are currently at 28 degrees South heading into tropical waters and it
definitely feels like it! After more than a month of clouds, rain and
grey we are finally being rewarded with some warmth and sunshine.
Luckily there is still a nice sea breeze blowing and a few clouds so it
isn't too hot. This week I am going to write a bit about a very cool
instrument that I have been running on board the ship, the Lowered
Acoustic Doppler Current Profiler (LADCP; the blue and yellow instrument
in the photo). That's a bit of a mouthful isn't it?
What is an LADCP?
Let's break it down: Lowered = the instrument is lowered in the ocean
with the CTD package; Acoustic= sound; Doppler = refers to the Doppler
shift. Current = ocean current; Profiler = measures a vertical profile.
So the LADCP measures ocean velocities using sound.
How does it work?
The Doppler effect is the change in frequency of a wave when the
receiver is moving relative to the source of the wave. This often
happens when a vehicle with a siren passes by. As it approaches the
sound waves are "bunched up" and the sound appears higher pitched to
someone listening than when the vehicle is moving away and the waves
appear more "spread out". In the case of the LADCP, it emits a ping
sound at a set interval (usually once or twice a second) into the water,
which is reflected by particles in the water about the same size as the
wavelength of the sound wave, usually plankton and other small
organisms. This reflected wave is read by the LADCP instrument and the
difference in frequency between the emitted wave and the reflected wave
indicates the motion of the water relative to the instrument. This is
complicated by the fact that the LADCP instrument is not motionless but
it moving with the ship and up and down with wave motions. To get
absolute ocean velocities this motion needs to be calculated and removed
from the Doppler signal. The LADCP contains a compass and sensors to
measure the pitch and roll of the instrument so its motion can be
calculated. There are several different methods used to get absolute
horizontal velocities and they each have pros and cons. There is even
some recent work on a method using LADCP data to calculate vertical
velocities in the ocean, which are very difficult to measure because
vertical velocities tend to be very small, compared to horizontal
velocities and vertical wave motion.
The Palmer and most other research ships also have an ADCP (not lowered
with the CTD) mounted on the ship bottom, which constantly measures
velocities in the upper ocean as the ship moves. ADCP's can be used in
many other ways, including measuring the discharge from rivers, locating
underwater "tornadoes" that might damage deep water oil drilling
activities, measuring flow through sewer pipes (gross but important!)
and measuring flow of meltwater from icebergs.
What do the results look like?
The resulting LADCP velocity profile can be noisy and have quite large
errors due to difficulties with removing the ships motion. However, you
can see the speed and direction of velocities from the surface to the
seafloor. The shipboard mounted ADCP measures surface currents so we can
see a map of the surface currents along the ship track and see when we
are crossing sharp fronts with strong currents and other features.
Why measure ocean velocities?
Finally, why do we measure ocean velocities in this complicated way?
Well it is actually very difficult to measure instantaneous velocities
in the ocean. Average velocities can be calculated from the trajectories
of drifters and floats. Also we can use a relationship between the
density gradient in the ocean to velocity to determine geostrophic
velocities in the ocean. But these methods are generally only useful
for very large-scale ocean currents. LADCP data is useful to see
detailed structure in velocity profiles and see how the velocity varies
over time and in space. This is important to understanding physical
processes in the ocean on small and larger scales. It can also be used
to calculate transports of water, which are important for determining
the amount of heat, nutrients, carbon and other properties transported
by the ocean.
Stay tuned next week, as I'll be wrapping up when we arrive in Tahiti
and will be sharing a video of the cruise that I have been working on
with some other scientists on the ship!
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