[ncl-talk] Question on moisture advection example
Dennis Shea
shea at ucar.edu
Fri Mar 1 09:52:50 MST 2019
[1] As indicated in the documentation, the advection is via:
*adv_X = U*(dX/dlon) + V*(dX/dlat)*
There is *no* minus sign*. *
Perhaps, something your are looking at has a convention*:*
* minus_adv_X_ = -( U*(dX/dlon) + V*(dX/dlat) )*
*[2]* As indicated in the documentation: If *opt_adv=1*, then:
adv_X = *advect_variable*(u,v,X,gridType,long_name,units,*opt_adv*)
; For clarity: explicitly extract the returned elements of the list
variable. All meta data is present.
Xadv = adv_X[0] ; advected quantity
Xgrx = adv_X[1] ; longitudinal gradient
Xgry = adv_X[2] ; latitudinal gradient
*printVarSummary*
<https://www.ncl.ucar.edu/Document/Functions/Built-in/printVarSummary.shtml>(Xadv)
; advection of 'X'
printMinMax(Xadv, 0)
print("---")
*printVarSummary*
<https://www.ncl.ucar.edu/Document/Functions/Built-in/printVarSummary.shtml>(Xgrx)
; longitudinal gradient
*printVarSummary*
<https://www.ncl.ucar.edu/Document/Functions/Built-in/printVarSummary.shtml>(Xgry)
; latitudinal gradient
*[3] *You can add the following
* meridional_advX = v*Xgry meridional_advX at long_name = "..."
meridional_advX at units = "..." copy_VarCoords(v, meridional_advX)*
* printVarSummary(meridional_advX) printMinMax(meridional_advX, 0)*
* zonal_adv_X = u*Xgrx ...*
On Thu, Feb 28, 2019 at 11:54 PM Lyndz <olagueralyndonmark429 at gmail.com>
wrote:
> Dear NCL-experts,
>
> I just want to clarify the following:
>
> I am calculating moisture advection from this example:
>
> https://www.ncl.ucar.edu/Document/Functions/Contributed/advect_variable.shtml
>
> Problems:
> [1] I am a little confused about the sign of the given equation here.
> Should the final output (advection) be multipled by -1? Not multiplying by
> -1 seems to give me the correct result, but if this is the case, all the
> terms should be negative in the equation:
> *adv_X = -U*(dX/dlon) -V*(dX/dlat)*
>
> [2] If I have to separate the zonal and meridional components, they should
> be also multiplied by -1?
>
> Sincerely,
>
> Lyndz
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