What do we measure and how?
ADCP attached to CTD frame
ADCP attached to CTD frame
Close up of ADCP
Acoustic Doppler Current Profilers
Submesoscales are not only associated with abrupt density fronts but also with strong, surface-intensified currents that flow parallel to them. Using acoustic technology we are now able to accurately measure current speeds and direction over large vertical distances and with high vertical resolution. The current meter we used during JR311 was an RDI Ocean Surveyor 75kHz acoustic Doppler current profiler, commonly referred to as an ADCP, that is permanently installed in the hull of the RRS James Clark Ross and points downwards into the water beneath.
In simple terms, the ADCP emits a series of acoustic pulses, referred to as pings, in specific directions at a frequency of 75kHz and listens to the echoes that are reflected back to the instrument by scatterers in the water column. The scatterers can be anything like plankton or suspended particles like sediment and are assumed to be moving at the same speed as the water that hols them in suspension. The frequency of an ADCP is important, as it is the change in frequency between the ping and its echo that we use to determine the current velocity. As the scatterers are moving in the water relative to the ADCP, the echo returns at a slightly different frequency, a phenomenon called the Doppler effect. An example of the Doppler effect is the change in pitch of a train heard by a person on the platform as the train passes. In the case of an ADCP, the change in frequency of the echo is proportional to the velocity of the scatterers either towards or away from the ADCP. Through the application of some relatively complicated trigonometry, the ADCP then converts the along-beam velocities to earth coordinates with the final output being eastward, northward and upward velocities.
A particularly powerful ability of the ADCP is its ability to measure current velocities at a range of distances from the instrument itself. As the speed of sound in water is known (and is about 1454 m s-1), the ADCP knows from how far away a ping has been echoed due to the time it takes for the instrument to hear the echo. The further away the scatterers are, the longer it takes for the echo to return. Thus the ADCP is able to measure currents in ‘bins’, which for the OS75 that we used in JR311 were 8 m in vertical extent. The ADCP is therefore equivalent to a strong of current meters spaced at 8 metre vertical intervals reaching up to 800 metres below the surface.
Whilst we don’t have a picture of the ADCP mounted in the hull of the ship, you can see in the photos above, a similar device that is attached to the CTD frame. This is referred to as a Lowered ADCP and works in a very similar way to the vessel mounted version.