[ncl-talk] vector plot with one axis scaled

[Wolfgang Langhans]

Hi all,

I'm seeking a vector plot with the vertical axis representing height in 
meter and the horizontal axis representing distance in nondimensional 
units. The original vector field is based on velocities u and w, both 
are defined in physical space and have units m/s. The distance r (meter) 
is transformed into a non-dimensional distance rs by rs=r/Rn, with 
Rn=1000 meters. The axis shall have units of meters (vertical) and 
nondimensional (horizontal). I transform the horizontal velocity u by 
u=u/Rn to get the horizontal speed in rs-space. Then I assign z and rs 
as coordinates to u and w and use gsn_vector to get the vector plot for 
the coordinate range 0<z<1000 and 0<rs<1. The angle of the vectors is 
fine (e.g., 45 deg for original data being u=2 and w=2). However, I did 
not succeed in getting the length of the vectors right. I want ncl to 
pick a reference vector that is based on the wind speed of the original 
vectors. Of course, ncl now computes the magnitude as sqrt(w^2+u^2) 
which in my case is very close to w (since u has been scaled). Only for 
a vector field with u=0 can I obtain the correct magnitude and the 
correct reference vector.

Is there any way to modify the underlying source code such that ncl can 
read in my scaling factor to reverse the applied normalization of u 
whenever the magnitude of the vector is computed. This would allow me to 
use vcRefMagnitudeF and vcRefLengthF with its normal meaning, i.e., 
RefMagnitudeF being magnitude in m/s? Or maybe there's an even more 
simple solution to this?

The simple test script is below.

Thanks!

Wolfgang
_______________________________________________________________

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/shea_util.ncl"

;coordinates
z=fspan(0.,800.,20)         ; height
r=fspan(0.,4000.,200)       ; radius
Rn=1000.
rs=r/Rn                   ; nondimensional radius

u=new((/dimsizes(z),dimsizes(rs)/),float)
w=new((/dimsizes(z),dimsizes(rs)/),float)
u!0="z"
u!1="rs"
u at z=z
u at r=rs
w!0="z"
w!1="rs"
w at z=z
w at r=rs

;create arbitrary velocity fields
u=10.00
w=0.00

;compute u in rs-space
u=u/Rn

   type              = "eps"                      ; create variable to 
hold info
   wks   = gsn_open_wks (type, "test_vec" )   ; open workstation

   setvalues wks
   "wkBackgroundColor" : (/1.,1.,1./)
   "wkForegroundColor" : (/0.,0.,0./)
   end setvalues

   vres=True
   vres at gsnDraw               = False             ; don't draw yet
   vres at gsnFrame              = False             ; don't advance frame yet
   vres at pmLabelBarDisplayMode     = "Never"
   vres at vcRefAnnoOn = True
   vres at vcRefAnnoOrthogonalPosF   = -0.994         ; move ref vector
   vres at vcRefAnnoParallelPosF   = 0.994
;  vres at vcRefLengthF              = 0.03
;  vres at vcRefAnnoString1          ="2 m s!S!-1!N!"
;  vres at vcRefMagnitudeF           = 2.0        ; define vector ref mag
;  vres at vcRefLengthF              = 0.01
   vres at vcGlyphStyle            = "LineArrow"     ; turn on curley vectors
   vres at vcLineArrowColor        = "black"           ; change vector color
   vres at vcLineArrowThicknessF   = 1.              ; change vector thickness
   vres at vcMinDistanceF=.03
   ; choose plotting range such that vectors point diagonally in case 
u=w for the original data
   vres at trXMaxF = 1.0
   vres at trYMaxF = Rn ;
   vres at trXMinF = min(rs)
   vres at trYMinF =  min(z)
   vres at vfXArray  = rs
   vres at vfYArray  = z

   plot = gsn_csm_vector(wks,u,w,vres)

   draw(plot)
   frame(wks)

_____________________________________________________________________


-- 
************************************************
Wolfgang Langhans

Postdoctoral Fellow
Earth Sciences Division
Lawrence Berkeley National Laboratory
1 Cyclotron Road 74R316C
Berkeley, CA 94720-8268

email:  wlanghans at lbl.gov
phone:  (510) 486-7175
fax:    (510) 486-7775
room:   84-118K
web:    http://esd.lbl.gov/about/staff/wolfganglanghans/
************************************************