Lowered ADCP
The lowered ADCP (LADCP) is a 150 kHz RDI Phase 3 broadband ADCP. The LADCP has custom 30 degree beam angles. The LADCP provides a full-depth profile of ocean current from a self-contained ADCP mounted on the CTD rosette. Using the conventional "shear method" for processing (e.g., Fischer and Visbeck, 1993), overlapping profiles of vertical shear of horizontal velocity are averaged and gridded, to form a full-depth shear profile. The shear profile is integrated vertically to obtain the baroclinic velocity and the resulting unknown integration constant is the depth-averaged or barotropic velocity. This barotropic component is then computed as the sum of the time-averaged, measured velocity and the ship drift (minus a small correction, less than 1 cm/s, to account for a nonconstant fall rate) (Fischer and Visbeck, 1993; Firing, 1998). Errors in the baroclinic profile accumulate as 1/ √ (N) where N is the number of samples (Firing and Gordon, 1990). This error translates to the lowest baroclinic mode and, for a cast of 2500 m depth, it is about 2.4 cm/s (Beal and Bryden, 1999). The barotropic component is inherently more accurate, because the errors result from navigational inaccuracies alone. These are quite small with P-code GPS, about 1 cm/s (2 to 4 cm/s without). Comparisons with Pegasus suggest that the LADCP can measure the depth-averaged velocity to within 1 cm/s (Hacker et al., 1996). The rms difference between Pegasus and LADCP absolute profiles are within the expected oceanic variability, 3-5 cm/s (Send, 1994), due primarily to high frequency internal waves.
References
- Beal, L. M., and H. L. Bryden. The velocity and vorticity structure of the Agulhas Current at 32 °S. J. Geophys. Res., 104:5151–5176, 1999. (doi:10.1029/1998JC900056)
- Firing, E. F., and R. Gordon. Deep ocean acoustic Doppler current profiling. In of the Ocean Engineering Society, C. M. T. C., editor, Proceedings of the IEEE Fourth International Working Conference on Current Measurements, pages 192–201, Clinton, MD, 1990.
- Firing, E. Erratum. It. WOCE Newsletter, 31:20, 1998.
- Firing, E. Lowered ADCP development and use in WOCE. It. WOCE Newsletter, 30:10–14, 1998.
- Fischer, J., and M. Visbeck. Deep velocity profiling with self-contained ADCPs. J. Atmos. and Oceanic Tech., 10:764–773, 1993.
- Hacker, P., E. Firing, W. D. Wilson, and R. Molinari. Direct observations of the current structure east of the Bahamas. Geophys. Res. Let., 23:1127–1130, 1996. (doi:10.1029/96GL01031)
- Send, U. The accuracy of current profile measurements - effect of tropical and mid-latitude internal waves. J. Geophys. Res., 99:16229–16236, 1994. (doi:10.1029/94JC01402)