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Numerical simulations of dense bottom currents in the Western Baltic Sea: Quantification of natural,

structure-induced and numerical mixing

Funded by:

Hannes Rennau

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Model area

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Model area

Hot spot of water mass transformation

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Model area

Hot spot of water mass transformation

Drodgen Sill (~8m)

Darss Sill (~20m)

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Main pathwaysOf dense bottom

currents

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Main pathwaysOf dense bottom

currents

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Main pathwaysOf dense bottom

currents

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Main pathwaysOf dense bottom

currents

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Main pathwaysOf dense bottom

currents

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Numerical Model

- typical ocean-circulation model GETM with state of the art turbulence model GOTM (code developers for both models at IOW)

Parallel execution on IOW Linux Cluster

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Simulation of bottom salinity over nine months

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Darss Sill Tracer vs. Drodgen Sill Tracer

Tracer release positions

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Darss Sill Tracer vs. Drodgen Sill Tracer

Tracer release positions

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Darss Sill Tracer vs. Drodgen Sill Tracer

Tracer release positions

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Darss Sill Tracer vs. Drodgen Sill Tracer

Tracer release positions

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Model Validation

MARNET Arkona Station

Burchard, H., F. Janssen, K. Bolding, L. Umlauf, and H. Rennau, Model simulations of dense bottom currents in the Western Baltic Sea, Cont. Shelf Res., 29, 205-220, 2009.

x

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Natural mixing in the numerical model

Significantly increased natural mixing:

in channels (Kriegers Flak, Bornholm Channel, …)

in the area of shallow sills (Drodgen Sill, Darss Sill, …)

Burchard, H., F. Janssen, K. Bolding, L. Umlauf, and H. Rennau, Model simulations of dense bottom currents in the Western Baltic Sea, Cont. Shelf Res., 29, 205-220, 2009.

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Physical mixing Numerical mixing

found method to analyse numerical mixing

Rennau, H., and H. Burchard, Quantitative analysis of numerically induced mixing in a coastal model application, Ocean Dynamics, submitted December 2008. Burchard, H., and H. Rennau, Comparative quantification of physically and numerically induced mixing in ocean models, Ocean Modelling, 20, 293-311, 2008.

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• Fundamental knowledge about propagation of dense bottom currents in the western Baltic Sea

• Model derived amount of physically induced mixing without offshore foundations

Conclusions

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• Fundamental knowledge about propagation of dense bottom currents in the western Baltic Sea

• Model derived amount of physically induced mixing without offshore foundations

Conclusions

• numerically induced mixing and physical mixing have same orders of magnitude but different horizontal distribution

• enhanced numerical mixing –> less physical mixing

• numerical techniques: adaptive vertical coordinates,…

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Impact of bridge piles

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Impact of bridge piles

Lass et al. (2008)

distance from pile / m

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Impact of bridge piles - modeling

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Impact of bridge piles - modelingH.U. Lass et al. (2008)

28

24

20

16

2000 1000 PILE 1000 2000distance from pile / m

distance from pile / m

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GETM 2D Slice Model

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Additional mixing due to Offshore windpark foundations

Local model at University of Hannover

Regional model at IOW

Parameterisaton?

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'Worst Case Study' with Offshore Windpark

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Influence of Offshore wind park: 10. April, 2004

With windpark without windpark

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windpark

„Worst Case“ Study – Simulation with Offshore WindparkSnapshot:

bottom salinity (without windpark) – bottom salinity (with windpark)

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windpark

„Worst Case“ Study – Simulation with Offshore WindparkSnapshot:

bottom salinity (without windpark) – bottom salinity (with windpark)

+0.5

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windpark

„Worst Case“ Study – Simulation with Offshore WindparkSnapshot:

bottom salinity (without windpark) – bottom salinity (with windpark)

+0.5

-0.2

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• - expected low additional mixing due to Offshore• Windpark foundations (needs further calibration • in parameterisation and longer time series)

• - strength of additional mixing mainly dependent on:• (1) where to be build • (2) windfarm distribution (how many, …)

Main conclusions

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Main Focus for last year:

- Final results concerning additional mixing of offshore windpark foundations in the Western Baltic Sea (publication in preparation)

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Main Focus for last year:

- Final results concerning additional mixing of offshore windpark foundations in the Western Baltic Sea (publication in preparation)

- Model nesting: 3D structure development of dense bottom currents in channels (Kriegers Flak North and Bornholm Channel) (publication in preparation)

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Main Focus for last year:

- Final results concerning additional mixing of offshore windpark foundations in the Western Baltic Sea (publication in preparation)

- Model nesting: 3D structure development of dense bottom currents in channels (Kriegers Flak North and Bornholm Channel) (publication in preparation)

- Passive Tracer study (correlations for propagation time of dense bottom currents) (publication in preparation)

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Main Focus for last year:

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Main Focus for last year:

- Final results concerning additional mixing of offshore windpark foundations in the Western Baltic Sea (publication in preparation)

- Model nesting: 3D structure development of dense bottom currents in channels (Kriegers Flak North and Bornholm Channel) (publication in preparation)

- Passive Tracer study (correlations for propagation time of dense bottom currents) (publication in preparation)

Rennau, H., and H. Burchard, Quantitative analysis of numerically induced mixing in a coastal model application, Ocean Dynamics, submitted December 2008.

Burchard, H., and H. Rennau, Comparative quantification of physically and numerically induced mixing in ocean models, Ocean Modelling, 20, 293-311, 2008.

Burchard, H., F. Janssen, K. Bolding, L. Umlauf, and H. Rennau, Model simulations of dense bottom currents in the Western Baltic Sea, Cont. Shelf Res., 29, 205-220, 2009.

Fehmarn Belt Project