Direct Observations and Quantification of vertical Buoyancy Fluxes:

1
Direct Observations and Quantification of
vertical Buoyancy Fluxes Results From Dye
Experiments
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PECS 2006 From Processes to Prediction Our
knowledge of estuarine, coastal and nearshore
physics is continually improving. We seek,
therefore, not merely to capture mechanisms and
quantify processes, but to predict future events
from dynamically realistic models. PECS2006,
convened 18-22 September 2006 at the beautiful
Columbia River Maritime Museum in Astoria, OR,
will bring together the world's leading
scientists in estuarine and coastal physical
oceanography to discuss and define the
state-of-the-art and future directions for the
field. We plan to use 1-2 dedicated issues of
Continental Shelf Research to publish refereed
papers resulting from the conference. Papers from
the 2000 PECS Conference in Norfolk, Virginia
appeared in this journal. Among the journals we
considered, CSR offered the best combination of
relevance, impact, and affordability.
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Merely capturing and quantifying estuarine
Buoyancy Flux.
JPO, Accepted
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Hudson Dye Experiments May,2002
0 5 10 15
m
0 0.5 1.0 1.5

km
Dye Injection Sites Moorings
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Salt section 1-day prior to neap tide injection.
km from the Battery

• Measure Buoyancy flux and determine mechanism.
• 2) Develop model for detailed analysis of
  buoyancy flux.
• 3) Use 1 and 2 to estimate mixing efficiency (Rf).

Normally inferred from turbulence velocity.
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River Discharge
River Discharge
Stratification and Tidal Range
DS
Tidal Range
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Dye Conc. 10-11
Boundary Layer Evolution on neap tide flood
1326-1429
Early Flood
1601-1656
Mid Flood
1853-1936
Late Flood
Dye moves upstream faster than isohalines due to
Mixing!
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Boundary layer is stratified. Boundary layer
extends To velocity maximum. Ri 0.25 1
in boundary Layer, Continuous increase
in stratification.
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Date Based on Salt Observed we
May 5th .23 mm/s .19 mm/s
May 22nd .41 mm/s .38 mm/s
Boundary Layer Grows due to Mixing Not Flow
Convergence
Hour
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Date Model dh/dt Observed dh/dt
May 5th .19 mm/s .17 mm/s
May 22nd .38 mm/s .38 mm/s
Issues
uncertainty in u
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c Dye Concentration w Vertical Velocity h
Boundary Layer thickness f- Structure Function
Trowbridge 1992 boundary layer model
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Vertical structure
Eddy Diffusivity
U 2 cm/s N.08 s-1
1
z
0.5
0
2
0
1
cm2/s
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Spring/neap variability in Buoyancy Flux
Friction Velocity
Dr
u2N
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Freshening rate of dye is direct measure of
boundary layer averaged buoyancy flux
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Shear Production (line) and 5 buoyancy Flux (dot)
a
b
c
Scatter plot of P and B. Slope is .14 (Rf) 90
confidence limits place .1ltRflt.18
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Flux Richardson Number From Model
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Blue and Green lines Peters Bokhorst
(2001) With functionality from Rohr et al.
1988 Red line this model
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Conclusions
1) Diapycnal mixing and boundary layer growth
primarily due to entrainment. 2) Tidal
straining plays minor role in defining flood-tide
boundary layer height (role of straining
increases with Horizontal Richardson
number). 3) Turbulent vertical buoyancy flux
0.11u2NBL. 4) Flux Richardson number
0.1-0.16 and 0.22(RiBL)1/2
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Decreases