1. Comparison of CTD/XBT Temperature Profiles and XBT/GDEM Sound Speed Profiles
LT Annie Laird
08 March 2006

2. Outline Background Data Collection Quality Control
Data results-temperature profiles
Data results-sound speed profiles
Naval relevance

3. Background Heinmiller (1983) Schmeiser (Summer 2000)
Roth (Winter 2001)
Boedeker (Summer 2001)
Fang (Summer 2002)
Dixon (Winter 2003)
Heinmiller’s study: Systematic errors in expendable bathythermograph (XBT) profiles. A comparison of 306 Sippican T-4 casts in the tropical Pacific and 139 Sippican T-7 casts in the Sargasso Sea and CTD profiles was performed to detect errors in XBT temperature and depth. The XBT temps were systematically higher than CTD temperatures. Isotherm depths from XBT’s were systematically less than CTD depth below an intermediate depth.
Schmeiser collected a series of 19 collocated CTD/XBT sites to identify any XBT temperature biases.
Roth’s study focused on XBT and CTD temperature measurement comparison and also the quality of the JJYY data and XBT data analysis of the mixed layer depth.
Boedeker had 28 pairs of collocated XBT/CTD’s for a temperature comparison.
Fang had 28 pairs of XBT/CTD for depth difference and temperature difference comparison. Some were T-4’s, others were T-7’s.
Dixon had 24 pairs of XBT/CTD for depth difference and temperature difference comparison. All XBT’s were T-7’s.
All of the student studies noted a warm bias with the XBT temperature readings. Those studies that included a study of depth comparisons noted that the mean XBT isotherm depths were greater (deeper) than the CTD measurements.

4. Re-named XBT and CTD casts for clarity in report.
Casts were conducted in Legs 1 and 2 of the cruise.
Most casts were obtained in water deeper than 1000m so could analyze data down to 760 meters (max depth for T-7 XBT’s) – Exception, Cast 12 only analyzed to 745 meters (shallow water).
GDEM data obtained from NAVO website, searched for the closest GDEM location to each XBT location.
Closest was 2.25 nm apart (site 13), furthest was 8.43 nm apart (site 1).

5. Quality Control
Plotted data from XBTs, CTDs, and GDEM for rough visual inspection.
After visual inspection, interpolated XBT temperature data to 383 depth levels already picked by CTD (after CTD pressure data was converted to depth data). Also interpolated XBT sound speed data to 44 depth levels already selected by GDEM.
A mean and standard deviation was computed for each level. All data run through an m-file. If a data point differed by more than 2 standard deviations from the level mean, it was flagged for further investigation.
All flagged points determined to be part of a logical sequence decreasing with depth, considered reasonable, and included in analysis.
For visual inspection, was expecting to see more errors with the XBT’s due to operator error than the CTD’s (i.e. XBT hitting the ship). See profile XBT 8, all data deeper than 630 meters was excluded. Hitting the bottom wasn’t an issue as most casts were >760 meters (Exception, Cast 12, 745 meters)

6. XBT #8 0.4 degree Celsius jump! XBT hit the ship.
- A 0.4 degree jump at 630 meters. All data below 630 meters excluded from analysis in XBT #8.

7. XBT 11 10 points in this profile needed further examination…..
10 data points to examine. All part of a logical decreasing sequence and included.
10 profiles had points that were outside of the 2 standard deviation curves.
207 from CTD profiles, 202 from XBT profiles. Out of a total 4899 data points (8.3% of the data points after exclusion of bad data from XBT #8), no bad data from GDEM.

8. Next step in analysis was to overlay CTD/XBT pairs, then compute CTD data point-XBT data point for each level in the water column (all 383 levels).
Note here the major variability in pair #12 is in the upper levels of the water column.

9. 25-125 meters Mean: -0.076 Sd: 0.1622 175-375 meters Mean: -0.0453
A mean and standard deviation were computed for the temperature difference CTD-XBT at each level in the water column down to 760 meters.
All measurements of mean and Standard deviation in degrees Celsius.

10. What the ?#%!! Mean: 2.358 m/s Std: 2.3352 m/s Ok…..
-Plot on the left overlays the data from the CTD, XBT and GDEM (extracted from the NAVO website), remember, site 13 was the location where the GDEM and XBT locations were the closest of any of the sites!
-Plot on the right takes the XBT data and subtracts it from the GDEM data. The GDEM data is noticeably faster than the XBT data.
-Major differences in the upper layers of the water column, specifically, meters.
-XBT trace indicates that the mixed layer depth is right at about 20 meters and the gradient of the curve is sharp. A target sub is going to use that relatively flat spot in the curve to calculate his “best depth” which appears to be ~30 meters shallower than the GDEM trace.
-This could make a difference for direct path propagation with a towed array if the array is placed at the wrong depth.
-If you use the GDEM trace, you would think you had a deeper layer with a shallow gradient, and you might even think you could passively detect a sub just below the “actual” layer with a hull-mounted sonar or shallow tail or using a medium freq. active sonar like SQS-56.
-Also, since GDEM is time-averaging, if we ran a comparison in a tropical climate or away from a large cold current we might not see such a difference between GDEM/XBT. GDEM looks like more of an “afternoon” profile than one that you would obtain with an XBT in the morning.

11. 0-200 meters Mean: 0.8156 Sd: 1.9236 0-700 meters Mean: 0.7272
This plot shows the composite average and standard deviation of the XBT data subtracted from the GDEM data for each of the 44 water levels (from 0m to 700m) designated from the NAVO database, averaged over all 13 pairs of difference computations.
Notice that the GDEM data has a warm bias as compared to the XBT data. Also most of the variability occurs in the upper water layers of the water column.
The units for the means and standard deviations are in meters per second.

12. Naval Relevance
XBT’s are the primary instrument for developing sound velocity profiles for use in USW operations.
The average warming bias introduced by the XBT in this study is °C.
A 1°C increase in temperature will roughly increase the sound speed by 4 m/s.
The average bias presented by the T-7 XBT in this study would increase the average speed of sound by only 0.51 m/s, about 0.034% of the average 1500 m/s sound speed.
Sound speeds are only nominally increased by the warm bias of the XBT’s.
So, the sound velocity measurements obtained by the XBT’s are not impacted significantly enough to impose an operational degradation upon the USW problem.

13. Naval Relevance
Instead of using an XBT, one could extract data from GDEM to obtain a SVP.
The spatial resolution of the data is not as high in GDEM as it is when firing an XBT, especially deeper in the water column.
The average bias presented by the GDEM data in this study would increase the average speed of sound as compared to the XBT measurements by only m/s, about 0.049% of the average 1500 m/s sound speed.
Even though not much difference in measurement of average sound speed, GDEM data can miss important features of the SVP, especially in the upper layers of the water column.
Tell ASWO to go ahead and launch an XBT!

14. QUESTIONS??