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Re: [ferret_users] calculate the density with Pressure



Hi all,
The arguments to the functions can be checked by "asking Ferret", in the output given by SHOW FUNCTION. We will clarify these definitions in the Users Guide.

yes? show function theta_fo
THETA_FO(salt,temp,p,ref)
   Fofonoff (1977) potential temperature
   salt: salinity (psu)
   temp: temperature (deg. C)
   p: pressure (decibars)
   ref: reference pressure (decibars)

yes? show function rho_un
RHO_UN(salt,temp,p)
   UNESCO state equation (density) for ocean H2O (kg/m^3)
   salt: salinity (psu)
   temp: temperature (deg. C)
   p: pressure (decibars)


Note that the syntax "y[g=v]" and "y[gy=v]" will give exactly the same result;

jeanbrazil@xxxxxxxxxxx wrote:
HI Billy, Jim and Dawn
I didn´t understand what you meant with "But the results are obviously wrong", but check the units you are using (they should match with the documentation) and replace "y[g=v]" by "y[gy=v]" and "z[g=v]" by "z[gz=v]". I dont know if it`ll help you but...

Luck

Jean

===========================================================

----- Original Message -----
From: "William S. Kessler" <William.S.Kessler@xxxxxxxx>
To: "James Orr" <James.Orr@xxxxxx>
Cc: "William S. Kessler" <William.S.Kessler@xxxxxxxx>; "ferret ocean" <ferretocean@xxxxxxxxx>; <oar.pmel.ferret_users@xxxxxxxx>
Sent: Tuesday, January 23, 2007 1:56 AM
Subject: Re: [ferret_users] calculate the density with Pressure

Hi Jim, Dawn -

No, I don't think Jim is correct. If a reference pressure is used, then you must use potential temperature theta instead of temperature. It does not make sense to calculate density at 0 pressure but with temperature at depth (where the pressure affects the temperature). Instead, bring the water parcel adiabatically to the surface by finding theta, then use your reference pressure:

let theta=theta_fo(salt,temp,z[gz=temp],0) ! potential temperature theta
let sigma_theta=rho_un(salt,theta,0)-1000

See the documentation for the THETA_FO function. For sigma-2, use 2000 instead of zero in the above.

But I think Dawn is asking something different. Instead of z, she wants to use pressure. Frankly, I don't know the answer to her question ("the results are obviously wrong" doesn't give me much of a clue). However, I think she is on the right track, because I believe the 3rd argument to THETA_FO really should be pressure, not depth (the documentation is unclear on this point). I suggest looking at the reference given under the THETA_FO function (Bryden 1973). Then tell us so the documentation can be fixed if necessary.

Billy K

On Jan 22, 2007, at 9:59 AM, James Orr wrote:

Dawn,

Your pressure in the 3rd argument of the rho_un function should be the reference pressure:

   0 for sigma_0
2000 for sigma_2

Cheers,

Jim


On Mon, 22 Jan 2007, ferret ocean wrote:

Dear ferreters,

I want to calculate the density with pressure instead of depth (more accuate way), but when I get the pressure with the following scipt to convert the depth to pressure,

  let x0 = sin(y[g=v]*3.1415926/180);
let c1 = 5.92E-3 + x0*x0 * 5.25E-3;
let pres = ((1-c1)-(((1-c1)*(1-c1))-(8.84E-6 * z[g=v]))^0.5) / 4.42E-6;

 then let rho=rho_un(salt,temp,pres)-1000;

 But the results are obviously wrong. Do you have any suggestions?

 Dawn




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