Direct Observations and Quantification of vertical Buoyancy Fluxes
Direct Observations and Quantification of vertical Buoyancy Fluxes: Results From Dye Experiments Bob Chant, Rocky Geyer 2, Bob Houghton 3 Eli Hunter 1, Jim Lerczak 2 Rutgers University 2 WHOI 3 Lamont Doherty 1
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. PECS 2006, 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-theart 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.
Merely capturing and quantifying estuarine Buoyancy Flux. Bob Chant, Rocky Geyer 2, Bob Houghton 3 Eli Hunter 1, Jim Lerczak 2 JPO, Accepted Rutgers University 2 WHOI 3 Lamont Doherty 1
Hudson Dye Experiments May, 2002 Channel Cross-section at mooring array Hudson River 0 m New York 1 2 3 4 5 10 15 0 0. 5 km Dye Injection Sites 3 1 New Jersey Atlantic Ocean Moorings 2 4 GWB 1. 0 1. 5
Salt section 1 -day prior to neap tide injection. km from the Battery 1) 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.
River Discharge Date-mean Discharge based on 50 year record Stratification and Tidal Range #1 #2 #3 DS Tidal Range #4
Boundary Layer Evolution on neap tide flood 13: 26 -14: 29 Early Flood 16: 01 -16: 56 Mid Flood Late Flood 18: 53 -19: 36 Dye moves upstream faster than isohalines due to Mixing! Dye Conc. 10 -11
Boundary layer is stratified. Boundary layer extends To velocity maximum. Ri # 0. 25 – 1 in boundary Layer, Continuous increase in stratification.
Boundary Layer Height May 22 nd 8 m Date Based on Salt May 5 th . 23 mm/s. 19 mm/s May 22 nd . 41 mm/s. 38 mm/s 0. 31 mm/s 6 m 4 m Observed we May 5 th 0. 12 mm/s h we=dh/dt 2 m Flood Hour Boundary Layer Grows due to Mixing Not Flow Convergence
(Trowbridge, 1992) -Defined at top of BL Date Model dh/dt Observed dh/dt May 5 th . 19 mm/s. 17 mm/s May 22 nd . 38 mm/s 1) Mixing driven by entrainment. 2) 2) Tidal straining only of minor 3) importance (more to that story*). Issues… 4) 3) Jet marks top of boundary layer 5) (Stacy and Ralston, 2005). 6) 4) 1 -D from Lagrangian perspective. *uncertainty in u*
c – Dye Concentration w – Vertical Velocity h – Boundary Layer thickness f- Structure Function f(z)=tanh(a(1 -z)) Trowbridge 1992 boundary layer model Kz- Vertical Eddy Diffusivity -Buoyancy Frequency top of BBL a-1
Vertical structure Eddy Diffusivity U*= 2 cm/s N=. 08 s-1 1 z 0. 5 0 0 20 240 cm /s 60 0 1 2
Spring/neap variability in Buoyancy Flux Friction Velocity Dr u*2 N
Freshening rate of dye is direct measure of boundary layer averaged buoyancy flux U(z)=az t=ru*2(1 -z/H) u t
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. 1<Rf<. 18
Flux Richardson Number From Model and
Blue and Green lines: Peters & Bokhorst (2001) With functionality from Rohr et al. 1988 Red line this model
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. 11 u*2 NBL. 4) Flux Richardson number ~ 0. 1 -0. 16 and = 0. 22*(Ri. BL)1/2
S 4 Salinity at top Of boundary layer Decreases Du b dds z S 1 a S 2 x Du/2 d d. M S 3 times faster than at bottom of bbl
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