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1

WPS is used to configure

real- data simulations

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WPS Program Flowchart 

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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

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Geogridprovides values for

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

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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 

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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

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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!)

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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

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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

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ARW Projections: Polar Stereographic 

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

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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

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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

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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 

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In ARW, (REF_LAT, REF_LON) can refer to

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

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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

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Geogrid: Program Output 

geo_em.d0n.nc
(where nis the domain ID number)

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GRIB is a WMO standard file format for 

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

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GRIB stands for

General Regularly-distributed Information in Binary

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

truncating numbers to a fixed number of digits 

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

record-based

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GRIB 

Fields in a file are identified only by

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

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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

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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 

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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

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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

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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 

 

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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

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 Rotate winds to WRF grid

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

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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!

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Metgrid: Masked Interpolation