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[ferret_users] integration along density axis



Hi,

    I have a question about integration between
density surfaces. For instance, if I want to calculate
the volume flux of ocean water between density
surfaces (i.e., 1027 and 1028), I may have to convert
the data in z (depth, meters) coordinate to the data
in sigma coordinate by using function ZAXREPLACE, then
get the data in sigma coordinate, then do integration
between density surface 1027 and 1028, however, the
resulting calculation bring a unit other than m^3/s.
The script for the above calculation follows:

    set data dataset.cdf
    
    let sigma = rho_un(salt[d=1],temp[d=1],0) - 1000
    define axis/z=21:29:0.05 axden

    let vonsigma = zaxreplace(v[d=1],sigma,z[gz=axden]
    
    set region/x=110w:20w/y=50n/l=1
    let volume_flux = vonsigma[x=@din,z=28:29@din]  !
Apparently the unit is not m^3/s but kilogram/(m*s)

    However, I want the unit of volume flux to be
ordinary, i.e., m^3/s, how can I do this?


    Other people may suggest to use density mask
method, like
    let v_new = if sigma ge 28 and sigma le 29 then v
else (0-(1e+34))
   let volume_flux = v_new[x=@din,z=@din]

    However, we do not know the error based on the
mask method for the grid datasets.

    Thank you for your attention and look forward to
hearing from you.

    Yangxing
    


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