A.  Boesch  and  S.  Müller-Navarra:  Reassessment  of  long-period  constituents  for  tidal  predictions

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www.ocean-sci.net/15/1363/2019/

Ocean  Sci.,  15,1363-1379,  2019

Residuals  of  high  and  low  water  times

0  50  100  150  200  250  300  350

Days  since  01.01.2016
Figure  14.  The  residuals  of  high-  and  low-water  times  for  the  tide
gauges  Cuxhaven  (a)  and  Hamburg  (b).

dictions  with  the  HRoI  for  German  tide  gauges  starting  with
the  BSH  tide  tables  for  the  year  2020.
In  future  work,  extensive  comparison  of  the  HRoI  with  the
common  harmonic  method  might  provide  more  insights  into
the  capabilities  of  both  tidal  analysis  techniques.  The  German ­
  Bight  would  be  an  ideal  area  of  investigation  with  its
large  network  of  tide  gauges  located  both  at  the  open  North
Sea  and  far  within  tidally  influenced  rivers.
Data  availability.  The  tide  gauge  observations  used  in  this  research
are  available  from  the  respective  authorities  (cf.  Table  Al  ).

7  Conclusions
Time  series  of  high-  and  low-water  records  from  111  German ­
  tide  gauges  were  analysed  to  determine  important
long-period  partial  tides.  Generalized  Lomb-Scargle  periodograms
  were  calculated  from  lunitidal  intervals  and
heights  for  all  tide  gauges,  and  spectral  peaks  were  identified
in  these  periodograms  above  noise  thresholds.  The  separate
analyses  of  lunitidal  intervals  and  heights  were  combined  to
realize  one  comprehensive  list  of  partial  tides.  An  application ­
  is  the  usage  of  these  constituents  in  tidal  analyses  and
predictions  with  the  HRoI.
The  new  set  of  39  partial  tides  largely  confirms  the  previously ­
  used  set  with  43  partial  tides.  It  can  be  seen  from
Table  2  that  nine  new  constituents  were  added  and  13  constituents ­
  were  removed.  Many  of  the  removed  angular  velocities ­
  are  close  to  strong  partial  tides,  such  as  the  anomalistic ­
  month  (Mm),  the  half  synodic  month  (MSf)  and  the  half
tropical  month  (Mf).  The  removed  constituents  might  have
been  artefacts  from  spectral  leakage,  which  are  most  prominent ­
  in  the  proximity  of  strong  spectral  lines  and  which  were
misidentified  as  true  signals  in  previous  studies.  The  unambiguous ­
  identification  of  partial  tides  is  very  difficult  at  angular ­
  velocities  below  approximately  1°  tn -1  because  the  noise
levels  in  the  periodograms  increase  towards  lower  angular
velocities  and  the  results  from  different  tide  gauges  are  less
consistent.
The  verification  based  on  observations  from  98  tide  gauges
in  the  year  2016  suggests  that  the  usage  of  the  new  constituents ­
  list  can  lead  to  slightly  better  predictions.  In  particular, ­
  the  average  standard  deviations  of  the  residuals  are
lower  and  four  frequencies  were  reduced.
This  study  presents  for  the  first  time  a  thorough  investigations ­
  of  the  long-period  constituents  used  with  the  HRoI.  The
new  list  of  constituents  will  be  used  in  tidal  analyses  and  pre ­