[ncl-talk] Relative vorticity

[Dennis Shea]

As noted in the



*documentation:https://www.ncl.ucar.edu/Document/Functions/Built-in/uv2vrF.shtml
<https://www.ncl.ucar.edu/Document/Functions/Built-in/uv2vrF.shtml>*
*[SNIP]*

   - input values must be in ascending latitude order <**********
   - input array must be on a global gri

========

Typically, NCEP, ECMWF, etc are ordered N=>S. You want S=>N

  ugrd = ugrd(:,:,::-1,:)   ; time,lev,lat,lon

  vgrd = ugrd(:,:,::-1,:)


or , 3D

  ugrd = ugrd(:,::-1,:)   ; time,lat,lon

  vgrd = ugrd,:,::-1,:)




On Fri, Aug 15, 2014 at 2:15 PM, David Wang <dw2116 at outlook.com> wrote:

> Hi,
>
> It is said in the documentation that uv2vrF is more accurate than
> uv2vr_cfd if the input velocity data are global and have no missing values.
> However, I found that using the 2.5x2.5-degree CFSR data, the relative
> vorticity field computed with uv2vr_cfd is almost identical to the CFSR
> output for absolute vorticity minus planetary vorticity (f); whereas the
> relative vorticity field computed with uv2vrF is quite different. I wonder
> if I missed anything obvious and didn't use uv2vrF correctly. Below is my
> NCL code.
>
> Thanks,
> David
>
> fobj =
> addfile("~/data/CFSR/cmd_grblow/1979/pgblnl.gdas.1979123118.grb2","r")
> absv = fobj->ABSV_P0_L100_GLL0({50000},:,:) ;absolute vorticity
> ugrd = fobj->UGRD_P0_L100_GLL0({50000},:,:) ; zonal velocity
> vgrd = fobj->VGRD_P0_L100_GLL0({50000},:,:) ; meridional velocity
>
> ; Compute relative vorticity from zonal/meridional velocity
> vort = ugrd
> vort = uv2vrF(ugrd,vgrd)
> ;vort = uv2vr_cfd(ugrd,vgrd,ugrd&lat_0,ugrd&lon_0,1)
>
> ; Compute relative vorticity from absolute vorcitity
> rad = 4.*atan(1.)/180.
> clat = fobj->lat_0
> f = 2*7.2921e-5*sin(rad*clat)
> relv = absv
> relv = absv - conform(absv,f,0)
> relv at long_name = "Relative vorticity"
>
> wks = gsn_open_wks("x11","relative.vorticity")
> res = True
> res at gsnDraw = False
> res at gsnFrame = False
> res at cnFillOn = True
> res at cnLinesOn = False
> symMinMaxPlt(relv,20,False,res)
> plot = new(2,graphic)
> plot(0) = gsn_csm_contour_map(wks,relv,res)
> plot(1) = gsn_csm_contour_map(wks,vort,res)
> gsn_panel(wks,plot,(/2,1/),False)
>
>
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