Direct Mixing Measurements using χpods in IWISE
Profiling Dissipation Measurements using χpods on Moored Profilers (Moum/Nash)
Shipboard LADCP/χpod profiling of Internal Wave Structure (Nash/Moum)
Jonathan D. Nash & James N. Moum
College of Earth, Ocean and Atmospheric Sciences
Oregon State University
with help from: Byungho Lim (OSU), Andy Pickering and Matthew Alford (APL-UW)and thanks to Ming-Huei Chang, Maarten Buijsman, Luc Rainville, Alexander Perlin, Ray Kreth, Mike Neeley-Brown, John Mickett, Eric Boget, Amy Waterhouse, Zoe Parsons, Jen MacKinnon, Harper Simmons
ObjectivesGeneral• quantify turbulence dissipation where large amplitude
internal waves are generated
Particular• capture the energetics of the largest scales that directly
extract energy from the barotropic tides • while simultaneously measuring mixing associated with the
turbulence that occurs at millimeter and millisecond scales. • through direct observation, to assess the means by which
waves form and break, elucidate the structure/evolution of the wave breaking, and quantify the dissipation that induces irreversible mixing
Methods• χpods on moorings
Stablemoor
Methods• χpods on moorings
• χpod-like devices on moored profilers
Methods• χpods on moorings
• χpod-like devices on moored
• χpod on shipboard CTD for full ocean depth turbulence profiling
χpod -LADCP
Methods• χpods on moorings
• χpod-like devices on moored profilers
• χpod on shipboard CTD for full ocean depth turbulence profiling
• fabricated and deployed 5-component array of moorings to capture the 2D evolution of the larger-scale dynamics
MP χpods
{
data example
Kraichnan theoretical spectrum
εχ=N2 χ/(2 Γ Tz
2)
Tested – Puget Sound 2009Refined for 2010 (MPN)Deployed 2011 on N1, N2 all data returned
Profiling Dissipation Measurements using χpods on Moored Profilers
Products → LT Thorpe (overturn) scalesχT temperature variance dissipation rateKT turbulence diffusivity (Osborn-Cox)ε TKE dissipation rate (indirect)Km turbulence viscosity
fast thermistors on APL MP
1st continuous deep-ocean profiling experimentmesoscale current (Kuroshio?) dominates 2nd half• elevated turbulence at base of current • is this friction on a western boundary current?
χpod
speed sensor at low f
ε=2x10-7 m2s-3
dissipation sensor at high f
mooring N1 – χpod at 2000 m
1 day time series
inexpensive, lightweight, low power, standalone velocity sensor
characterization of sensor includes tests in • wind tunnel • tidal channel• P / T chambers
New mean speed / dissipation sensor for use on χpods and in general on moorings
compensated pitot
tube
leading to a new GustT combination probe
not acoustic, hence requires no
scatterersquiet
2nd continuous deep turbulence profiling time series
N2 (1830m water depth)
2011
2 units constructed and deployed – both worked – only 1 MP profiled
Chipod-LADCP-CTD
Above: TKE dissipation rate from LADCP/chipods (green) and Thorpe analyses (blue) at one of the most energetic stations sampled during IWISE.
directturbulence
(green)
inferredturbulence
(blue)
OSU Ocean Mixing χ-pod/LADCPdirect measurements of abyssal turbulence from standard shipboard CTD. permits rapid deep profiling direct turbulence differs from that inferred from overturnslow noise-floor (but N2-dependent)
fast-T3-axis accel3-axis gyrocompassUSB-data
Nash & Moum
MPchipods
1) broadly-distributed dissipation on the east ridge
2) big breaking lee waves on the west ridge
eastwest
contrasting structures from detailed measurements at 2 ridges
mid-column dissipation not dominated by a single breaking wave…
A1 – mid-column dissipation at the generation site 1440 m water depth
Byungho Lim
A1 – mid-column dissipation at the generation siteobservation / model comparison (MITgcm /
Buijsman)
similar tidal fields, but water-column instabilities are not captured by MITgcm and model dissipation is mostly near the bottom.
Byungho Lim
T-Chains on the West Ridge
Buijsman et al
T1T2
T3T4
N2
30-50 m sensor spacing to detect overturns
2-sec sampling to capture inertial subrange
Vertical synopticity (test sampling schemes of other platforms)
3 months data
700 m
500 moverturns
waves
west
east
eastwardwestward
Jonathan Nash
Mooring T3 during spring + diurnal inequality
T1T2
T3T4N2
T-Chains on the West Ridge spring tides / diurnal inequality
Mooring T3 during neap/semidiurnal period
T1T2
T3T4N2
neap tides / semidiurnal period
T-Chains on the West Ridge
T1T2
T3T4N2
Dissipation tied to lee waves
Strong spring/neap changes
Isopycnals displaced down in the mean?
Lee-wave shifts closer to ridge crest during neaps?
Springs(diurnal)
Neaps(semidiurnal)
Time – mean structure / east ridge
T1T2T3T4
N2
T1T2
T3T4N2
dissipation scales with u3bt
(nonlinear!)
consistent with Klymak et al (2010)’s “recipe” for ε over a supercritical ridge
… u3 because flux into trapped lee waves ~( ubt x u2
bt ) …
ε~ u3bt
Dissipation evolution and scaling
Summary Results
• 1st continuous turbulence profiling away from ship-based upper ocean measurements
• χpod-CTD measurements have led to beginning of contribution to Global Repeat Hydrogaphy Program
• NEW VELOCITY SENSOR - speed + turbulenceleading way to new possibilities
• observational confirmation of Klymak etal (2010) ε scaling
• breaking waves: vertically-integrated ε O(1 W/m2)comparable to flux divergence 5 kW/5 kmsuggests significant local dissipative losses
• vertically-distributed turbulence may be difficult to model but significant to water mass mixing through vertical flux divergence
Summary Results
continued contributions to NRL field scienceMORT Mixing Over Rough TopographyBWE Breaking Wave Effects in High Winds
technological:• loan of OSU-developed instrumentation• technical-level analysis
scientific:• participation in science-level analysis• contribution to publications
LT – large-eddy length scale statistic simply computed from 1D profiles - but an imperfect statistic in an evolving 3D field
Lo – large-eddy length scale defining buoyancy limit on turbulenceLo = √(ε/N3)
if LT = Lo, then ε = LT2
N3
is LT = Lo ?
Moored profiler χpod estimates of turbulence dissipation rate, ε
Moored profiler χpod estimates of turbulence dissipation rate, ε
LT – large-eddy length scale statistic simply computed from 1D profiles - but an imperfect statistic in an evolving 3D field
Lo – large-eddy length scale defining buoyancy limit on turbulenceLo = √(ε/N3)
if LT = Lo, then ε = LT2
N3
is LT = Lo ?
same data – different definition of N2
How do we know χpods work?
5 χpods on TAO mooring yields 5 time series of χ, ε
7 χpods on EQUIX mooring yields 7 time series of χ, ε
24h continuous profiling of χ, ε 6-10 profiles/h
16-day experiment at 0, 140WOct/Nov 2008
Equatorial Internal Wave Experiment 2008
Perlin & Moum, 2012 JAOTech
pod
How do we know χpods work?
Perlin & Moum, 2012 JAOTech
χ ε
profilerχpodsχpods
pod
Comparison of ε computed from χpod and from pitot tube
A1 – mid-column dissipation at the generation site
Observation / Model comparison at T3
Andy Pickering
Observation / model comparison at T3
model does a pretty good job with the vertical distribution and daily-averages
details are a little different
Andy Pickering
T1T2T3T4
N2
MITgcm / Buijsmann et al 2013
Dissipation evolution / compare to MITgcm
T1T2
T3T4N2
mesoscale current (Kuroshio?) dominates 2nd half elevated turbulence at base of current is this friction on a western boundary current?
First continuous deep turbulence profiling time series
MP-N 2010
diurnal composite / spring
semidiurnal composite / neap
T-Chains on the West Ridge
spring diurnals vs. neap semidiurnals
ConclusionsBuijsman et al
log10 εobserved
Integrated Dissipation from big breaking waves contributes O(1 W/m2) vertically-integrated ε this suggests ΔFε= 5 kW/m in 5km… ε is significant to FE!
ε~ u3bt
Distributed Mixing (detached from bottom) is difficult to model
ε~ u3bt
Can models accurately capture mid-column ε and
its vertical distribution?
Can we assign errorbounds on model ε?
Summary Results
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