Collecting and Utilizing an XBT/CTD Comparison Reference Database for XBT Bias Estimation Tim Boyer, John Antonov, Syd Levitus, Ricardo Locarnini, Dan Seidov, Melissa Zweng Ocean Climate Laboratory National Oceanographic Data Center Silver Spring, MD March 10, 2008

Introduction Gouretski and Koltermann (2007) demonstrate a time-dependent XBT bias vs. CTDs Various statistical methods have been introduced to correct for this bias for ocean heat content calculations [Gouretski and Koltermann, 2007, Wijffels et al. 2008, Levitus et al. in preparation]

Problem All these statistical methods rely on large time/space differences between XBT/CTD Geographically biased XBT/CTD comparisons Possibly large errors in statistical approach

Ideal Solution Use historical XBT/CTD side-by-side comparisons to calculate XBT bias correction (drop rate correction) This has been done before on a more limited basis (Hanawa et al., 1995)

Less Ambitious Solution Gather together and disseminate all available data from XBT/CTD side-by- side experiments. Use these test data as validation/error estimation of various statistical methods

Challenge Can enough geographically diverse data be gathered for all different time periods?

Answer In my opinion yes, but it will take a community effort to assemble the datasets. Key: Identifying and collecting data from all side-by-side XBT/CTD drops (original data, not compiled statistics)

Data from Hanawa et al. (1995)

1970s – Early 1980s MODE-I and POLYMODE made numerous side- by-side XBT/CTD drops Flierl and Robinson (1977), Federov et al. (1978), McDowell et al. (1978), Seaver et al. (1982), Heinmiller et al. (1983) Geographically limited (western Atlantic) Woods, Eastern Pacific 1975

Late 1980s – Early 1990s Many tests Many gathered in Hanawa et al. (1995) Hallock and Teague (1992) Boyd AXBT/CTD comparisons (data destroyed in Hurricane Katrina)

Late 1990s – 2000s Thadathil et al. (2002) Coral Sea/R. V. Yellowfin Recorder Comparison (courtesy Gronell- Thresher) U. S. Naval Postgraduate School (NPS) Monterrey ( )

NPS Side-by-side test July 25 – August 1, 2000 February 5-12, 2001 August 2-9, 2001 July 15-22, 2002 January 27- February 3, 2003 January 19-26, 2006 January 23-30, 2007

Other Navy Data Naval Underwater Systems XBT tests 1977 and tests included XBTs from each year Only have reports, data needs to be found

National Oceanographic Partership Data ( ) XBT CTD

NOPP Cruise Alternating XBTs and CTDs

Conclusion I There are sufficient XBT/CTD side-by-side comparison data over time and maybe geographically for useful statistics to be compiled It will take a sustained community effort to identify and assemble the data

Side-by-side XBT/CTD drops used to check statistical bias correction of Levitus et al. Notes: Bias corrections are made to standard level data No other quality control done (just for this test)

XBT/CTD Comparison data detailed in Thadathil et al. (JAOT, 2002) Courtesy P. Thadathil XBT CTD

March 3, °E 61.0°S (South Indian Ocean) from MV Polar Bird (P. Thadathil et al.) UncorrectedCorrected

XBT/CTD comparison detailed in Federov et al. (Polymode Newsletter 1978) XBT CTD

August 18, °W 29.0°N (off US East Coast) from Akademic Kurchatov (Federov et al.) UncorrectedCorrected

XBT/CTD comparison detailed in Reseghetti et al. (Ocean Sciences, 2007) XBT CTD

January 24, °E 41.0°N (Mediterranean Sea) from Urania (Reseghetti et al., Ocean Science, 2007) UncorrectedCorrected

XBT/CTD comparison from R/V Yellowfin during an XBT Recorder Comparison Experiment, June 2005 XBT CTD

June 21, °W 33.6°N (off of US West Coast) from Yellowfin (XBT Recorder Comparison) UncorrectedCorrected

Conclusion II Tests on side-by-side drops can tell a lot about the application of statistical bias corrections on an individual basis A lot of investigation is necessary

MBTs Note: Work done by John Antonov 1992 (unpublished work)

Points MBTs drop at a constant rate (~2 meters/second) so there should be no drop rate error At least at Weather Station Charlie, there is a seasonal pattern to MBT/CTD differences, largest at 50 meters depth. At least partially due to thermistor response time Need more data: There are MBT/CTD comparisons

Thank You