[ncl-talk] overlaying a hodograph onto a sounding

[Joe Grim]

Hi,

Could someone please help me with a conceptual skewT + hodograph plot?  My
hope is to create a skewT plot with a hodograph overlaid, much like what
you see from other skewT plots, such as this one:
http://weather.rap.ucar.edu/upper/dnr.gif.  I couldn't find any examples on
the NCL website, so I thought I could create my own, but couldn't get it to
plot correctly.

Conceptually, I imagine the script would be created in this order:
1) Draw the background using skewT_BackGround
<https://www.ncl.ucar.edu/Document/Functions/Skewt_func/skewT_BackGround.shtml>
.
2) Draw the hodograph using gsn_csm_xy
<https://www.ncl.ucar.edu/Document/Graphics/Interfaces/gsn_csm_xy.shtml>
   - making sure to set gsnDraw and gsnFrame to False
   - set vpXF, vpYF, vpWidthF, and vpHeightF to only plot in the
upper-lefthand side of the plot
3) Use the overlay procedure to overlay the two, like
this: overlay(skewt_bkgd,hodo_plot)
4) Draw the plot, like this: draw (skewt_bkgd)
5) Use the skewT_PlotData
<https://www.ncl.ucar.edu/Document/Functions/Skewt_func/skewT_PlotData.shtml>
to
overlay the skewT data.
6) Advance the frame: frame(wks)

I almost got it to work, except the hodograph only plots on the bottom of
the image the portion of the hodograph with positive V component (I assume
the rest is "off screen" below); I assume it is using the native coordinate
system of skewT_BackGround
<https://www.ncl.ucar.edu/Document/Functions/Skewt_func/skewT_BackGround.shtml>
instead
of having its own independent coordinate system.

Does anyone have any thoughts or ideas of how I can do this?  Thanks!

Joe

P.S.  Here is the current version of my code:

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/skewt_func.ncl"
begin
; Read in WRF data
 in_file =
"/d3/ATEC/wrfout_misc/ATC/wrfout_d01_2017-09-02_05:00:00.ATC_P+FCST"
 ncdf_in = addfile(in_file+".nc","r")
 P_tot      = wrf_user_getvar(ncdf_in,"pressure",0)
 z_tot      = wrf_user_getvar(ncdf_in,"z",-1)
 U          = wrf_user_getvar(ncdf_in,"U",-1)
 U_unstag = wrf_user_unstagger(U,U at stagger)
 V          = wrf_user_getvar(ncdf_in,"V",-1)
 V_unstag = wrf_user_unstagger(V,V at stagger)
 TC         = wrf_user_getvar(ncdf_in,"tc",-1)
 TD         = wrf_user_getvar(ncdf_in,"td",-1)

; Create skewT background
 PlotType = "png"
 PlotName = "Sounding_MartinAF"
 wks = gsn_open_wks(PlotType,PlotName)
  loc = wrf_user_ll_to_ij(ncdf_in,-76.42,39.33,True)
  locX = loc(0) - 1
  locY = loc(1) - 1
 skewtOpts          = True
 skewtOpts at vpXF              = 0.07    ; controls off-set from left
 skewtOpts at vpYF              = 0.92    ; controls off-set from top
 skewtOpts at vpWidthF          = 0.85    ; controls size of plot
 skewtOpts at vpHeightF         = 0.85    ; controls size of plot
 skewtOpts at tiMainString = "Sounding at 39.33N, 76.42W"
 skewt_bkgd             = skewT_BackGround (wks, skewtOpts)
 draw (skewt_bkgd)

; Draw hodograph here???
 hodo_res = True
 hodo_res at vpXF      = 0.07
 hodo_res at vpYF      = 0.92
 hodo_res at vpWidthF  = 0.15
 hodo_res at vpHeightF = 0.15
 hodo_res at gsnDraw   = False
 hodo_res at gsnFrame  = False
 hodo_plot =
gsn_csm_xy(wks,U_unstag(0,:,locY,locX),V_unstag(0,:,locY,locX),hodo_res)
 overlay(skewt_bkgd,hodo_plot)
 draw (skewt_bkgd)

; Draw the skew-T data
 dataOpts           = True
 dataOpts at Parcel          = 1
 dataOpts at WspdWdir        = False  ; wind speed and dir [else: u,v]
 skewT_data = skewT_PlotData(wks, skewt_bkgd, P_tot(:,locY,locX), \
                                               TC(0,:,locY,locX), \
                                               TD(0,:,locY,locX), \
                                            z_tot(0,:,locY,locX), \
                                         U_unstag(0,:,locY,locX), \
                                         V_unstag(0,:,locY,locX), \
                                             dataOpts)
 frame(wks)

end
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