1. A Comparison of Computed Sound Speed Profiles from CTD and GDEM Data
LT Chris Bryan
OC 3570
Winter 2008

2. Data Sources ● CTD From November 2007 Off central California coast
34 CTD casts
● Generalized Digital Environmental Model (GDEM)
- NAVOCEANO created
- derived from the 1995 Master Oceanographic Observation
Data Set (MOODS)
- NetCDF files of gridded monthly means of temperature and
salinity
- Global coverage at 0.25° resolution (latitude/longitude)

3. CTD Locations

4. Sound Speed Calculation
● Many Methods Exist
Medwin (6 terms; only good to 1km)
Mackenzie (9 terms)
Chen and Millero (41 terms)
● Del Grosso
- chosen for use here
- Medwin says Del Grosso gives best values
terms with 18 coefficients having 12 significant digits
each

5. b002
CTD
GDEM

6. b002 SSP key features CTD GDEM SST 13.26 13.53 SS@ bottom 1481.22
MLD 13 25
MLSS
DSCD
741
700
DSCSS
1480.6
Depth Max
1020
967

7. b014
Notice bottom depth difference.
CTD
GDEM

8. b014 SSP key features CTD GDEM SST 14.96 15.99 SS@ bottom 1481.79
MLD 40 35
MLSS
DSCD
704
600
DSCSS
1480
Depth Max
1018
4037

9. b014 - modified
CTD
GDEM

10. b017
Notice bottom depth difference.
CTD
GDEM

11. b017 SSP key features CTD GDEM SST 15.17 16.11 SS@ bottom 1481.37
MLD 29 35
MLSS
DSCD
544
700
DSCSS
1480
Depth Max
1027
4395

12. b017 - modified
Notice bottom depth difference.
CTD
GDEM

13. b025
Notice bottom depth difference.
CTD
GDEM

14. b025 SSP key features CTD GDEM SST 14.93 15.05 SS@ bottom 1480.70
MLD 34 30
MLSS
1504.4
DSCD
552
600
DSCSS
1479.3
Depth Max
1025
4338

15. b025 - modified
Notice bottom depth difference.
CTD
GDEM

16. b031
Notice bottom depth difference.
CTD
GDEM

17. b031 SSP key features CTD GDEM SST 12.88 13.29 SS@ bottom 1488.93
MLD 15 30
MLSS
DSCD 73
600
DSCSS
1480.2
Depth Max
118
1728

18. b031 - modified
Notice bottom depth difference.
CTD
GDEM

19. Why Don’t They Match locations (GDEM only in .25° increments)
● NONE of the GDEM points exactly match CTD
locations (GDEM only in .25° increments)
● GDEM is an AVERAGE and not always
representative of current environment
● Last depth in profiles are extremely different for most
profiles (affects bottom parameters)

20. is a good representation of CTD data for the cases examined,
Conclusion
GDEM
is a good representation of CTD
data for the cases examined,
especially in shallower portion of profile

21. QUESTIONS