1. What makes an ocean model coastal ?
Hans Burchard
Leibniz Institute for Baltic Sea Research Warnemünde
„Coastal oceans“= „shelf seas + coastal regions + estuaries“
Coastal Ocean Modelling Workshop, May 18, 2015

2. Basic equations for almost all ocean models
Typically, ocean models are hydrostatic.
Jerlov class …
e.g., TEOS-10
Coastal Ocean Modelling Workshop, May 18, 2015

3. Turbulent flux parameterisations
Typically, down-gradient parameterisations are used:
eddy viscosity / eddy diffusivity
Note that coastal ocean models resolve
parts of the internal wave spectrum and
parts of the mesoscale/submesoscale dynamics.
This has strong implications on the eddy coefficient calculations.
Coastal Ocean Modelling Workshop, May 18, 2015

4. Is a coastal ocean model simply an ocean model …
… with high resolution?
structured
unstructured
ocean model
MPI-OM (Mathis et al.)
FESOM (Wang et al., 2011)
coastal ocean model
GETM (Gräwe et al., in prep.)
FVCOM (Grashorn et al., 2015)
Coastal Ocean Modelling Workshop, May 18, 2015

5. + + + + Coastal ocean model characteristics I:
Surface elevation variation is relatively large.
Geopotential (z) coordinate + + X + + X X z x
Coastal Ocean Modelling Workshop, May 18, 2015

6. + + + + + + + + + + + + + + + + Coastal ocean model characteristics I:
Surface elevation variation is relatively large.
s / z* coordinate + + X + X + X + + + X + X X + + X + + X X + + + + X X X z x
Coastal Ocean Modelling Workshop, May 18, 2015

7. Tide in Wadden Sea model
Coastal ocean model characteristics I:
Surface elevation variation is relatively large.
Drying & flooding is essential for many coastal applications.
Tide in Wadden Sea model
Care is needed with
implicit-surface models:
large time steps would
damp the tides.
Gräwe et al. (in prep.)
Coastal Ocean Modelling Workshop, May 18, 2015

8. Coastal ocean model characteristics II:
Topographical variation matters a lot.
Topographic variation in Western Baltic Sea
BBL-related processes in the Baltic Sea
Coastal Ocean Modelling Workshop, May 18, 2015

9. Sigma coordinate problem
Coastal ocean model characteristics II:
Topographical variation matters a lot.
Sigma coordinate problem
Inflows
Pressure gradient problem of sigma coordinates
Coastal Ocean Modelling Workshop, May 18, 2015

10. Geopotential coordinate problem
Coastal ocean model characteristics II:
Topographical variation matters a lot.
Geopotential coordinate problem
Inflows
Inflow approximation: problem of geopotential coordinates
Additionally, both coordinate types share the problem of numerical mixing.
There is a need for generalised (or hybrid) vertical coordinates.
Coastal Ocean Modelling Workshop, May 18, 2015

11. Coastal ocean model characteristics III:
Boundary layers are relatively thick.
Simple turbulence closure
models (e.g., PPMix)
would not work.
Two approaches:
Two-equation turbulence
closure models with
second moment closure
(see, e.g., GOTM).
versus
KPP (K profile parameterisation)
with various constructed add-ons.
Northern North Sea
Surface
boundary
layer
Bottom
boundary
layer
Burchard et al. (2002)
Note that in the coastal ocean wind waves
often affect the BBL. This has strong impacts on their dynamics.
Coastal Ocean Modelling Workshop, May 18, 2015

12. Coastal ocean model characteristics IV:
Gradients are relatively sharp.
Strong gradients of non-negative
tracers (such as salinity near
river plumes) require
monotone
non-negative and
non-diffusive
advection schemes.
Coastal ocean models typically use
TVD or FCT schemes.
Large scale ocean models often use
central differences plus smoothing.
Hofmeister et al. (2013)
Coastal Ocean Modelling Workshop, May 18, 2015

13. Coastal ocean model characteristics V: Open boundaries are important.
Coastal ocean models transfer boundary
information to the interior, either via
nesting of structured grids
or via unstructured grids.
Gräwe et al. (in prep.)
De Brauwere et al. (2011)
Coastal Ocean Modelling Workshop, May 18, 2015

14. Conclusions for coastal ocean models
+ A coastal ocean model is not simply a global ocean model with Dx  small.
You need to add specific physical & numerical features.
+ There are efforts to bring large scale ocean models to the coast.
E.g., MOM / NEMO / FESOM / MPI-OM.
+ Structured and unstructured models are two major options for the coastal ocean.
Both have their pros & cons.
+ Multi-purpose community models applicable to many regions are needed.
This allows for comparable results across systems.
Coastal Ocean Modelling Workshop, May 18, 2015