Hydrodynamics
The “hydrodynamic” module of the Mohid system is able to simulate the flow in water masses where the flow velocity is lower than the celerity of the pressure wave.
This module has been used to simulate hydrodynamic processes in oceans (Atlantic NE and Atlantic SW), in coastal areas (several areas along the Portuguese and Brazil coasts), in more than 30 estuaries and lagoons (European, African and Brazilian estuaries and lagoons) and in water dams (south of Portugal).
All these study areas have complex flows and intense ecological activity with a strong relation with hydrodynamic processes. The “hydrodynamic” module aims to be a numerical tool oriented to help understanding biogeochemical processes and resolve ecological problems associated with human activity.
Approximations
Numerical Characteristics
Main characteristics 
Spatial discretisation  Finite volumes 
Horizontal Grid  Orthogonal 
Vertical Grid  Generic coordinates 
Computation points distribution  Arakawa C 
Time discretisation  ADI – 2D mass balance; explicit – horizontal momentum; implicit – vertical momentum 
Forces discretisation 
Forces computed explicitly  Coriolis, tide potential, baroclinic pressure gradient, atmosphere forcing (wind stress and pressure), horizontal advection and diffusion of momentum 
Forces computed implicitly  Barotropic pressure gradient, bottom friction, vertical advection and diffusion of momentum 
Baroclinic pressure spatial discretisation  Cartesian referential (or z level referential) 
Horizontal advection of momentum 
 Upwind 1º order
 TVD (upwind 1º order + central differences or higher order upwind)

Vertical advection of momentum  Hybrid (upwind + central differences) 
Diffusion of momentum  Central differences 
Boundary conditions 
Barotropic pressure gradient:
 Water level
 Barotropic velocity

 Imposed
 Null gradient
 Cycle
 Radiation (Flather, 1976)
 Radiation (Blumberg & Kantha, 1985)
 Flow relaxation
The last three boundary conditions use a reference solution that can be imposed using two methodologies:
 Input data: the solution is imposed as model input data
 One way nesting: the solution is compute by a courser grid model

Baroclinic pressure gradient:
 Baroclinic velocity
 Temperature and salinity

 Imposed
 Null gradient
 Radiation (Marchesiello et al., 2001):
 Celerity constant
 Celerity according to Orlansky (1976)
 Flow relaxation:
 Input data
 One way nesting

Coriolis, horizontal advection and diffusion of momentum:

 Imposed null value
 Null gradient

Turbulence 
Horizontal


Vertical

 Constant
 Coupled with GOTM. Traditional as well as stateoftheart parameterisations (e.g. kε, kΩ) for vertical turbulent mixing – see http://www.gotm.net/

3D

Smagorinsky 3D

Waves 
Solution imposed via input files

 Variable in space constant in time ASCII
 Variable in space and time HDF5

Fetch type model


Two way runtime coupling with SWAN

The user can run MOHID and SWAN together. In run time MOHID updates the sea level, velocities and bathymetry while SWAN updates the wave properties (e.g. Hs, Tp, Dir., gradients of radiation stresses).
