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Profiling Dissipation Measurements using %pods on
Moored Profilers in Luzon Strait
James N. Mourn
Jonathan D. Nash
College of Oceanic & Atmospheric Sciences
Oregon State University
Corvallis, OR 97331-5503
ph: (541) 737-2553 fx: (541) 737-2064 email: firstname.lastname@example.org
http://mixing. coas. oregonstate. edu/
The long-tenn goal of this program is to understand the physics of small-scale oceanic processes and
how they affect the larger scales of ocean circulation. Ongoing studies within the Ocean Mixing
Group at OSU emphasize observations, interaction with turbulence modelers and an aggressive
program of sensor / instrumentation development and integration.
The principal objectives of this project are to:
• quantify the energy losses to turbulence dissipation in the Luzon Strait in a systematic,
comprehensive and extended way;
• quantify the spring-neap variation in these energy losses;
• obtain meaningful, long-tenn observations of the turbulent heat and momentum flux
profiles in Luzon Strait, from which useful parameterizations can be derived;
To accomplish these objectives, we have:
1. modified 2 McLane Moored Profilers MPs for direct and extended measurements of
2. built, deployed and analyzed data from additional fixed-point turbulence measurements
on IWISE moorings
We worked with engineers from the Applied Physics Lab at University of Washington to modify 2
MPs to house new pressure cases with analog electronics, fast thermistor sensors, analog-to-digital
conversion electronics and batteries (Figure 1).
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Profiling Dissipation Measurements using χpods on Moored
Profilers in Luzon Strait
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Oregon State University,College of Oceanic & Atmospheric
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Initial engineering tests in December 2009 in Puget Sound prompted refinements which were
incorporated into the unit deployed in Luzon Strait for the 2010 field campaign. Following this
successful deployment (Figure 2), an additional MP was modified and 2 units deployed in Luzon Strait
for the 2011 field campaign.
Figure 1 - turbulence-resolving /pods on one (of 2) Moored Profilers deployed in Luzon Strait in
summer 2011. Two chipods point UP on the upper part of the MP, two point DOWN on the lower
part. The UP/DOWN concept permits a measurement of undisturbed flow when the MP
profiles both UP and DOWN.
An additional 5 individual moored chipods were deployed in the IWISE 2011 mooring array (Al, Nl,
N2). Two of these were deployed at 2000 m depth below the MP chipod units, a depth greater than all
previous chipod measurements. These were equipped with a new pitot sensor designed to provide
mean flow speeds (as well as turbulent velocity fluctuations) for incorporation into chipod temperature
variance dissipation rate calculations.
Summary examples of data results are shown in Figs. 2-5.
m 0 04
21 Jul 11 21 Aug
Figure 2 -Continuous deep-ocean turbulence profiling measurements using turbulence resolving
chipods on a moored profiler. The upper panel shows vertically-averaged kinetic energy and the
bottom panel turbulence kinetic energy dissipation rate (s). These measurements were made on
mooring N2 in Luzon Strait in summer 2011.
Figure 3 -Summary of measurements from moored profiler at N2 in Luzon Strait summer 2011
shown in Fig. 2. Upper panel shows vertically-averaged KE in 200 in intervals. Beneath is shown
vertically-averaged dissipation rate over the same intervals.
21 Jul 10 20 Aug
Figure 4 -Summary of measurements from mooring A1 in Luzon Strait summer 2011. Upper panel
temperature with locations of 2 chipods indicated by the dashed lines. Beneath are shown vertically-
averaged kinetic energy plus dissipation rates from each chipod.
• Chipod at 600 m
• Chipod at 680 m
•• . .
• + •
• •. \ *
• • -
.• •• •
log 10 (l4) [nAs 3 ]
Figure 5 -Comparison of daily-averaged dissipation rates from Fig. 4 and cubed
A new velocity measurement: A fundamental requirement to compute /j and e x from temperature
gradient measurements is the flow speed past the sensor tip. This is necessary to convert measured
frequency spectra to wavenumber spectra to which a universal form is compared by scaling (Mourn &
Nash 2009). A significant part of this signal is the motion of the cable itself (Perlin and Mourn 2012),
but it also requires a measure of the current speed, which to date has required xpods to be deployed
near a stand-alone velocity sensor. In some cases, this requirement has been inconvenient as it controls
the xpod placement, and it is possibly the weakest part of the measurement, only partly because the
velocity sensors are typically sampled too slowly. To address this issue, we have devoted considerable
effort to develop a local measure of velocity using a differential pressure sensor - a pitot tube. The
design is based on that used by Mourn (1990) to measure turbulent velocities with our vertical profiler
Chameleon. The basic problem with previous incarnations was the absolute pressure-dependence of
almost all differential pressure sensors - that is, increased ambient pressure causes an increased
voltage in the absence of any differential pressure. This essentially excludes the sensor from
measurement of mean speed.
We have recently found a new differential pressure sensor that minimizes this common-mode pressure
signal, have characterized its (small) common-mode pressure dependence and temperature dependence
and have carried out extensive wind tunnel testing. The first deployment was on a mooring in Luzon
Strait in summer 2011 at 2000 m depth (Fig. 6). The comparison of velocity from a nearby ADCP with
large spatial and temporal averaging is very good (Fig. 7). Furthermore, the high-frequency part of the
signal (sampled at 50 Hz) indicates the presence of an inertial subrange coincident with turbulence
sensed by the temperature sensor on the %pod. Comparisons of s computed directly by scaling the
inertial subrange of the velocity spectra to e x derived indirectly from x show statistical agreement on 5-
minute spectral estimates within 20%. Another advantage of the complementary measurement of
velocity fluctuations is evidenced just past 18:00 in Fig. 6 where the bottom boundary layer became
well-mixed, reducing temperature stratification and thereby precluding any assessment of turbulence
based on temperature gradient fluctuations alone
-pitot 100 Hz
- adcp 5 minute data
pitot 5 minute average
03:00 06:00 09:00 12:00 15:00 18:00 21:00
06 August 2011
Figure 6-A single day of a 2-month time series from a xpod moored at 2000 m depth on mooring
N1 in Luzon Strait, a) temperature; b) time derivative of temperature (dT/dt), from which // is
computed; c) velocity from a nearby ADCP (red) and from a pitot tube on the xpod (black at 50 Hz,
yellow averaged to match ADCP). The pitot velocity was derived from a static lab calibration of
pressure and Bernoulli, u — J2p/p.
Figure 7 - Mean and turbulent velocities from a high-speed pitot tube on xpod. a) direct
comparison of ADCP and pitot velocities; b) velocity spectrum computed from 30 minutes of data
from the pitot tube starting at 18:00. Estimates of s derived from inertial subrange fits to the velocity
spectrum agree statistically to within 20% of estimates of s y from temperature gradient
measurements on the same xpod (Figure 6b).
The additional estimate of s from velocity measurements on %pods is a bonus. However, the most
significant result is that the local measurement of current speed makes %pod a standalone measurement.
Where velocity measurements do not exist, this provides them.
The development of a chipod for moored profilers offers a new way to obtain long time series of
turbulence in the deep ocean. Development of a new, small, inexpensive, low power velocity sensor
offers simultaneous measurements of both current speed and turbulence.
The dissipation measurements derived from the chipods on moorings Al, N1 and N2 are being used to
validate inferred estimates of turbulence from other platforms. Together, these are contributing to
larger IWISE effort by providing a temporal and spatial assessment of dissipation and mixing in Luzon
Strait. Some of the measurements are currently being used by Byungho Lim as part of his MS thesis
project. This project has relied on the IWISE mooring project of Matthew Alford (APL/UW).
The development of our new velocity measurement of both mean+turbulence scales has led to
additional deployments of two new chipods (plus 6 high-resolution pressure pods) in Cordova Channel
off SE Alaska as part of a Naval Research Lab experiment on Breaking Wave Effects under High
Winds (BWE; David Wang, Paul Hwang, Hemantha Wijesekera). Deployments will take place
Mourn, J.N., 1990. Profiler measurements of vertical velocity fluctuations in the ocean, J. Oceanic
Atmos. Techno/., 7, 323-333.
Mourn, J.N., and J.D. Nash, 2012. Mixing measurements on an equatorial ocean mooring, J.Atmos.
Oceanic Techno/. 26, 317-336.
Perlin, A. and J.N. Mourn, 2012: Comparison of thermal dissipation rate estimates from moored and
profiling instruments at the equator. J.Atmos. Oceanic Techno/., 29, 1347-1362
Smyth, W.D. and J.N. Mourn, 2012: Ocean Mixing by Kelvin-Helmholtz instability. Oceanography,
25(2), 140-149. [published, refereed]
Nash, J.D., S. Kelly, E.L. Shroyer, J.N. Mourn and T. Duda, 2012. The unpredictable nature of internal
tides on the continental shelf. J. Phys. Oceanogr., doi:10.1175/JPO-D-12-028.1. [published,
Stoeber, U. and J.N. Mourn, 2011. On the potential for automated realtime detection of nonlinear
internal waves from seafloor pressure measurements. Appl. Ocean Res., 33, 275-285.
doi:10.1016/j.apor.2011.07.007. [published, refereed]
James N. Mourn, Oregon State University, Fellow, American Geophysical Union, 2012