)  was  called  “HRSST-2”.  In  total,
46  such  HRSST-2  buoys  fitted  with  all  three  technical  advances, ­
  as  well  as  including  each  a  barometer,  were  deployed
between  2012  and  2017.  These  buoys  are  listed  in  Table  1  below. ­
  They  were  manufactured  by  Metocean  (Petólas,  2016),
using  Yellow  Springs  Instrument  Company  (YSI  Inc.)  sensors ­
  described  in  the  table.  One  buoy  was  redeployed  after
running  ashore.
In  addition,  several  other  HRSST-2  buoys  were  manufactured ­
  for  experimental  purposes,  also  by  Metocean.  Each
buoy  carried  a  conductivity-temperature  (CT)  probe  manufactured ­
  by  Sea-Bird  Electronics  (SBE)  in  order  to  measure ­
  salinity.  Each  HRSST-2  SVP  buoy  with  barometer  and
salinity  (SVP-BS)  hence  included  two  individual-calibrated
SST  probes:  one  integrated  with  the  buoy  hull  (around  17  cm
depth),  and  one  in  the  CT  probe  (around  45  cm  depth).
This  twin-sensor  configuration  offered  near-optimal  horizontal ­
  and  temporal  co-location  by  virtue  of  the  buoy  design.  The
only  major  differences  between  the  two  sensors  were  the  vertical ­
  positioning  and  the  housing  of  the  sensors  (one  digital
SST  sensor  integral  with  the  hull,  the  other  CT  sensor  immersed ­
  entirely  in  water).  In  total,  there  were  19  such  buoys
deployed  between  2012  and  2015  (one  buoy  was  redeployed
after  beaching).  Table  2  shows  the  list  of  such  buoys,  the  de-Number

  of  points

275  280  285  290  295  300  305
Hull  SST  (K)
Figure  1.  Density  plot  of  the  scatter  between  hull  SST  measurements ­
  (horizontal  axis)  and  CT  SST  measurements  (vertical  axis)
from  HRSST-2  SVP-BS  buoys.

ployment  areas,  and  the  mission  dates.  Most  buoys  were  deployed ­
  in  the  North  Atlantic.
2.2  HRSST-2  SVP-BS  data  record  revisited
In  order  to  exploit  the  co-located  information  from  two  individually ­
  calibrated  SST  probes,  the  data  record  from  the  second ­
  set  of  HRSST-2  buoys,  SVP-BS  fitted  with  CT  probes,  is
addressed  here.  The  record  consists  of  about  87  000  data  reports ­
  between  2012  and  2016.  Figure  1  shows  a  scatter  density ­
  plot  of  the  two  temperatures.  The  twin  measurements
are  highly  correlated,  and  the  robust  standard  deviation  of
the  difference  is  0.03  K.  This  result  is  compatible  with  uncertainty ­
  in  a  difference  of  two  sensors  with  total  uncertainties ­
  better  than  0.05  K  (or  possibly  0.02  K).  However,  Fig.  1
shows  a  small  fraction  of  outliers  in  both  directions,  especially ­
  for  warmer  temperatures.  In  fact,  the  root  mean  square
(rms)  of  the  differences  is  quite  large,  at  0.36  K.
The  differences  between  the  two  measurements  are  not
only  due  to  sensor  accuracy  but  also  to  the  placement  of  the
sensors:  vertical  location  and  housing  (one  integral  with  the
buoy  hull,  the  other  underneath  the  buoy).  To  better  understand ­
  the  sources  of  differences.  Fig.  2a  shows  the  differences
between  the  two  sensor  temperatures  as  a  function  of  solar  elevation ­
  angle.  Differences  that  are  out  of  range  (below  —IK
or  above  1  K)  are  also  shown  for  completeness  (at  —  1  and
+  1  K,  respectively);  they  represent  about  0.5  %  of  the  entire
data  record.  We  find,  as  expected,  that  most  large-magnitude
differences  (absolute  value  greater  than  0.2  K)  are  positive