Comparison of CTDXBT Temperature Profiles and XBTGDEM Sound Speed Profiles

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Comparison of CTD/XBT Temperature Profiles and
XBT/GDEM Sound Speed Profiles

• LT Annie Laird
• 08 March 2006

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Outline

• Background
• Data Collection
• Quality Control
• Data results-temperature profiles
• Data results-sound speed profiles
• Naval relevance

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Background

• Heinmiller (1983)
• Schmeiser (Summer 2000)
• Roth (Winter 2001)
• Boedeker (Summer 2001)
• Fang (Summer 2002)
• Dixon (Winter 2003)

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(No Transcript)
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Quality Control

• Plotted data from XBTs, CTDs, and GDEM for rough
  visual inspection.
• After visual inspection, interpolated XBT
  temperature data to 383 depth levels already
  picked by CTD (after CTD pressure data was
  converted to depth data). Also interpolated XBT
  sound speed data to 44 depth levels already
  selected by GDEM.
• A mean and standard deviation was computed for
  each level. All data run through an m-file. If a
  data point differed by more than 2 standard
  deviations from the level mean, it was flagged
  for further investigation.
• All flagged points determined to be part of a
  logical sequence decreasing with depth,
  considered reasonable, and included in analysis.

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XBT 8
0.4 degree Celsius jump! XBT hit the ship.
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XBT 11
10 points in this profile needed further
examination..
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25-125 meters Mean -0.076 Sd 0.1622
175-375 meters Mean -0.0453 Sd 0.0992
0-760 meters Mean -0.0407 Sd 0.0936
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What the ?!!
Mean 2.358 m/s Std 2.3352 m/s
Ok..
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0-200 meters Mean 0.8156 Sd 1.9236
0-700 meters Mean 0.7272 Sd 1.6515
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Naval Relevance

• XBTs are the primary instrument for developing
  sound velocity profiles for use in USW
  operations.
• The average warming bias introduced by the XBT in
  this study is 0.0407C.
• A 1C increase in temperature will roughly
  increase the sound speed by 4 m/s.
• The average bias presented by the T-7 XBT in this
  study would increase the average speed of sound
  by only 0.51 m/s, about 0.034 of the average
  1500 m/s sound speed.

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Naval Relevance

• Instead of using an XBT, one could extract data
  from GDEM to obtain a SVP.
• The spatial resolution of the data is not as high
  in GDEM as it is when firing an XBT, especially
  deeper in the water column.
• The average bias presented by the GDEM data in
  this study would increase the average speed of
  sound as compared to the XBT measurements by only
  0.7272 m/s, about 0.049 of the average 1500 m/s
  sound speed.
• Even though not much difference in measurement of
  average sound speed, GDEM data can miss important
  features of the SVP, especially in the upper
  layers of the water column.
• Tell ASWO to go ahead and launch an XBT!

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QUESTIONS??