CTDXBT Comparison, Quality of JJYY Data and XBT Data Analysis of the Mixed Layer Depth

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CTD/XBT Comparison,Quality of JJYY DataandXBT
Data Analysis of theMixed Layer Depth

• by
• LT Mike Roth
• OC3570
• 20MAR01

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Purpose
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XBT-CTD Comparison

• Insruments
• Sea Bird CTD
• T-7 Sippican XBTs

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XBT-CTD Comparison

• Location of collocated XBTs and CTDs

8 10 cruise three
1-7 cruise one
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XBT-CTD Comparison

• Data Processing

Converted CTD dbars to m (Saunders, 1981)
Data deeper than deepest depth of collocated
instrument discarded
XBT had smaller sampling interval (0.7 m 1.98 m)
Linear interpolation of XBT to standard CTD levels
383 levels / 3,326 data pts each/ total6,652
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XBT-CTD Comparison

• Quality Control of the data
• Causes
• Spikes due to XBT copper wire hitting hull
• Bad XBTs
• Upper 4 m (16.28 )
• 3 Stage Process
• 0.2 C criteria to flag points
• Visual
• Final re-run

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XBT-CTD Comparison

• Quality Control of the data
• Example of a bad XBT

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XBT-CTD Comparison

• Quality Control of the data
• Results of QC
• Reduced sample size to 9 CTD/XBTs
• None of CTD data flagged
• Removed 501 pts (1,002)
• 2,825 pts (5,650) or 85 remained for
  statistical analysis

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XBT-CTD Comparison

• Data Analysis
• XBT subtracted from CTD temp at each depth for
  each collocated XBT/CTD

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XBT-CTD Comparison

• Data Analysis
• Composite Plot

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XBT-CTD Comparison

• Findings
• XBT generally gt CTD
• Greatest T-diff in upper 100 m
• 60-80 m was largest T-diff (-0.2915 ºC at 69.5 m)
  with max variability as well (0.5007 ºC at 67.5
  m)
• CTDgtXBT ( T-diff) between 90-105 m (max 0.0538
  ºC at 91.3 m)
• 105 to 760 m mean T-diff -0.077 ºC and smaller
  std

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XBT-CTD Comparison

• Comparison to Similar Studies

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JJYY-XBT/CTD Comparison

• Results of QC
• Much easier to perform
• XBT-6 salvaged for upper 100 m
• Only had 7 collocated JJYY-XBT/CTD
• Started with 2,247 pts (4,494) after
  interpolation
• Ended up with 2,245 pts (4,490) 99 remained for
  statistical analysis

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JJYY-XBT/CTD Comparison

• Results of Data Analysis
• NAVO SVPG software with user interface performs a
  pre-Quality control of the data and
• With a linear interpolation of the data points
  does perform well as representing the full XBT
  profile .

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Analysis of MLD using XBTs
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Analysis of MLD using XBTs
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Analysis of MLD using XBTs
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Conclusions

• Good Quality Control is a necessity, start below
  4m

• Sometimes you are limited to small sample sizes
  which can have a significant impact on final
  results

• XBT/CTD comparison did reveal similar results as
  past studies (ie. XBTgtCTD bias, upper 125 m)

• JJYY-XBT/CTD comparison showed SVPG is a useful
  tool in representing the original XBT profile and
  makes QC much easier

• Ungridded XBT data is not ideal for MLD analysis,
  but does show effects of thermal and mechanical
  forcing