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pregrid: Overview


 Introduction

REGRID is a preprocessor of the Penn State/NCAR Mesoscale Modeling system (MM5). The primary purpose of REGRID is to read archived meteorological analyses on pressure levels and interpolate these analyses in the horizontal to the model grid as defined by program TERRAIN. The output from REGRID is generally used as a first-guess for objective analysis by program RAWINS, or to make model initial and boundary conditions through program INTERP. For most applications, the objective analysis step (program RAWINS) is the normal route taken.

Thus, the basic program flow for the MM5 modeling system looks like:

TERRAIN (Define model grids; put terrestrial fields on grids)

REGRID (Put first-guess analyses on model grids)

RAWINS (Get observations and perform objective analysis)

INTERP (Interpolate vertically to model sigma levels, make initial conditions for each domain, and boundary conditions for the outermost domain)

MM5 (Integrate the model)

REGRID is intended to replace the DATAGRID program of the MM5 modeling system.

 

REGRID Structure

REGRID has been partitioned into two main parts: PREGRID, which reads analyses from a variety of data sources, and REGRIDDER, which interpolates those analyses to the model grid. Files written in a simple, intermediate format are the means of passing data from PREGRID to REGRIDDER.

This approach of partitioning the functional pieces of REGRID into two sections was selected for several reasons:

  1. Separation of input tasks from the interpolation tasks removes the complicated code handling many types of input from the relatively straightforward task of interpolation.

  2. Access programs could be (and have been) written for individual data sets, keeping the code for the input tasks relatively focused.

  3. Data from different sources are easy to combine. This is most applicable to surface and soil data sets which are not archived in all atmospheric analysis data sets.

  4. All fields output from the pregrid programs are interpolated by REGRIDDER and passed along to the rest of the modeling system.

  5. Separate programs and the simple intermediate format allow users to readily introduce into the MM5 modeling system data sets which are not included in the set of supported data
    sources.

  6. Users introducing data from unsupported sources need only be concerned with the parts of the package required to decode their own particular data sets.

  7. When user intervention is necessary, the modifications are located solely in an area where the user is a presumed expert - with the new data source.

The Unix tarfile "regrid.tar" has been set up to reflect the bipartite structure of the REGRID package, with a top-level directory named "regrid", and subdirectories named "pregrid" and "regridder".

 

A) PREGRID:

PREGRID is not a single program, but rather a suite of programs to read from various sources of gridded data in various formats, and write the data out again in a simple, specified format (described in section C). The primary purpose of this suite of programs is to prepare gridded
analyses for horizontal interpolation to an MM5 model grid by program REGRIDDER.

  1. General description

    A separate PREGRID program has been written for a number of data sources. In addition, many data sets in the GRIB format may be processed by the program in the "grib.misc" directory. Some data sets in GRIB format may be handled more easily be a separate program, rather than by trying to make the grib.misc code handle everything. If users want to import code from some other data source or in some other data format, they will need to create their own pregrid program, which simply reformats the data to be read by REGRIDDER. There currently exists a stand-alone shell for running PREGRID alone (without REGRIDDER). A single shell for running both PREGRID and REGRIDDER will be available sometime in the future.

  2. The "pregrid" directory

    The pregrid directory has the following files and subdirectories:

    Doc:

    Some documentation files, which you've obviously already found

    ecmtoga:

    Tables needed to run the ecmwf toga archives through pregrid

    era :

    Tables needed to run the Ecmwf Reanalysis dataset

    grib.misc:

    Fortran code to run many GRIB datasets

    navysst:

    Program and table needed for Navy SSTs

    ncep.grib:

    Program and tables needed for NCEP GDAS archives in GRIB

    nnrp:

    Tables needed to access NCAR/NCEP Reanalysis project archives.

    on84:

    Program and Tables needed to access old NCEP GDAS archives in
    ON84 format.

    pregrid.deck:

    Deck used to submit a pregrid job to NCAR's crays

    pregrid.csh:

    C-shell script used to run pregrid locally or interactively

    util:

    Fortran code for many functions common to many of the data access programs

     

  3. How to use PREGRID

    The pregrid programs take several different kinds of input:

    1) The namelist, defining times you want to process.
    2) The archived analysis files.
    3) the Vtable file, which describes which fields to extract from the archive files, and what variable names are assigned. Vtables are explained in more detail below.

    The pregrid program scans through each of the input analysis files, checking on the dates. For any dates which fall between your start time and end time (inclusive), pregrid scans through the archived fields and pulls out those which have been specified in the Vtable. From these fields, pregrid builds (or adds to) preliminary output files, and the moves on to the next input file. When all input files have been processed, and all pertinent data between the start time and end time have been written to the preliminary output files, pregrid rereads the output files, and builds in certain fields which may be missing. The pregrid program then scans through all the files, searching for dates which are missing. If it finds a missing date, it fills it in by temporal interpolation. Then the final output files are written.

    1. pregrid shell

      The pregrid shell "pregrid.csh", is provided to run pregrid interactively on a workstation.

      To run the "pregrid.csh" script:
      1) Get the analysis archives and put them in some directory.
      2) Set the script variables.
      3) Fill out the namelist.
      4) Make sure you've got the right Vtables, especially if you're getting analyses other than the standard ones.
      5) Make sure "pregrid.csh" is executable ("chmod u+x pregrid.csh").
      6) Run pregrid.csh, by entering "pregrid.csh"

      1. Script variables

        DATADIR:

        The local directory where you've placed the original
        analysis archive files.

        SCR3D:

        The source of 3D analyses. Select from one of the
        following:

        ON84 :

        NCEP GDAS analyses, in ON84 format
        (before 1997 Mar).

        NCEP :

        NCEP GDAS analyses, in GRIB format
        (beginning 1997 Mar).

        GRIB :

        A catch-all for many other datasets in the
        GRIB format.

        InFiles:

        Full pathname to the files with 3D Analyses.

        SRCSST:

        Source of the SST Analyses. Select from among
        the following:

        ON84 :

        NCEP GDAS analyses, in ON84 format
        (before 1997 Mar).

        NCEP :

        NCEP GDAS analyses, in GRIB format
        (beginning 1997 Mar).

        NAVY :

        Low-resolution Navy archives
        (roughly 2.5x2.5 degrees)

        GRIB :

        A catch-all for many other datasets in the
        GRIB format.

        $SCR3D:

        Same source as the 3D analyses

        InSST:

        Full pathname to the files with SST analyses, if different
        from the files with 3D analyses.

        SRCSNOW:

        Source of snow-cover analyses.

        InSnow:

        Full pathname to the files with Snow-cover analyses, if
        different from the files with 3D analyses.

        SRCSOIL:

        Source of soil (i.e. ground) analyses (e.g., soil moisture,
        ground temperature, etc.)

        InSoil:

        Full pathname to the files with Soil analyses, if
        different from the files with 3D analyses.

         

         

        Next, there are a few script variables you need to set if you
        have set any of the above to GRIB. These script variables
        denote the tables the GRIB pregridder is going to need to know
        which variables to extract from the GRIB files.

        VT3D:

        The Vtable for the 3D analyses

        VTSST:

        The Vtable for the SST analyses

        VTSNOW:

        The Vtable for the Snow-cover analyses

        VTSOIL:

        The Vtable for the soil analyses.

         

      2. namelist

        There is a small namelist built by the "pregrid.csh" script. This namelist file is named "pregrid.namelist", and defines the dates you want to process:

        The starting date of the time period you want to process is set by
        START_YEAR, START_MONTH, START_DAY, and START_HOUR: eg:

        START_YEAR = 1987 # Year (YYYY)
        START_MONTH = 08 # Month ( 01 - 12 )
        START_DAY = 2 # Day ( 01 - 31 )
        START_HOUR = 12 # Hour ( 00 - 23 )

        DELTA_TIME = 6 # Time interval (hours) to process.
        # This is most sanely the same as the time interval for
        # which the analyses were archived, but you can really
        # set this to just about anything, and pregrid will
        # interpolate in time and/or skip over time periods for
        # your regridding pleasure.

        NTIMES = 2 # Number of analysis times.
        # Don't forget to count both the first and last times.
        # E.g.: A 24-hour period with with 6-hour time interval
        # needs 5 analysis times (00Z, 06Z, 12Z, 18Z, 00Z).

    2. Vtables

      The Vtables are the means by which the pregrid programs are told which fields to extract from the analysis files. Tables like these are expedient, because the archives tend to label fields by code numbers, and different archives frequently use different code numbers. The Vtables also allow users to request that additional fields be extracted from the archives, without having to modify the pregrid programs themselves.

      The Vtable files have a fairly strictly defined format:

      ++ Any lines before the first line beginning with '-' are ignored.

      ++ All lines after the first line beginning with '-', and up to the next line beginning with '-', are interpreted.

      ++ Any subsequent lines after the next line beginning with '-' are ignored, until the next line beginning with '-' is hit, and then the following lines are interpreted. This pattern continues until the end of the file.

      ++ For each line to be interpreted, there are the following fields, separated by "|":
      - A code number associated with the variable to extract
      - A level code number associated with the type of level to extract. This generally refers to pressure levels, surface, or ground.
      - A "Level 1" value, denoting which level to extract (* indicates all levels of the type of level described by the level code).
      - A "Level 2" value, needed for those fields which represent averages or sums over more than one level.
      - A REGRID Name, up to eight characters, which is what the field will be called in the regridder output, and in subsequent programs of the MM5 system.
      - REGRID Units, a string of up to 16 characters which describes the units of the field.
      - REGRID Description, up to 54 characters describing the field.

      If the Units and Description strings are missing, pregrid will read the named field from the archives, but will not write it to the final output (for input to regridder). This is necessary for those situations where one field which we desire to output is derived from fields which we may not want to output. For example, some archives are missing surface RH, but have both surface T and surface dewpoint. So we have to read in the dewpoint, but we don't want to write it out, we want to compute RH and write RH.

      If you need to make your own Vtable, there are plenty of examples in the grib.misc directory.

 

B) REGRIDDER:

REGRIDDER is a program which reads the horizontal fields output from PREGRID and interpolates them horizontally to the MM5 model grid

 

C) INTERMEDIATE DATA FORMAT

  1. General format description

    Data are passed from the PRIGRID programs to REGRIDDER via files written in the format described herein. Any number of horizontal slabs may be written in to single file. The slabs in a given file are not necessarily all from the same data source, or all on the same map projection, but they all represent the same time.

  2. File Naming conventions

    Each file contains data for a single time. The file names consist of a prefix (ideally, but not necessarily, denoting the source of data), followed by a colon, followed by a time-stamp in the form YYYY-MM-DD_HH.
    REGRIDDER uses the file names as discussed in section 4.

    For example, analyses from the ON84-format data from NCEP for 3 January 1998 at 12 UTC would be written to a file called "ON84:1998-01-03_00"

  3. File format:

    The files are written as unformatted FORTRAN 32-bit IEEE records. Four
    records are used for every horizontal slab.

     

    Record 1:

    IFV

     

    Record 2:

    HDATE, XFCST, FIELD, UNITS, DESC, XLVL, NX, NY, IPROJ
     

    if (IPROJ == 0) (Cylindrical equidistant grid)

     

    Record 3:

    STARTLAT, STARTLON, DELTALAT, DELTALON

    if (IPROJ == 3) (Lambert conformal grid)

     

    Record 3:

    STARTLAT, STARTLON, DX, DY, XLONC, TRUELAT1, TRUELAT2

    if (IPROJ == 5) (Polar-stereographic grid)

     

    Record 3:

    STARTLAT, STARTLON, DX, DY, XLONC, TRUELAT1
     

     

    Record 4:

    SLAB

    Where:

     

     

     

     

    integer

    ::

    IFV

    !

    The PREGRID format version number, currently 2

    character*24

    ::

    HDATE

    !

    The time, in format "YYYY-MM-DD_HH:mm:ss "
    (only the first 19 characters are used)

    real

    ::

    XFCST

    !

    Forecast time (in hours) of the data in the slab

    character*8

    ::

    FIELD

    !

    A field name. Names with special meaning are described in section 4.

    character*16

    ::

    UNITS

    !

    Units describing the field in the slab

    character*56

    ::

    DESC

    !

    Text description of the field.

    real

    ::

    XLVL

    !

    Pressure-level (Pa) of the data.
       200100 Pa indicates surface data;
       201300 Pa indicates sea-level pressure

    integer

    ::

    NX

    !

    Slab dimension in the X direction

    integer

    ::

    NY

    !

    Slab dimension in the Y direction

    integer

    ::

    IPROJ

    !

    Flag denoting the projection. Recognized values are:
      0: Cylindrical Equidistant (Lat/lon) grid.
      3: Lambert-conformal projection
      5: Polar-stereographic projection

    real

    ::

    STARTLAT

    !

    Starting latitude (degrees)

    real

    ::

    STARTLON

    !

    Starting longitude (degrees)

    real

    ::

    DELTALAT

    !

    Latitude increment (degrees) for lat/lon grid

    real

    ::

    DELTALON

    !

    Longitude increment (degrees) for lat/lon grid

    real

    ::

    DX

    !

    Grid-spacing in x (km at TRUELAT1 (and TRUELAT2))

    real

    ::

    DY

    !

    Grid-spacing in y (km at TRUELAT1 (and TRUELAT2))

    real

    ::

    XLONC

    !

    Center longitude of the projection.

    real

    ::

    TRUELAT1

    !

    Extra latitude used for defining Polar Stereographic and Lambert conformal projections

    real

    ::

    TRUELAT2

    !

    A second extra latitude used for defining Lambert conformal projection

    real

    ::

    SLAB(NX,NY)

    !

    Two-dimensional array of data.



  4. Special field names

    The program variable FIELD indicates the physical variable in the slab.
    Certain values of FIELD are recognized by REGRIDDER for specific treatment.
    Slabs identified by an unrecognized values of FIELD are simply interpolated
    horizontally and written out by REGRIDDER. Recognized field names are:
    'T '
    'U '
    'V '
    'RH '
    'HGT '
    'PMSL '
    'SOILT010'
    'SOILT200'
    'SOILT400'
    'SOILM010'
    'SOILM200'
    'SST '
    'SEAICE '
    'LANDSEA '
    'SOILHGT '
    'WEASD '
    'SNOWCOVR'

  5. Sample read program:

    program ddread

    C This is a simple program to read data in the PREGRID output format.
    C It is included mainly as an aid to understanding said format.

    implicit none

    C Declarations:

    integer IFV
    character*24 HDATE
    real XFCST
    character*8 FIELD
    character*16 UNITS
    character*56 DESC
    real XLVL
    integer NX
    integer NY
    integer IPROJ
    real STARTLAT
    real STARTLON
    real DELTALAT
    real DELTALON
    real DX
    real DY
    real XLONC
    real TRUELAT1
    real TRUELAT2
    C
    C So as not to need dynamic memory allocation, we dimension SLAB as
    C large as we're likely to need. In order to read the appropriate
    C size of SLAB, we use the read routine READSLAB. If you want to do
    C anything with the array SLAB, subroutine READSLAB is the place to
    C do it.
    C
    integer MAXNX, MAXNY
    parameter (MAXNX=500, MAXNY=500)
    real SLAB(MAXNX,MAXNY)

    integer IUNIT

    IUNIT = 10

    100 CONTINUE

    read (IUNIT, END=101) IFV
    if (IFV.EQ.2) THEN
    read (IUNIT) HDATE, XFCST, FIELD, UNITS, DESC, XLVL, NX, NY,
    & IPROJ
    print*, HDATE, XLVL, FIELD
    if (IPROJ .EQ. 0) then
    read (IUNIT) STARTLAT, STARTLON, DELTALAT, DELTALON
    elseif (IPROJ .EQ. 3) then
    read (IUNIT) STARTLAT, STARTLON, DX, DY, XLONC, TRUELAT1,
    & TRUELAT2
    elseif (IPROJ. EQ. 5) then
    read (IUNIT) STARTLAT, STARTLON, DX, DY, XLONC, TRUELAT1
    endif

    call readslab(IUNIT, SLAB, NX, NY)
    else
    print*, 'Unrecognized format version IFV: ', IFV
    stop 00001
    endif

    GO TO 100

    101 CONTINUE
    stop 99999
    end



    subroutine readslab(IUNIT, SLAB, NX, NY)
    implicit none
    integer iunit
    integer nx, ny
    real slab(nx,ny)

    read (iunit) SLAB

    C If you want to do anything with SLAB, this is as good place as any
    C for it.

    return
    end
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