Introduction

Full depth velocity profiles of ocean currents are useful for many applications in physical oceanography ranging from direct observations of the large scale ocean circulation, over investigations of individual processes that control ocean dynamics down to aspects of small scale mixing. For large scale flows in the oceans interior one can assume that the currents are in geostrophic balance and deduce them indirectly from the density filed. However in the equatorial region, close to boundaries, within straits and in general on small scales the geostrophic assumption might not hold and direct velocity measurements become increasingly important. Several methods exist to obtain deep velocity profile one of which is the lowered acoustic Doppler Current profiler (LADCP) that allows to measure full ocean depth velocity profiles during a Hydrographic station. It takes no extra ship time to obtain the measurements since the profiler is part of the CTD/water-sampler package and requires minimal operating effort. The low demands on ship time and running costs makes the LADCP system attractive and consequently a large increase in usage was observed over the last few years.

Figure caption: A few photographs are displayed from a recent test cruise for the new LADCP-2 system based on two RDI-WHM300 work horse instruments. The standard system RDI-BB150 is the bigger instrument in the center of the package pointing downward. The sketch illustrates roughly how individual velocity profiles are patched together to obtain the full ocean depth final velocity profile.


The LADCP system

The history of the LADCP system is quite young but the low cost of operation and ease of use made it attractive to many sea going researchers. Traditionally one self contained ADCP with a frequency of either 150 or 300 kHz has been attached to a CTD package with the acoustical transducers facing toward the ocean floor (See figure above). Every second an acoustical pulse is transmitted into the water and the backscattered signal along each of the four beams is analyzed. The strength of the signal allows to map changes in the backscatter environment (e.g. Fischer and Visbeck, 1993b) and is also useful to monitor the instrumental performance. The Doppler frequency shift of the the backscatter is transformed into a velocity estimated and the range gating allows to probe at different distances for the transducer heads. The range of useful velocity data for each single ping is about 100--300~m depending on backscatter conditions. A velocity profile is obtained along each of the four beams which are inclined from the vertical by 20 or 30 degrees. Using pitch, roll and a compass these profiles are then converted to earth coordinates and stored internally. Often 2--10 profiles are averaged in order to reduce the data volume during each station of 2--6 hours duration. This raw data set is then down loaded from the instrument once it is back on the research vessel. The data processing is usually done right after the cruise and within 30 minutes after the cast the final velocity profile can be calculated in the following way.

Martin Visbeck is now developing an improved LADCP2 system.

How to process the data

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 outliers.

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: