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Re: [ferret_users] calculate the density with Pressure

Billy is right. I was thinking that the "temperature" was actually already potential temperature, as in the case of model output that I've worked with. Sorry for the unclear statement.


On Mon, 22 Jan 2007, William S. Kessler wrote:

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:


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

  0 for sigma_0
2000 for sigma_2



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?


James Orr
Marine Environment Laboratories (MEL-IAEA)
4, Quai Antonie 1er
MC-98000 Monaco

Tel:  +377 9797 7229
Fax: +377 9797 7273
Email: j.orr@xxxxxxxx

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