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

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

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

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

+ + + + 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

+ + + + + + + + + + + + + + + + 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

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

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

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

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

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

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

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

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