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 LaboratoryNational 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 (2000-2007)

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 1990

• 1977 tests included XBTs from each year 1967-1975

• Only have reports, data needs to be found

National Oceanographic Partership Data (1997-2000)

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 inThadathil et al. (JAOT, 2002)

Courtesy P. Thadathil

XBT CTD

March 3, 1999 23.8°E 61.0°S (South Indian Ocean)

from MV Polar Bird (P. Thadathil et al.)

Uncorrected Corrected

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

XBT CTD

August 18, 1977 69.98°W 29.0°N (off US East Coast)

from Akademic Kurchatov (Federov et al.)

Uncorrected Corrected

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

XBT CTD

January 24, 2004 7.5°E 41.0°N (Mediterranean Sea)

from Urania (Reseghetti et al., Ocean Science, 2007)

Uncorrected Corrected

XBT/CTD comparison from R/V Yellowfin during an XBT Recorder Comparison

Experiment, June 2005

XBT CTD

June 21, 2005 118.4°W 33.6°N (off of US West Coast)

from Yellowfin (XBT Recorder Comparison)

Uncorrected Corrected

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