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

[Dennis Shea]

Using Example 3 at:  http://test.www.ncl.ucar.edu/Applications/wind.shtml

I created the attached. Looks good to me.

D

On Thu, Mar 24, 2016 at 10:43 AM, whc Ren <ren.whc at gmail.com> wrote:

> 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
>     f10 = addfile("uwnd.mon.mean.nc","r")
>
> 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)
> f20 = addfile("vwnd.mon.mean.nc","r")
> 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=addfile("sf.nc","c")
> f70->sf=sf
>     system("rm -f "+"vp.nc")
>     f80=addfile("vp.nc","c")
> 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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