Turbulent Mixing During an Admiralty Inlet Bottom Water Intrusion Philip Orton Hats off to the...
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Transcript of Turbulent Mixing During an Admiralty Inlet Bottom Water Intrusion Philip Orton Hats off to the...
Turbulent Mixing During an Admiralty Inlet Bottom
Water Intrusion
Philip Orton
Hats off to the A-Team:
Sally, Erin, Karin and Christie!
Profs extraordinaire: Rocky and Parker!
Motivation - Why Study Mixing/ Dissipation
sigma-t (kg m-3)
Echo Sounder Backscatter, 120 kHz, 04-Aug-2006, 11:28h
• Power/ importance • Difficulty for modeling
sorted profile raw profile
Plan-of-Attack
• Methods - dissipation/mixing estimation• Along- and across-channel comparisons• Consistency check: Observed dissipation vs
Expected?• Dynamical explanation for weak mixing
H0: Mixing during our study was spatially uniform
test: Compute buoyancy flux at many locations in along- and across-channel surveys
Field Program
88W
300kHz ADCP
Seabird 19 CTD
Echo Sounder
Full transect
Two half-transects
Cross-channel survey
Bush Point
Fine-Structure Instability Turbulence Analysis
A “Thorpe scale” analysis of ~138 CTD density profiles
The Thorpe scale (LT) is the rms re-sorting distance of all points in an overturning “patch”.
Method gives comparable results to microstructure instrumentation (e.g. Klymak and Gregg, JPO 34:1135, 2004).
Matlab mixing toolbox for CTD fine-structure and Lowered-ADCP
sorted profile raw profile
Mixing & Dissipation from Thorpe Scales
322 NLa T
NLaK T22
where a ≈ 1 (Klymak and Gregg; Peters and Johns, 2004)
We assume a mixing efficiency, ≈ 0.22, reasonable for stratified conditions (discussion in Macdonald and Geyer, JGR 109: C05004, 2004).
buoyancy frequency, N = [(g/d/dz)]0.5, is computed over overturn patch heights.
Dissipation of turbulent kinetic energy:
eddy diffusivity:
Station 16, 8/4 15:17h, slack after greater flood
Assume: (a) LO = LT, (b) LO is length-scale for TKE, (c) N is time-scale for dissipation.
Richardson Number, Ri = N2/Shear2
Ricrit= 0.25
Transect #1
FLOOD!
Transect #2
weak ebb
Transect #3
weak flood
Buoyancy Flux, B = N2K
Transect #1
FLOOD!
Transect #2
weak ebb
Transect #3
weak flood
Along-Channel Variability?
W/kg
Across-Channel Variability?
W/kg
Consistency Check: Tidal Dissipation
• Dissipation mean (away from bed) over entire study was 6.4 x 10-4 W/m3
• Hudson has mid-water column values of 10-2 (spring) to 10-3 W/m3 (neap; Peters, 1999)
• NOAA study (Lavelle et al., 1988) showed total tidal dissipation averages ~500 MW
• I estimate the total dissipation during our study as overturns + loglayer = 12 + 112 = 124 MW– assumed log layer dissipation ( ~ U*
3)– quad drag law: CD = 0.002 for velocity at 5-10m height
• This is reasonable, as our tidal range was ~3/4 the mean, U ~ range, ~ U3, and (3/4)3 = 0.4
Why Weak Mixing in Most Places?
Results suggest low mixing because tidal straining is overcoming mixing
horizontal
Richardson
(Stacey)
number, Rix
ebb EBB
Summary
• Was mixing during our study spatially uniform?– Cross-channel variability: results were inconclusive– Along-channel variability: No -- mixing was elevated
by a factor of O(10) in at least one hotspot
• Tidal dissipation estimates were consistent with a prior study, downscaled for below avg. tidal range
• Tidal straining can explain the low mixing that occurred in most of the estuary
• Excellent conditions for a bottom water intrusion!
Overturn Analysis: Quality Control
To avoid mistaking noise for overturns, each “resorting region” must pass various tests:
1) the rms (t,sort - t,raw) in a patch must be greater than the instrument noise ( = 0.002 kg m-3)
2) the T-S space tests of Galbraith and Kelley (J-Tech, 13:688, 1996)
a) near-linearity in the T- relationshipb) near-linearity in the S- relationship
3) rms run-length of overturn patch must be longer than 7 points total
Ambient Conditions
• Tides - end of a ~5 day period of weaker than normal tidal currents– Semidiurnal tidal range near annual low
– Diurnal tidal range on the rise, but below average
• Winds light• Riverflow into Puget Sound - [likely had an above
average summertime flow]