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wrf_ll_to_ij

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

Prototype

	function wrf_ll_to_ij (
		lon  : numeric,  
		lat  : numeric,  
		opt  : logical   
	)

	return_val  :  float or double

Arguments

lon
lat

Longitude and latitude arrays in ARW WRF.

opt

A logical scalar containing attributes. See description below.

Description

This function replaces wrf_latlon_to_ij.

The return value is a multi-dimensional float or double array that holds the closest I, J positions to the input lon, lat values. The dimensions will be 2 x N:

  • N represents the dimensions of lon and lat
  • loc(0,:,...,:) --> I values
    loc(1,:,...,:) --> J values

The opt variable can contain the following attributes, many of which are included as global attributes on the WRF output file. Attributes are case-insensitive:

  • MAP_PROJ - Model projection [1=Lambert, 2=polar stereographic, 3=mercator, 6=lat-lon]  (required)

  • TRUELAT1 - required for MAP_PROJ = 1, 2, 3 (defaults to 0 otherwise)

  • TRUELAT2 - required for MAP_PROJ = 6 (defaults to 0 otherwise)

  • STAND_LON - Standard longitude used in model projection (required)

  • REF_LON, REF_LON - A reference longitude and latitude (required)

  • KNOWNI, KNOWNJ - The I and J locations of REF_LON and REF_LAT (required)

  • POLE_LAT - optional for MAP_PROJ = 6 (defaults to 90 otherwise)

  • POLE_LAT - optional for MAP_PROJ = 6 (defaults to 0 otherwise)

  • DX, DY - required for MAP_PROJ = 1, 2, 3 (defaults to 0 otherwise)

  • LATINC, LONINC - required for MAP_PROJ = 6 (defaults to 0 otherwise)

Questions on this function should be sent to wrfhelp@ucar.edu.

See Also

wrf_user_ll_to_ij, wrf_user_ij_to_ll, wrf_ij_to_ll

Examples

Example 1

   opt = True
   opt@MAP_PROJ          = 1
   opt@TRUELAT1          =  30.0
   opt@TRUELAT2          =  60.0
   opt@STAND_LON         = -98.0
   opt@REF_LAT           =  34.83
   opt@REF_LON           = -81.03
   opt@KNOWNJ            =  37.0
   opt@KNOWNI            =  30.5

   opt@DX                = 30000.
   opt@DY                = 30000.

   loc = wrf_ll_to_ij( -80.0, 30.0, opt )

Example 2

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

  opt = True
  opt@MAP_PROJ  = nc_file@MAP_PROJ
  opt@TRUELAT1  = nc_file@TRUELAT1
  opt@TRUELAT2  = nc_file@TRUELAT2
  opt@STAND_LON = nc_file@STAND_LON
  opt@DX        = nc_file@DX
  opt@DY        = nc_file@DY

  XLAT  = nc_file->XLAT(0,:,:)
  XLONG = nc_file->XLONG(0,:,:)

  opt@REF_LAT = XLAT(0,0)
  opt@REF_LON = XLONG(0,0)
  opt@KNOWNI  = 1.0
  opt@KNOWNJ  = 1.0

  loc = wrf_ll_to_ij ( -80.0, 30.0, opt )

Example 3

The function wrf_user_ll_to_ij, (available in the $NCARG_ROOT/lib/ncarg/nclscripts/wrf/WRFUserARW.ncl script) can be used to locate i,j points given specific lon, lat references. 

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

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

  opt = True
  opt@returnInt = False    ; Return real values (set to True for integer values - integer is default)
  loc = wrf_user_ll_to_ij (a, -80.0, 30.0, opt)

Example 4

You can see another specific example of using wrf_user_ll_to_ij to get temperature at a specific location: http://www2.mmm.ucar.edu/wrf/OnLineTutorial/Graphics/NCL/Examples/SPECIAL/wrf_meteo_5.ncl

There are other example scripts and their resultant images at:

http://www2.mmm.ucar.edu/wrf/OnLineTutorial/Graphics/NCL/