rhopot, but also rhoins?
Added by Anne Moree almost 3 years ago
Hello,
Would there be an (undocumented?) alternative to the rhopot function that calculates in-situ seawater density instead (as a function of in-situ temperature, salinity and pressure?)?
Thanks in advance for the reply!
Anne
Replies (2)
RE: rhopot, but also rhoins? - Added by Ralf Mueller almost 3 years ago
hi Anne!
I am not aware of any such method in CDO. But the point is: I am not even sure that such a formula exists. Oceanographers might jump here. Do you have a reference like a paper of a book for such a thing?
In case you do the computation on your own, please keep in mind that numerics might have a big influence on the results. Usually such methods are polynomials in T,S and P and they look easy written down in an analytic way. unfortunately a straight forward computation leads to non-neglectable errors. That's why the implementation in CDO (like this) does not look like a simple polynom any more.
If you find a proper reference please post it here - we might fight a way to incorporate it into CDO.
best wishes
ralf
RE: rhopot, but also rhoins? - Added by Anne Moree almost 3 years ago
Hi Ralf,
Thanks for the reply! I am an oceanographer/marine biogeochemist myself, so I can explain some more.
In situ density is relevant for me as a biogeochemist when I for example want to correctly convert units at depth of oxygen concentration from mol/m3 to mol/kg.
A nice general explanation is given in the Sarmiento and Gruber (2006) book on page 34:
''Density is a function of temperature T, salinity S (defined in table 1.2.1), and pressure obeying the equation of state rho = f(T, S, p). Oceanographers always report density as sigma = (rho - 1000), so that, for example, a density of rho = 1027 kg m m-3 becomes sigma = 27. Figure 2.3.1 shows how density at the surface of the ocean varies as a function of temperature and salinity. The full equation of state, given by Fofonoff [1985], is quite complex. A simple rule of thumb is that sigma increases by ~1 when temperature increases by 5 degrees C and when salinity increases by 1. Because water is compressible, the density also increases as the pressure increases, at a rate of about 1 sigma unit per 200 db (about 200 m depth).''
However, since 2010, the 'new' TEOS (Thermodynamic Equation of State) describes the properties of seawater (http://www.teos-10.org). For Python, one can calculate insitu density using gsw.density.rho(SA, CT, p) (https://teos-10.github.io/GSW-Python/density.html).
Still, I still see the 'old' EOS-80 being used most. It is available in the Python seawater package (https://pythonhosted.org/seawater/eos80.html?highlight=dens#seawater.eos80.dens).
Either/both could be implemented in CDO. But they are tricky, and precision problems like you describe could arise. Nevertheless, in oceanography calculating of the insitu density is a relevant step in many calculations I think.
References
Fofonoff, N. P. (1985), Physical properties of seawater: A new salinity scale and equation of state for seawater, J. Geophys. Res., 90( C2), 3332– 3342, doi:10.1029/JC090iC02p03332.