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WPS is used to configure

real- data simulations


WPS Program Flowchart 


We use the geogridprogram to define:

§  Map projection (all domains must use the same projection) 
§  Geographic location of domains 
§  Dimensions of domains 
§  Horizontal resolution of domains


Geogridprovides values for

static (time-invariant) fields at each model grid point 


Geogridprovides values for static (time-invariant) fields at each model grid point

Compute latitude, longitude, map scale factor, and Coriolisparameters at each grid point 
§  Horizontally interpolate static terrestrial data (e.g., topography height, land use category, soil type, vegetation fraction, monthly surface albedo) from global datasets 


Geogrid: Defining model domains 

First, we choose a

map projection to use for the domains; why? 
§  The real earth is (roughly) an ellipsoid 
§  But WRF computational domains are defined by rectangles in the plane


ARW can use any of the following projections:

1.  Lambert conformal 
2.  Mercator 
3.  Polar stereographic 
4.  Latitude-longitude (for global domain, you must choose this projection!)


ARW Projections: Lambert Conformal 

  • Well-suited for mid-latitudes
  • l  Domain cannot contain either pole
  • l  Domain cannot be periodic in west-east direction
  • l  Either one or two true latitudesmay be specified
    • §  If two are given, the order doesn’t matter


ARW Projections: Mercator 

  • Well-suited for low-latitudes
  • l  May be used for “channel” domain (periodic domain in west-east direction)
  • l  A single true latitude is specified
    • §  Cylinder intersects the earth’s surface at +/- truelat


ARW Projections: Polar Stereographic 

  • Good for high-latitude domains,especially if domain must contain a pole
  • l  A single true latitude is specified


ARW Projections: Cylindrical Equidistant 

  • Required for global domains
  • l  May be used for regional domains
  • l  Can be used in its normal or rotated aspect


Geogrid: Defining Model Domains 
l  Define projection of domains using a subset of the following parameters 

MAP_PROJ: ‘lambert’, ‘mercator’, ‘polar’, or ‘lat-lon’ 
§  TRUELAT1: First true latitude 
§  TRUELAT2: Second true latitude (only for Lambert conformal) 
§  POLE_LAT, POLE_LON: Location of North Pole in WRF computational grid (only for ‘lat-lon’) 
§  STAND_LON: The meridian parallel to y-axis 

All parameters reside in the file namelist.wps


Geogrid: Defining Model Domains 
l  Define the area covered (

dimensions and location) by coarse domain using the following: 
§  REF_LAT, REF_LON: The (lat,lon) location of a known location in the domain (by default, the center point of  the domain) 
§  DX, DY: Grid distance where map factor = 1 
§  For Lambert, Mercator, and polar stereographic: meters 
§  For (rotated) latitude-longitude: degrees 
§  E_WE: Number of velocity points in west-east direction 
§  E_SN: Number of velocity points in south-north direction 


In ARW, (REF_LAT, REF_LON) can refer to

an arbitrary point in the domain by using the variables REF_X and REF_Y 


Geogrid: Interpolating Static Fields 

Given definitions of all computational grids, geogrid interpolates terrestrial, timeinvariant fields 
§  Topography height 
§  Land use categories 
§  Soil type (top layer & bottom layer) 
§  Annual mean soil temperature 
§  Monthly vegetation fraction 
§  Monthly surface albedo


Geogrid: Program Output 

(where nis the domain ID number)


GRIB is a WMO standard file format for 

storing regularly-distributed (e.g., gridded) fields


GRIB stands for

General Regularly-distributed Information in Binary


Fields within a GRIB file are compressed with a 
§  Think of

truncating numbers to a fixed number of digits 


GRIB file is a ............. format




Fields in a file are identified only by

code numbers 
§  These numbers must be referenced against an external table to determine the corresponding field 


The ungribprogram 

  • Read GRIB Edition 1 and GRIB Edition 2 files 
  • l  Extract meteorological fields 
  • l  If necessary, derive required fields from related ones 
    • §  E.g., Compute RH from T, P, and Q 
  • l  Write requested fields to an intermediate file format


Ungrib: Vtables 

How does ungrib know which fields to extract? Using Vtables (think: Variable tables) 
l  Vtables are files that give the GRIB codes for fields to be extracted from GRIB input files 
l  One Vtable for each source of data 


Ungrib: Intermediate File Format 

After extracting fields listed in Vtable, ungribwrites those fields to intermediate format 
l  For meteorological data sets not in GRIB format, the user may write to intermediate format directly 
§  Allows WPS to ingest new data sources; basic programming required of user 
§  Simple intermediate file format is easily read/written using routines from WPS


Ungrib: Program Output 

Output files named FILE:YYYY-MM-DD_HH
§  YYYYis year of data in the file; MMis month; DDis day; HHis hour 
§  All times are UTC


The metgridprogram 

Horizontally interpolate meteorological data (extracted by ungrib) to simulation domains (defined by geogrid) 

 Rotate winds to WRF grid 

 ARW Grid Staggering 

Masked Interpolation

Wind Rotation 



Horizontally interpolate meteorological data (extracted by ungrib) to simulation domains (defined by geogrid) 

Masked interpolation for masked fields 
§  Can process both isobaric and native vertical coordinate data sets


 Rotate winds to WRF grid

  i.e., rotate so that U-component is parallel to x-axis, V-component is parallel to y-axis


Metgrid: ARW Grid Staggering 

For ARW, wind U-component interpolated to “u” staggering 
l  Wind V-component interpolated to “v” staggering 
l  Other meteorological fields interpolated to  “θ” staggering by default (can change this!


Metgrid: Masked Interpolation