Home Contact Us Site Map  
 
       
    next up previous contents
Next: 3.16.7 Momentum equations Up: 3.16 Customizing MITgcm Previous: 3.16.5 Computational domain, geometry   Contents

3.16.6 Equation of state

First, because the model equations are written in terms of perturbations, a reference thermodynamic state needs to be specified. This is done through the 1D arrays tRef and sRef. tRef specifies the reference potential temperature profile (in $ ^{o}$C for the ocean and $ ^{o}$K for the atmosphere) starting from the level k=1. Similarly, sRef specifies the reference salinity profile (in ppt) for the ocean or the reference specific humidity profile (in g/kg) for the atmosphere.

The form of the equation of state is controlled by the character variables buoyancyRelation and eosType. buoyancyRelation is set to 'OCEANIC' by default and needs to be set to 'ATMOSPHERIC' for atmosphere simulations. In this case, eosType must be set to 'IDEALGAS'. For the ocean, two forms of the equation of state are available: linear (set eosType to 'LINEAR') and a polynomial approximation to the full nonlinear equation ( set eosType to 'POLYNOMIAL'). In the linear case, you need to specify the thermal and haline expansion coefficients represented by the variables tAlpha (in K$ ^{-1}$) and sBeta (in ppt$ ^{-1}$). For the nonlinear case, you need to generate a file of polynomial coefficients called POLY3.COEFFS. To do this, use the program utils/knudsen2/knudsen2.f under the model tree (a Makefile is available in the same directory and you will need to edit the number and the values of the vertical levels in knudsen2.f so that they match those of your configuration).

There there are also higher polynomials for the equation of state:

'UNESCO':
The UNESCO equation of state formula of Fofonoff and Millard [15]. This equation of state assumes in-situ temperature, which is not a model variable; its use is therefore discouraged, and it is only listed for completeness.
'JMD95Z':
A modified UNESCO formula by Jackett and McDougall [33], which uses the model variable potential temperature as input. The 'Z' indicates that this equation of state uses a horizontally and temporally constant pressure $ p_{0}=-g\rho_{0}z$.
'JMD95P':
A modified UNESCO formula by Jackett and McDougall [33], which uses the model variable potential temperature as input. The 'P' indicates that this equation of state uses the actual hydrostatic pressure of the last time step. Lagging the pressure in this way requires an additional pickup file for restarts.
'MDJWF':
The new, more accurate and less expensive equation of state by McDougall et al. [41]. It also requires lagging the pressure and therefore an additional pickup file for restarts.
For none of these options an reference profile of temperature or salinity is required.


next up previous contents
Next: 3.16.7 Momentum equations Up: 3.16 Customizing MITgcm Previous: 3.16.5 Computational domain, geometry   Contents
mitgcm-support@dev.mitgcm.org
Copyright © 2002 Massachusetts Institute of Technology