# [ncl-talk] Relative vorticity

David Wang dw2116 at outlook.com
Fri Aug 15 14:15:55 MDT 2014

```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 vorticityugrd = fobj->UGRD_P0_L100_GLL0({50000},:,:) ; zonal velocityvgrd = fobj->VGRD_P0_L100_GLL0({50000},:,:) ; meridional velocity
; Compute relative vorticity from zonal/meridional velocityvort = ugrdvort = uv2vrF(ugrd,vgrd);vort = uv2vr_cfd(ugrd,vgrd,ugrd&lat_0,ugrd&lon_0,1)
; Compute relative vorticity from absolute vorcitityrad = 4.*atan(1.)/180.clat = fobj->lat_0f = 2*7.2921e-5*sin(rad*clat)relv = absvrelv = absv - conform(absv,f,0)relv at long_name = "Relative vorticity"
wks = gsn_open_wks("x11","relative.vorticity")res = Trueres at gsnDraw = Falseres at gsnFrame = Falseres at cnFillOn = Trueres at cnLinesOn = FalsesymMinMaxPlt(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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