Post on 30-Dec-2015
description
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
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