# Presentation: 919706

## 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 (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

- NOPP Cruise Alternating **XBT**s and **CTD**s

## 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, 1999 23.8°E 61.0°S (South Indian Ocean) from MV Polar Bird (P. Thadathil et al.)

**Uncorrected**

**Corrected**

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

**XBT/CTD comparison detailed in****Reseghetti 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 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)**

## Slide 28

## Slide 29

## Slide 30

## 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