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TURBULENCE @ OCEAN OBSERVATORIES
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Turbulence and observatories
Vertical turbulent transports of momentum, mass, chemical species and
particles play major, often dominant, roles in a range of processes spanning all
the sub-disciplines of oceanography – processes as fundamental and diverse as
sediment resuspension, biological primary production, and particle/contaminant
dispersion, to name but a few [Want more information? #1]. Significant turbulent
transfers are strongly variable in time, not only in estuarine and shelf
environments, but also in offshore surface and bottom boundary layers, and in
tidally-driven deep-sea regimes. Attempts to understand such fundamental
questions as the response of marine ecosystems to anthropogenic forcing,
whether imposed on local (ie changes in coastal environments caused by landuse changes) or global (ie climate change) spatial scales, require the
characterization of turbulence over the full range of operative time scales. Shipbased observational programs, typically of order a few to several weeks, can't
provide the information necessary to fully describe the modification of turbulence
and its effects by seasonal or longer term variability of atmospheric forcing, much
less by highly episodic, extreme events that may dominate such processes as
sediment resuspension and on/offshelf tranports of bio-active material.
[#1 For more information on the fundamental importance of turbulence to a wide
variety of ocean processes, see Ocean Sciences at the New Millenium
www.ofps.ucar.edu/joss_psg/publications/decadal ), a report of the National
Science Foundation 2001]
Present techniques for measuring ocean turbulence are often extremely
labor-intensive (hence expensive), and use sensors that are highly sensitive to
marine fouling. As a result, these techniques are not suitable for the long-term
monitoring that is needed to support studies of the role of turbulence in both
natural and anthropogenic time variability of marine systems. What is needed are
turbulence tools designed for deployment at long-term ocean observatories [want
more information? #2], cabled or moored platforms that can provide required
levels of power supply and data transmission rate. Ideally such tools would be
relatively insensitive to biofouling, run continuously and cheaply with minimal
human intervention, and provide estimates of turbulence parameters over a
range of depths, rather than at a single point. A system consisting of a single 5beam acoustic Doppler current profiler (VADCP) can potentially provide this
essential ability, a potential inherent in a number of recently developed
techniques which use standard acoustic Doppler measurements in non-standard
ways. Our work aims to move these new techniques out of the low-noise, high-
signal environments in which they have been developed, first into coastal
environments and eventually into deep-ocean environments.
[#2 For more information on ocean observatories, see
Illuminating the Hidden Planet: The Future of Seafloor Observatory Science
www.nap.edu/books/0309070767/html/), a report of the National Research Council,
2001]
Why cabled observatories first?
Our first step is deployment of a VADCP at a cabled observatory, a
subsea junction box (node) that is connected to shore by an electro-optical cable
buried under the sea floor. Cabled deployment is essential at this stage because
of the power required and data rates produced by continuous operation of the
Doppler system. Continuous operation over several months is highly desirable in
order to provide multiple realizations of "normal" turbulence-generating
mechanisms, such as storms, tidal flows etc, while even longer records may be
necessary to illuminate the relative importance of episodic extreme events.
Eventually, extensive experience with temporally well-resolved fields at cabled
observatory sites will make it possible to suggest and evaluate statistically
meaningful measures of turbulent and ecosystem quantities that can be made
under the power and storage constraints associated with non-cabled systems.
Deployment at LEO-15
Starting in April 2003, the VADCP has been deployed at the outermost (B)
node of LEO-15 (http://marine.rutgers.edu/mrs/LEO/LEO15.html), a cabled
observatory off the coast of New Jersey that is operated by NOAA's Mid-Atlantic
Bight National Undersea Research Center and Rutgers University.
The shore station for LEO is the Rutgers University Marine Fieldstation
(http://marine.rutgers.edu/rumfs), located in a refurbished Coast Guard Station
near the mouth of Great Bay near Tuckerton, New Jersey.
Despite being at the end of ~ 7 km of cable, the topography (or lack of it!) seen in these
above-water photos continues offshore, so that Node B is in only 15m of water. This
water depth allows resolution of the entire water column with the range of a 1.2MHz
ADCP.
The location of the LEO observatory is characteristic of the very gently sloping
continental shelves that extend along most of the eastern seaboard of the United
States, in places extending 100s of km offshore. LEO is also characteristic of strongly
surface-wave-influenced shelf environments with significant supply of sediment from
the bordering land. The gently undulating subsurface topography that surrounds Node B
can undergo substantial re-organization as bottom sediments remobilize during storm
events. For this reason, "bottom-mounted" instruments are actually attached to pipes
driven ~ 2 m into the bottom.
Instruments deployed at LEO-15
A package containing a 1.2MHz 5-beam VADCP is mounted near the edge of the
area (~ 25 m radius) around Node B that is protected by guard buoys, and connected to
the node by a short underwater cable. At installation, the height of the transducers off
the bottom was ~ 0.6m. However since this depth can change with time, the package
also contains a higher frequency (5.0MHz) acoustic backscatter sensor (ABS) oriented
downwards to track bottom location (provided it stays beneath the package - if it doesn't
we have other problems!).
The water column at LEO is unstratified during winter, but becomes strongly
stably stratified during the summer heating season. Proper interpretation of turbulence
measurements in a stratified fluid requires a quantitative measure of stratification, which
will be provided by the combination of a string of closely spaced thermistors, sampling
frequently in time, and a profiling CTD, operated a few times a day.
See the latest data from the LEO-15 installation
This link is to the most recent data received from the VADCP, displayed here to allow us
to check on performance of the installation. You will see 6 color-coded fields, beam*
velocities from all 5 beams, as well as the backscatter amplitude from the vertical beam,
which is used to track the position of the ocean surface. Each measurement (ping) is
displayed sequentially in time (horizontal). Bin number increases with distance from the
transducer.
* Beam velocity = an estimate of the water velocity parallel to a Doppler beam, positive
if towards the transducer. Beam velocities can be put together in various ways to
produce estimates of the mean velocity components, as well as a number of turbulence
characteristics.