wrf_latlon_to_ij

Finds the nearest model grid indices (i,j) to the specific location(s) in latitude and longitude.

Prototype

	function wrf_latlon_to_ij (
		lat_wrf     : numeric,  
		lon_wrf     : numeric,  
		lat_pts [*] : numeric,  
		lon_pts [*] : numeric   
	)

	return_val  :  integer

Arguments

Latitude and longitude arrays in ARW WRF, whose rightmost dimensions are ny x nx.

Scalars representing the lat/lon locations. (In version 4.3.1 the interface was upgraded to allow these variables to be 1D arrays.)

If any of the input values are out of data range in lat_wrf and/or lon_wrf, then the appropriate return values will be set to missing (-999).

Description

This function returns the i-th, j-th grid location of the given location(s) (lat, lon). This is basically same as "get_ij_lat_long" in "wrf_user_fortran_util_0.f".

Note: this function was available in V4.3.0, but it was overhauled in V4.3.1 to do the correct thing with values outside the range of the input, so the 4.3.0 version should not be used.

See Also

wrf_user_latlon_to_ij

Examples

The function wrf_user_latlon_to_ij, (available in the $NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRFUserARW.ncl script) can be used to locate x/y point given specific long/lat references.

Example 1

  a=addfile("wrfout_d01_2000-01-24_12:00:00.nc","r")
  wlat=a->XLAT(0,:,:)
  wlon=a->XLONG(0,:,:)
  print(min(wlat)+","+max(wlat))  ; 7.97334,55.7424
  print(min(wlon)+","+max(wlon))  ; 98.3065,177.803

; A sample observation site
  olat=36.5
  olon=121.4

; Find the closest point to the site in WRF grids
  obsij=wrf_latlon_to_ij(wlat,wlon,olat,olon)
  print(obsij)    ; => 63,29

  b=addfile("wrfout_d01_2000-01-24_12:00:00.nc","r")
  wlat=b->XLAT(0,:,:)
  wlon=b->XLONG(0,:,:)

  c=addfile("Wprf.nc","r")        ; Wind profiler observations
  locs=charactertostring(c->id)   ; station names
  nloc=dimsizes(locs)             ; 36 stations
  lats=c->lat                     ; float [station | 36]
  lons=c->lon                     ; float [station | 36]

;
; The following will only work in version 4.3.1.
;
  loc_ij =wrf_latlon_to_ij(wlat,wlon,lats,lons)   ; 36 x 2
  print(locs+":"+loc_ij(:,0)+","+loc_ij(:,1))

  load "$NCARG_ROOT/lib/ncarg/nclscripts/gsn/gsn_code.ncl"
  load "$NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRFUserARW.ncl"

  a = addfile("wrfout_d01_2000-01-24_12:00:00.nc","r")

  loc  = wrf_user_latlon_to_ij(a, 40.0, -100.0)
  print("X/Y location is: " +loc)