# Presentation: 520425

## Looking for ship-speed dependency of XBT fall rates

- XBT Fall Rate Workshop
- NOAA/AOML, Miami, Florida
- March 10-12, 2008

- John Gilson and
- Dean Roemmich
- Scripps Institution of Oceanography
- Greg Johnson
- Pacific Marine Environmental Laboratory

- As ships attain greater velocities, it is natural to wonder whether XBTs fall rate is affected.
- Here 2 comparisons are attempted:
- 	- P18: ~0 kn vs ~10 kn with near 		XBTs (Deep Blues)‏
- 	- HRX: 14-25 kn XBT Deep Blues 		vs Argo

## NOAA/NSF funded 2007/2008 US Repeat Hydrography

- Program reoccupation of WOCE Section P18 in support of CLIVAR and CO2 programs.

-  Thanks to:
- For leg 1 (San Diego - Easter Island) the chiefs and co-chiefs were John Bullister and Dong-Ha Min.
- On leg 2 they were
- Gregory C. Johnson and Alex Orsi.
- The graduate student CTD watchstanders helped on both legs, as well as Pedro Pena,
- Carlos Fonseca, Andrew Stefanick, and
- Kyle Seaton from AOML.

- Sippican Deep Blue XBTs launched at 3 speeds, 'full' speed, 'half' speed, and 'zero'.
- Data received on March 5th

- R/V Ronald H. Brown
- Dec 2007-Feb 2008

- P18

## Method:

- - use 1/10th second data
- - difference, slightly filtered XBT 	profile
- - shift 'slow' profile +/- 30m to 		find offset that minimizes 		variance.
- - variance computed over 150m 	window
- - average of 26 pairs of XBTs

- Example:    Requires deepening of 8.1m at 700m for profile deployed at a 'slow' speed.
- XBT Pair at 2 30.00 S:  Good example with deep structure
- 7.3 kn vs 0.5 kn

## The effect on fallrate of Fast vs. Slow deployment speed 

- The effect on fallrate of Fast vs. Slow deployment speed

- - Gradient features in the 'Slow' deployment profile are shallow by a few meters, thus the 'fast' profile is warmer on a each recorded depth level
- - The sense of the offset implies the XBT takes a bit more time to reach a depth level from an XBT deployment at a 'fast' speed
- - At 500m, 6 'slow' deployments are deep vs. 14 that are shallow
- - Need more data

- Standard error

- Average

## Scripps High Resolution XBT (HRX)‏

- Scripps High Resolution XBT (HRX)‏

- Might expect fall rate to be affected more strongly by faster ships
- 	-  HRX ships travel up to 25 knots (P08)‏
- Compare Sippican Deep Blue XBTs, corrected fallrate by Hanawa et al. 1995,  against nearby Argo floats (2004-2007)‏

- - Mid Ocean              - Argo floats averaged within 1o and 15 days
- - Excluded XBTs in regions with weak temperature gradients (WOCE Global    	Hydrographic Climatology, Gouretski and Koltermann, 2004)‏
- 3651 comparisons

## (XBT – Argo) is shown

- XBT is warm/deep : consistent with recent studies (e. g. Goretski and Koltermann, 2007)‏
- Slight change with speed. From the present data does not appear significant
- Lack of data at 'slower' speeds
- standard error (n/6)‏

- at 700m

- at 700m

- uses WGHC

## “Conclusions” is too strong a word...

- “Conclusions” is too strong a word...
- This very limited analysis suggests that a ship-speed dependent fallrate error might exist in todays 'Deep Blue'...
- ...at least it warrants further investigation
- The HRX data does not show a significant change in fall rate as ship speed increases from 14 to 25 knots as might be expected.
- More nearly concurrent stopped and slow XBT deployments would be helpful

## (XBT – Argo) is shown

- XBT is warm/deep : consistent with recent studies (e. g. Goretski and Koltermann, 2007)‏
- Slight change with speed. From the present data does not appear significant
- Lack of data at 'slower' speeds
- standard error (n/6)‏

- at 700m

- at 700m

- uses WGHC