XBT Fall Rate Equation – A ReviewPankajakshan and Gopalkrishna, (pankaj@nio.org)National Institute of Oceanography, Goa, India

“Oceangraphic data are like infants. Unless care is taken, it is susceptible to injuries(errors).

However, unlike infants data are immortal.

Once it is collected, it remains as a source of information for ever.” …Thadathil

http://www.nio.org

Why XBT Data susceptible to errors?

No depth sensor

Z=6.472 t – 0.00216 t2 Operated when ship is underway

Data transmitted through a thin wire

http://www.nio.org

History…….

Based on XBT-CTD Experiments ( simultaneous Observations) it is reported in a number of studies that Manufacture’s fall rate equation underestimate XBT depth, causing XBT depth error.

Subsequently, IOC task team proposed a ‘new depth equation’ for correcting the XBT depth error ( Hanawa et. al. 1995).

Since the new depth equation was developed based on XBT-CTD data sets mainly from Pacific, we evaluated the applicability of the new depth equation for XBT data from the Indian Ocean.

http://www.nio.org

XBT-CTD Controlled Data Sets for the Indian Ocean

Cruises1. SK 110 (29 Feb.-10 Mar., 1996)2. SK-95(1-25 Sep., 1994)3. SK-119(12-18 Dec., 1996)4. SK-120(27-Dec. 31-Jan.,1997)

http://www.nio.org

Depth vs Estimated Elapsed Time

# Red broken line shows the manufacturer’s equation.

# Solid line represent mean depth-time relation

based on the present data from the Indian Ocean

a-b Coefficients of the present data set

http://www.nio.org

• Linear trend is visible

• Depth difference w:r:t reference equation does not vary much

Mean Depth Error for XBT Data of Different Cruises

• Mean error for our cruises falls outside the manufacturer’s stipulated error bar (2 percent of the depth).

• Coefficients supplied by the manufacturer are not adequate.

The mean depth error varied between -2 to -27m.

a & b coefficients of our data do not differ much from Hanawa et al.

Hanwa et al fall rate equation is suitable for the Indian Ocean.

http://www.nio.org

Analytical depth error proposed by Seaver and Kuleshov (1982)

http://www.nio.org

Applicability of new depth equation for correcting fall rate in Antarctic Waters???????

Temperature - Viscosity Relation

• Inversely proportional

• Decrease of temperature increases viscosity

• Increase of viscosity retards probe free fall velocity

Station locations of XBT-CTD data sets from Antarctic waters

http://www.nio.org

XBT - CTD data collection

Dec 1998–Mar 1999&

April 1993

Composite XBT and CTD Temperature profiles from the two data sets

XBT : Red Lines CTD: Black lines

http://www.nio.org

Mean Depth Error for XBT data from Antarctic region

http://www.nio.org

Mean depth error is positive.

Depth error is in agreement with theoretical value.

Probe descends at a slower rate than expected.

Latitudinal Variation of XBT Depth Error

http://www.nio.org

Conclusion

Existing correction scheme is valid only for the tropical and sub-tropical regions where the depth error is negative.

It is reasonable not to correct XBT profiles from the extreme climatic regions.

More experimental studies are essential to further substantiate this result.

http://www.nio.org

Ongoing Indian XBT Transects

http://www.nio.org