# [ncl-talk] Fwd: A question about the sign of velocity potential according to function:uv2sfvpf

whc Ren ren.whc at gmail.com
Sat Mar 26 09:07:13 MDT 2016

```Dear All:
I met a problem when I try to caculate the velocity potential according
to function:uv2sfvpf.
The u wind and v wind used in the study is the NCEP/NCAR reanalysis I
monthly dataset, The 1000 hPa wind in 1948:1:1:0 is shown in Fiugre 1, and
the corresponding stream function and velocity potential is shown in Figure
2 and 3.
It is shown that in Figure 1, there is an anticyclone around the
Northwestern Pacific(marked as A in red), and the wind over Tibetan
Plateau (marked as B in purple) is significantly divergence.
Figure 2 shows the stream function calculated by the u and v wind in
Figure 1. The positive and negative of stream function is in accordance
with the cyclonic/anticyclonic center in Figure 1.
However, the sign of velocity potential is incorrect( Figure 3).  The
wind over Tibetan Plateau is significantly divergence(Figure 1, mark B)
,but the velocity potential over Tibetan Plateau(Figure 3, mark C) is
negative ,which indicates the convergence circulation over this region.
The result of velocity potential confused me ,and I can't find if there
are mistakes in my ncl program. Looking forward to your kindly help.
The ncl program is shown as follows:

begin

uwnd = f10->uwnd(0:792,0:11,::-1,:)         ; latitude of NCEP reanalysis
is 90:-90, so I reversed the latitude to meet the request of function:
uv2sfvpf
print(uwnd&lat)
vwnd = f20->vwnd(0:792,0:11,::-1,:)

sf = new((/dimsizes(uwnd)/),float,uwnd at _FillValue)
vp = new((/dimsizes(uwnd)/),float,uwnd at _FillValue)

copy_VarCoords(uwnd,sf)
copy_VarCoords(uwnd,vp)
uv2sfvpf(uwnd,vwnd,sf,vp)
system("rm -f "+"sf.nc")
f70->sf=sf
system("rm -f "+"vp.nc")
f80->vp=vp
end

[image: 内嵌图片 1]
Figure 1. NCEP/NCAR reanalysis wind of 1000 hPa in 1948:1:1:0
[image: 内嵌图片 2]
Figure 2. Stream Function obtained by function:uv2sfvpf, using the wind in
Figure 1(1948:1:1:0).
[image: 内嵌图片 3]
Figure 3. Velocity Potential obtained by function:uv2sfvpf, using the wind
in Figure 1(1948:1:1:0).
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