hans burchard leibniz institute for baltic sea research warnemünde
DESCRIPTION
Numerical model applications to lakes and estuaries with focus on transport and mixing of tracers. . Hans Burchard Leibniz Institute for Baltic Sea Research Warnemünde [email protected]. Programme Thermohaline circulation & sediment transport in the Wadden Sea - PowerPoint PPT PresentationTRANSCRIPT
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Hans Burchard
Leibniz Institute for Baltic Sea Research Warnemünde
Numerical model applications to lakes and estuaries with focus on transport and mixing of tracers.
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Programme
1. Thermohaline circulation & sediment transport in the Wadden Sea
2. Basin-wide mixing in lakes due to seiches
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www.rapid.ac.uk
Wadden Sea … and … thermo-haline circulation?
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Warming Precipitation
Weak tidal mixing: vertically stratified
Strong tidal mixing: horizontally stratified
Land Ocean
Downward surface buoyancy flux
Estuarine circulation
Sea bed
River
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Global ocean:Spatially inhomogeneous surface buoyancy fluxes plus internal mixing leads to global overturning circulation.
Wadden Sea:Spatially homogenous surface buoyancy fluxes over sloping bathymetry plus tidal mixing should lead to redidual overturning circulation.
But does it really happen?
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Locations of five automatic monitoring poles in theWadden Sea of theGerman Bight, recordingtemperature and salinity,(and thus density).
How can we approach this with observations ?
Burchard et al. (JPO 2008)
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Climatology: Salinity difference HW-NW
Burchard et al. (JPO 2008)
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Climatology: Temperature difference HW-LW
Burchard et al. (JPO 2008)
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Climatology: Density difference HW-LW
Burchard et al. (JPO 2008)
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Suspended matter concentrationsare substantially increased in theWadden Sea of the German Bight, withouthaving significantsources at the coast.
Why ?
Total suspended matter from MERIS/ENVISAT on August, 12, 2003.
Implications for sediment transport
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Model approach:
1. Simulating a closed Wadden Sea basin (Sylt-Rømø bight) with small freshwater-runoff and net precipitation. 2. Spin up model with variable and with constant density until periodic steady state.3. Then initialise both scenarios with const. SPM concentration.4. Quantify SPM content for control volume.
Burchard et al. (JPO 2008)
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Computer simulationsin Sylt-Rømø Bight
Wassertiefe
Burchard et al. (JPO 2008)
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Surface salinity at high and low water
Burchard et al. (JPO 2008)
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Total water and SPM volume
With density differences
V /
km3
Burchard et al. (JPO 2008)
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Total water and SPM volume
Without density differences
V /
km3
Burchard et al. (JPO 2008)
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Sea level rise & tidal flat growth (Danish Wadden Sea)
Data and graphics from Morten Pejrup, Copenhagen University
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Model system based on GETM:
NA: 5.4 km X 5.4 km (2D)NSBS: 1.8 km X 1.8 km (3D)SNS, WBS: 600 m X 600 m (3D)Wadden Sea: 200 m X 200 m (3D)
PACE project (NWO-BMBF):„The future of the Wadden Sea sediment fluxes: Still keeping pace with sea level rise?“ (2011-2014)
Wadden Sea model
Gräwe et al., in prep.
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Sealevel
Temperature
SaSalinity
Model validation(600 m resolution)
Gräwe et al., in prep.
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Tides in the Wadden Sea (as seen in 200 m resolution model)
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Wadden Sea model:
M4 tidal elevations(phase and amplitude)as validation data.
Gräwe et al., in prep.
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Sea surface salinity in the Wadden Sea (as seen in 200 m resolution model)
Personal communication Matias Duran Matute (NIOZ)
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Simulation Lake Alpnach (Switzerland)
Becherer & Umlauf (2011)
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Simulation Lake Alpnach (Switzerland)
Becherer & Umlauf (2011)
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Simulation Lake Alpnach (Switzerland)
Becherer & Umlauf (2011)
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Simulation Lake Alpnach (Switzerland)
Becherer & Umlauf (2011)
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Basin-Scale Mixing
deep-water average of mixing (depth > 15 m)
Becherer & Umlauf (2011)
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Baltic Sea Tracer Experiment (BATRE)
• Goal: quantify deep-water mixing in the central Baltic Sea
• Pilot study for new inert tracer gas (CF3SF5, now standard)
• 5 tracer surveys within 2 years
• Mooring arrays and turbulence measurements
• High-resolution nested 3-D model (GETM)
• 600 m lateral resolution• 200 sigma-type layers (vertically adaptive, Hofmeister et al. 2010)
• Second-moment turbulence closure model (GOTM, www.gotm.net)
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Mixing processes in the Baltic Sea
Reissmann et al. 2009
Courtesy Peter Holtermann
Principle of basin-wide mixing
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Investigation of deep water mixing during a stagnation period
Reissmann et al. 2009
Courtesy Peter Holtermann
Principle of basin-wide mixing
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Reissmann et al. 2009
Boundary Mixing
Internal Mixing
Courtesy Peter Holtermann
Investigation of deep water mixing during a stagnation period
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Interior mixing
Vertical Mixing Rates
Intrusions
Boundary mixing
Late stage(after boundary contact): ~ 10-5 m2 s-1
𝜕𝑐𝜕𝑡= 1
𝐴 [ 𝜕𝜕 𝑧 (𝐴 𝜕𝑐𝜕 𝑧 )]
Initial stage(before boundary contact): 10-6 m2 s-1
𝜕𝑐𝜕𝑡= 𝜕
𝜕 𝑧 [𝐼𝜕𝑐𝜕𝑧 ]
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Numerical Model Results
October 2007 January 2008 August 2008 February 2009
datamodel
model feels boundary mixing to early
Holtermann et al. (submitted)
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Take home:Differential buoyancy losses (over sloping topography) drive overturning circulation in coastal seas and lakes.
This causes net sediment fluxes into the Waden Sea whichmay explain why the Wadden Sea survived past and may survivefuture sea level rise.
Seiches in lakes and other stratified basins cause boundary mixingtypically increases effective mixing by about one order of magnitude.
Question: Can we make a 3D model for a deep lake such that we can properly predict the effective basin-wide mixing? For the Baltic Sea this worked (Holtermann et al., in revision), but lakesare narrower and often even deeper.