First the velocity profiles are differentiated with respect to depth
to eliminate the CTD-package's motion. Then a depth record is obtained
by either using the CTD-pressure record or integrating the vertical velocity
in time. Now the shear profiles are averaged together within depth bins.
Here one of the crucial steps is to carefully edit shear estimates to reject
outliers. The average shear profile is then integrated vertically to obtain
a baroclinic velocity profile. The missing barotropic correction can be
calculated if precise position of the start and end point of the cast is
known (from GPS, for example) and the velocity record is continuous in
time (Fischer and Visbeck, 1993a). Several problems are known for the LADCP
system. First, some levels close to the bottom of the profile, but still
several hundreds of meters above the sea floor, will not have any useful
data if the ping rate is constant. At those levels the bottom reflection
of the previous ping arrives at the same time as the backscatter from the
current ping and results in ambiguous Doppler shifts. This problem can
be reduced by employing asynchronous ping rates (T. Chereskin, pers. comm.
1996). Furthermore, if the CTD package approaches the sea floor the velocity
recording fails altogether since the minimum distance for velocity profiling
is about 20--30~m. Both of those data gaps can significantly degrade the
final velocity estimate. Finally, the range of the individual profiles
in the deep ocean is significantly reduced due to dramatically decreased
abundance of scatterer. As a consequence the number of shear estimates
per depth cell drop and problems arise in the the detection of statistical
Note that version 5 and later of the MATLAB software uses a new approach without
ever computing velocity shears. It also allows to incorporate bottom track
data (Visbeck, 2002).
The LADCP system and data processing is still under development and this
web page is meant to be a forum of discussion for new ideas and approaches.
Individual researcher have written analysis software packages and links
to those products will be soon made available:
Some LADCP data to look at
AOML section from Dominica passage , Grenada passage , Guadeloupe passage , Antigua passage , Anegada passage , Trinidad waters , Barbados waters .
WOCE ADCP Data