Hans Burchard1,3, Frank Janssen2, Karsten Bolding3, Lars Umlauf1, and Hannes

Rennau1

1. Baltic Sea Research Institute Warnemünde2. Bundesamt für Seeschifffahrt und Hydrographie

3. Bolding & Burchard Hydrodynamics

hans.burchard@io-warnemuende.de

Estimates of vertical mixing due to dense bottom currents

in the Western Baltic Sea

Motivation: wind farms in the Western Baltic Sea

Kriegers Flak

Goal: Quantify natural mixing for evaluating additional mixing effects due to offshore constructions. Method: Three-dimensional numerical modelling.

GETM is a 3D numerical model for estuarine,

coastal and shelf sea hydrodynamics with • Coupling to GOTM Turbulence Module• Generalised vertical coordinates• High-resolution advection schemes• Parallel execution•…

GETM Western Baltic Sea hindcast

Model setup:

Meteo: 7 km (DWD)

BC/IC: 3 nm Baltic Seamodel based on MOM-3

Period: Sep 2003-May 2004

Resolution: 0.5 nm

# of layers: 50 GVC

GETM Western Baltic Sea hindcast

GETM Western Baltic Sea hindcast

Western Baltic Sea monitoring stations

Darss Sill: 19 m

+

Drogden Sill: 8 m

+MARNET (IOW/BSH)

Farvandsvæsenet

+

Arkona Buoy: 48 m

Model validation: Darss Sill

Model validation: Drogden Sill

Model validation: Arkona Buoy

Model validation:

Section across

Kriegers Flak

Quantification of vertical mixing

Non-averaged salinity equation:

Reynolds decomposition:

Mean salinity equation

Micro-structure salinity variance equation:

Vertically integrated mean salinity variance:

Vertically integrated salinity variance equation:

Model derived annual mean vertically integrated salinity variance decay in Western Baltic Sea

Vertically integrated turbulent salt flux

Alternative measure for vertical mixing:

Model derived annual mean vertically integrated vertical turbulent salt flux in Western Baltic Sea

Conclusions:

Density currents in Western Baltic Sea are highly variable,

show a complex transverse structure (see Umlauf et al.)

and induce substantial natural transports and mixing.

A suitable model-based quantification of vertical mixing

is given by the

decay of the vertically integrated mean salinity variance.

Next question:

How big is the additional numerically-induced mixing ?

RV Gauss leaving Warnemünde for its last research cruise