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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: moum@coas.oregonstate.edu 

http://mixing. coas. oregonstate. edu/ 

Award#: N00014-09-1-0280 


LONG-TERM GOALS 

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. 

OBJECTIVES 

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; 

APPROACH 

To accomplish these objectives, we have: 

1. modified 2 McLane Moored Profilers MPs for direct and extended measurements of 
turbulence, and 

2. built, deployed and analyzed data from additional fixed-point turbulence measurements 
on IWISE moorings 

WORK COMPLETED 

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 

Sciences,Corvallis,OR,97331-5503 

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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. 


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RESULTS 


Summary examples of data results are shown in Figs. 2-5. 


_ 0.08 

CM 

> 0.06 

m 0 04 
0.02 



E 


Q- 

a> 

Q 


1200 

1300 

1400 

1500 

1600 

1700 



21 Jul 11 21 Aug 

2011 




CO 


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. 


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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 

2011 


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. 


4 








































10 - 


10 : 


10 - 


10 


• Chipod at 600 m 

• Chipod at 680 m 


• •* 

•• . . 
• + • 

• •. \ * 

• • - 


•• • 

.• •• • 
• • 


10 


log 10 (l4) [nAs 3 ] 


Figure 5 -Comparison of daily-averaged dissipation rates from Fig. 4 and cubed 

barotropic currents. 

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- 


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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 


_l_L 




dT/dt 

.Ik...., 


-pitot 100 Hz 

- adcp 5 minute data 

pitot 5 minute average 

- S\-Iit*s****^^ 



-0.05 

c 

0.5 

'to 

"e 0.25- 


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. 


a 


0.6 



E 


■o 

0 

0 

Q. 

</) 



0 




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). 


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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. 

IMPACT/APPLICATION 

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. 

RELATED PROJECTS 

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 
October 2012. 

REFERENCES 

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 

PUBLICATIONS 

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, 
refereed] 

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] 

HONORS/AWARDS/PRIZES 

James N. Mourn, Oregon State University, Fellow, American Geophysical Union, 2012 


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