Re: Trying to get correct map over rotated lat-lon data

Hi David,

I think that rotated lat/lon projections in WRF is a confusing subject, especially in the context of NCL, because I suspect that the NCL WRF routines may not always map the attributes defined in the WRF file correctly into the resource settings required for NCL (in the rotated lat/lon case only). I will add the caveat that I do not fully understand the WRF attributes defined in your file either. What I do know is based on looking through this web page: http://www.mmm.ucar.edu/wrf/users/docs/user_guide_V3/users_guide_chap3.htm#_Description_of_GEOGRID.TBL, which may or may not be the most current description of the WRF projections and corresponding namelist parameters.

Since I do not have your data file I cannot run your script for myself, but I have attempted to approximate the map projection of your data based on the WRF parameters you list in the script, the documentation listed above, and the appearance of the surface pressure field plot you attached. I am attaching my test script along with the output. I think it is fairly close to the projection of your data, but of course it would be better if I had the locations of the lower left and upper right corners of your data. If you use gsn_csm_contour_map instead of wrf_map_overlays you should be able to plot the correct map projection for your data. One thing the documentation states is that CEN_LON and CEN_LAT do not apply for rotated lat/lon projections, but I have to wonder why they are set at all in that case. I am also not sure what the role of ret_lat and ref_lon are. I do note that ref_lat - pole_lat is almost equal to CEN_LAT and - STAND_LON is not too far from CEN_LON, but I don't know if that has any significance.

It would be helpful if you would upload your data file. It uses a projection and a domain we have not often encountered. If you need instructions on how to do that, see http://www.ncl.ucar.edu/report_bug.shtml. Thanks,
 -dave

On Nov 28, 2012, at 11:50 AM, David Jones wrote:

> Dear NCL talk help,
>
> I would like to kindly ask for some advice regarding the plotting of rotated lat-lon output. I am currently trying to plot the output from a rotated lat-lon grid using NCL...
>
> When I make the contour plot of my variable (psfc), all of the contoured data appears where I expect and want it to be, the land-sea boundaries can even be detected as the value of the variable changes. So everything seems fine in that respect.
>
> However, when I try to plot the national/geographical borders using a map overlay, the borders are all in the wrong places. Please see the attached image, in which it is clearly visible.
>
> The variable I am plotting (PSFC) is made up of (time, west_east, south_north), with west_east and south_north corresponding to a point on the computational grid (i.e. they're defined by a lat and lon from the computational grid).
>
> I also have 2D arrays (XLAT and XLONG) giving the geographical latitude and longitude for every point on the computational grid. I tried to use these to plot the geographical boundaries in the correct location, but sadly to no avail.
>
> I have looked through the NCL talk archives and on the NCL website, but unfortunately nothing I've tried has worked so far.
>
> I'll include my script below and also attach a copy of the output from an ncl_filedump on my data. You'll notice in the script below that lots of the map options are commented out; that's because several tries showed them to make no difference whatsoever to the plot I received, whether they were included or commented out (I did try plotting with them uncommented though, in several different combinations). The only one of those options that did make a difference was setting mpres@mpCenterLatF = a@CEN_LAT. But that only changed it from one incorrect plot to another incorrect one, albeit the new looks slightly better (attached).
>
> If you had any guidance as to where I might be going wrong I would be most appreciative.
>
> Many thanks for any advice,
> Dave.
>
> ----------------------SCRIPT-----------------------------
>
> ; Basic script for plotting model output
>
> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
> load "$NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRFUserARW.ncl"
> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
>
> begin
>
> a = addfile("wrfout_d01_2009-12-01_00:00:00.nc","r")
>
> wks = gsn_open_wks("x11","PSFC")
>
> ;---------------------------------------------------------------
> ; Set basic resources
> res = True
> res@MainTitle = "PSFC" ; Give a main title
> pltres = True ; Plotting resources
> mpres = True ; Map resources
>
> ;---------------------------------------------------------------
> ; Get variables
> ;---1. WRF------------------
>
> psfc = a->PSFC(0,:,:)
> psfc_size = dimsizes(psfc(:,:))
> print("dimsizes(psfc) = " + psfc_size)
>
> ;---2. Lat/Lon arrays-------
>
> lat2d = a->XLAT(0,:,:)
> printVarSummary(lat2d)
> lon2d = a->XLONG(0,:,:)
> printVarSummary(lon2d)
>
> ;--------------------------------------------------------------
> ; Plotting options
> opts = res ; Add basic resources
> opts@FieldTitle = "PSFC"
> opts@cnFillOn = True ; Shaded plot
> opts@gsnSpreadColorEnd = -8 ;
>
> contour_psfc = wrf_contour(a,wks,psfc,opts)
>
> ; Set some map options
>
> ; mpres@tfDoNDCOverlay= True
>
> ; pltres@sfXArray = lon2d
> ; pltres@sfYArray = lat2d
> ; pltres@LatLonOverlay = True
>
> ; mpres@mpCenterLonF = a@CEN_LON ;= -16.84654
> mpres@mpCenterLatF = a@CEN_LAT ;= 53.4331
>
> ; mpres@mpProjection = "CylindricalEquidistant"
> ; mpres@mpLimitMode = "Corners"
> ; mpres@mpLeftCornerLatF = lat2d(0,0) ; bottom left
> ; mpres@mpLeftCornerLonF = lon2d(0,0) ; bottom left
> ; mpres@mpRightCornerLatF= lat2d(268,568) ; top right
> ; mpres@mpRightCornerLonF= lon2d(268,568) ; top right
>
> mpres@mpGeophysicalLineColor = "Black"
> mpres@mpGridLineColor = "Black"
> mpres@mpLimbLineColor = "Black"
> mpres@mpNationalLineColor = "Black"
> mpres@mpUSStateLineColor = "Black"
> mpres@mpPerimLineColor = "Black"
> mpres@mpGeophysicalLineThicknessF = 1.0
> mpres@mpGridLineThicknessF = 1.0
> mpres@mpLimbLineThicknessF = 1.0
> mpres@mpNationalLineThicknessF = 1.0
> mpres@mpUSStateLineThicknessF = 1.0
>
> ; Namelist map projection information:
> ; map_proj : "lat-lon" (cylindrical equidistant)
> ; ref_lat : 88.5 (geographical lat of grid point (ref_x,ref_y)
> ; ref_lon : -5.0 (geographical lon of grid point (ref_x,ref_y)
> ; ref_x : 285 (total x grid points = 570)
> ; ref_y : 265 (total y grid points = 270)
> ; pole_lat : 35.0 (lat of geog N hem pole on comp grid)
> ; pole_lon : 180.0(lon of geog N hem pole on comp grid)
> ; stand_lon: 17.5 (rotation about geographic poles)
>
> ; MAKE PLOTS
>
> plot = wrf_map_overlays(a,wks,(/contour_psfc/),pltres,mpres)
>
> delete(res)
>
> end
>
> <ncl_filedump.txt><psfc_rotlatlon.ncl><PSFC.000001.png>_______________________________________________
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Received on Thu Nov 29 21:57:00 2012