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FNL Archive Information

NCEP Model Output -- FNL ARCHIVE DATA TD-6141                                  
archive began: January 1, 1997                           
archive ended: December 31, 2006                                          
Prepared for National Climatic Data Center (NCDC)
by Barbara J.B. Stunder
NOAA-Air Resources Laboratory
1315 East-West Highway
Silver Spring, MD 20910  (301-713-0295)        
[See separate documentation for the MRF Archive,TD-6140, for the period 1991 - 1996.]   
June 17, 1997 

The data in the files called fnl.xh.mmmyy.00# contain either the
first 15 days of the month (00#=001) or the rest of the month
(00#=002), where xh refers to the northern (x=n) or southern (x=s)
hemisphere, mmm is the month (e.g. jul) and yy is the year
(97).  The data are archived on tape and sent to the National 
Climatic Data Center (NCDC) for distribution to anyone.  

NCDC can be reached at:

      Climate Services Branch
      National Climatic Data Center
      151 Patton Avenue
      Asheville, NC 28801
      Phone:  828-271-4800
      Fax:    828-271-4876

Note that the FNL Archive began January 1, 1997.  Global data for the
period 1991 - 1996 are called the MRF Archive and are also available
from NCDC (TD-6140).

The following is a document describing the global FNL ARCHIVE (TD-6141)
that is available from NCDC.  It explains what the data are and what 
data are available.  



     The National Weather Service's National Centers for Environmental 
Prediction (NCEP) runs a series of computer analyses and forecasts
operationally (Petersen and Stackpole, 1989).  One of the operational 
systems is the Global Data Assimilation System (GDAS, Kanamitsu, 1989), 
which uses the spectral Medium Range Forecast model (MRF) for the forecast 
(Sela, 1980).  Another system is the EDAS (Eta Data Assimilation System), 
covering the U.S.

     At NOAA's Air Resources Laboratory (ARL), NCEP model output are
used for air quality transport and dispersion modeling.  ARL archives 
both EDAS and GDAS data using a 1-byte packing method.  Both archives 
contain basic fields such as the u- and v-wind components, temperature, 
and humidity.  However, the archives differ from each other because of 
the horizontal and vertical resolution, as well as in the specific 
fields, provided by NCEP. 


     The 6-hourly archive data come from NCEP's GDAS.  The GDAS is the 
final run in the series of NCEP operational model runs; it therefore is 
known as the Final Run at NCEP and includes late arriving conventional 
and satellite data (Petersen and Stackpole, 1989).  It is run 4 times 
a day, ie, at 00, 06, 12, and 18 UTC.  Model output is for the analysis 
time and a 6-hour forecast.  Some fields such as precipitation and 
surface fluxes are only available at the forecast time.  Details of the 
GDAS are described by Kanamitsu (1989), Derber et al. (1991), and 
Parrish and Derber (1992).  

     NCEP post-processing of the GDAS converts the data from spectral 
coefficient form to 1 degree latitude-longitude (360 by 181) grids and 
from sigma levels to mandatory pressure levels.  The data are written 
to the NIC (NOAA Information Center) FTP server ( in 
GRIB (GRIdded Binary) format.


     The ARL archiving program converts data from the 1 degree latitude-
longitude grid to hemispheric 129 by 129 polar stereographic grids.  
Model output from both the analysis and 6-h forecast are written to 
the archive file, so as to include the fields only available in the 
forecast period, and to enable the short-term forecast data to 
"fill-in" for a missing period if model output from any single NCEP 
run is not available.  These archived data are referred to as the 
"FNL archive."


     The archive data file contains the data in synoptic time
sequence, without any missing records (missing data will be
represented by nulls and the forecast hour by negative 1). 
Therefore it is possible to position randomly to any point within
a data file.  Each file contains data on one hemisphere for 
approximately half a month: days one through 15, and 16 through the 
end of the month.  At each time period, an index record and surface 
data come first, followed by all data in each mandatory pressure 
level from the ground up.

Cartridge Specifications (beginning January 1, 1997)

     Data for one hemisphere, covering a one month period, are archived 
          on a 3480 cartridge and sent to NCDC.
     2 files per month: day 1-15 and 16-end of month.
     Files are copied to cartridge with the UNIX "dd" command.

     TYPE           3480 cartridge, ASCII*
     RECORD LENGTH  16691
     BLOCK SIZE     16691

* Note that each data record is composed of a 50-character header
in ASCII, followed by the binary packed data.  Therefore, an
ASCII-EBCDIC conversion on the entire data record or cartridge
file is not possible. 

Data Grid

     The data are on hemispheric 129 by 129 polar stereographic
grids.  In Table 1 each data grid is identified 
by the model that produced the data, a grid identification
number, the number of X and Y grid points, the grid spacing (Delta)
which is true at the indicated latitude, the longitude to which
the Y axis is aligned, and the pole position in grid units.  The
given pole position results in the lowest left grid point to have
a value of (1,1).  The northern and southern hemisphere grids
have ID #12 and #13, respectively.  

                Table 1. Data Grid Specifications
Model     ID   X    Y    Delta     True Align     X    Y
Type      #    Max  Max  Km        Lat. Lon.   Pole    Pole
FNL       12  129  129   190.5     60N   80W   65.0    65.0
FNL       13  129  129   190.5     60S  100E   65.0    65.0

Meteorological Fields and Vertical Structure

     The archived data file only contains some of the fields
normally produced by the model at NCEP.  These were selected
according to what is most relevant for transport and dispersion
studies and disk space limitations.  In Table 2, the fields are
identified by a description, the units, and a unique four
character identification label that is written to the header
label (see Data Grid Unpacking Procedure in a later section) 
of each record.  Data order in the file is given by
a two digit code.  The first digit indicates if it is a surface
(or single) level variable (S) or an upper level variable (U).
The second digit indicates the order in which that variable
appears in the file.  The upper level FNL data are output on the
following mandatory pressure surfaces: 1000, 925, 850, 700, 500, 
400, 300, 250, 200, 150, 100, 50, and 20 hPa.  Table 3 gives the 
level number, corresponding to each data level, which is also 
written to each header label.  

      Table 2. Meteorological Fields for FNL Archive Data.
Field                                    Units     Label     Data 
Pressure at surface                       hPa       PRSS      S1
Pressure reduced to mean sea level        hPa       MSLP      S2
Temperature at surface                    K         TMPS      S3
Total precipitation (6 h accumulation)    m         TPP6      S4
Momentum flux, u-component at surface     N/m2      UMOF      S5@
Momentum flux, v-component at surface     N/m2      VMOF      S6@
Sensible heat net flux at surface         W/m2      SHTF      S7@
Latent heat net flux at surface           W/m2      LHTF      S8@
Downward short wave radiation flux        W/m2      DSWF      S9@
  at surface
Temperature at 2 m AGL                    K         T02M      S10
Relative humidity at 2 m AGL              %         RH2M      S11
U-component of wind at 10 m AGL           m/s       U10M      S12
V-component of wind at 10 m AGL           m/s       V10M      S13
Volumetric soil moisture content          fraction  SOLW      S14
  of layer 0-10 cm below ground
Total cloud cover, entire atmosphere      %         TCLD      S15@
U-component of wind with respect to grid  m/s       UWND      U1
V-component of wind with respect to grid  m/s       VWND      U2
Geopotential height                       gpm*      HGTS      U3
Temperature                               K         TEMP      U4
Pressure vertical velocity                hPa/s     WWND      U5**
Relative humidity                         %         RELH      U6***
@ 6-hour average
* geopotential meters
** Vertical velocity available only through 100 hPa.
*** Relative humidity available only through 300 hPa.

              Table 3. Description of Vertical Levels
		Level     Height
		13     20 hPa
		12     50 hPa
		11    100 hPa
		10    150 hPa
		 9    200 hPa
		 8    250 hPa
		 7    300 hPa
		 6    400 hPa
		 5    500 hPa
		 4    700 hPa
		 3    850 hPa
		 2    925 hPa
		 1   1000 hPa
		 0   surface

Missing Data

     Missing data are written as an array of nulls with a forecast hour 
of -1 in the header label.  Usually the associated field label is "NULL".  

     If data from one GDAS run (analysis and +6 h) are not available, the 
fields available only at +6 h (TPP6, UMOF, VMOF, SHTF, LHTF, DSWF, and TCLD)
will be the only fields missing from the archive.  Data at the analysis time
will be a 6-h forecast from the previous run.  Data at the  time +6 h
will be the analysis from the next run, except for the missing 6-h 
accumulated or averaged values.

Definition File

     The definition file summarizes the grid specifications and data 
fields.  The northern hemisphere file is given in Appendix A; the 
southern hemisphere file in Appendix B.  The format is such that the 
first 20 characters are the dummy ID field followed by the data.  
Much of the information is written into the index record of
each time period.  

Record 1 consists of a four character string that identifies the source 
of the meteorological data.  

Record 2 is the integer identification of the meteorological data grid
(Table 1).  

Record 3 is an integer number that identifies the vertical coordinate 
system.  Only four coordinate types are recognized: 1-pressure sigma; 
2-pressure absolute; 3-terrain sigma; 4-hybrid sigma.

Records 4 & 5 identifies the pole position of the grid projection.  
Most projections will either be defined at +90 or -90 depending upon 
the hemisphere.  The longitude would be the point 180 degrees from 
which the projection is cut.

Records 6 & 7 is the reference position at which the grid spacing is defined.

Record 8 is the grid spacing in km at the reference position.

Record 9 is the grid orientation or the longitude of the meridian which 
is parallel to the up-down direction of the grid.

Record 10 is the angle between the axis and the surface of the cone.  
For regular projections it is equal to the latitude at which the grid 
is tangent to the earth's surface.  A polar stereographic grid would be 
tangent at either 90 or -90, while a Mercator projection is tangent
at 0 latitude.   A Lambert Conformal projection would be in between the 
two limits.  An oblique stereographic projection would have a cone angle 
of 90.
Records 11 & 12 are used to equate a position on the grid with a position 
on the earth as given in Records 13 & 14.  For the FNL archive, the 
position indicated is the center of the grid located over the North Pole.  
Any position is acceptable.  It need not even be on the grid.
Record 15 is not currently used.

Records 16 & 17 identify the number of grid points in each direction.

Record 18 is the number of levels in the vertical, including the surface 

Record 19, through the number of levels, identifies the height of each 
level in appropriate units according the definition of the vertical 
coordinate, the number of variables at that level, and the four character 
identification string for each variable.  The height coordinate is as 
follows for each type of vertical coordinate: 1-sigma (fraction); 
2-pressure (mb); 3-terrain (fraction); 4-hybrid (mb-offset.fraction)

Index record

     The key to reading the meteorological files is decoding the ASCII
index record, the first record of each time period.  The first 50 
characters of the index record contain the same "header" information as 
do the other records in the given time period.  The four-character label 
is "INDX".  The format for this record is given below.  Complete 
descriptions are similar to the variables in the discussion above of the 
Definition File.

     Format of the Index Record

     A4   Data Source
     I3   Forecast hour 
     I2   Minutes associated with data time
     12F7.     1) Pole Lat, 2) Pole Long, 3) Tangent Lat, 4) Tangent Long, 
               5) Grid Size, 6) Orientation, 7) Cone Angle, 8) X-Synch pnt, 
               9) Y-Synch pnt, 10) Synch pnt lat, 11) Synch pnt long, 
               12) Reserved
     3I3  1) Numb x pnts, 2) Numb y pnts, 3) Numb levels
     I2   Vertical coordinate system flag
     I4   Length in bytes of the index record, excluding the first 50 bytes

          LOOP ===> number of data levels

          F6.  height of the first level
          I2   number of variables at that level

               LOOP ===> number of variables

               A4   variable identification 
               I3   rotating checksum of the packed data
               1X   Reserved space for future use


Data Grid Unpacking 

     NCEP typically saves their model output in GRIB format.  
However, here the data are stored differently because the ARL format 
is a bit more compact and it can be directly used on a variety of 
computing platforms with direct access I/O. 

     The data array is packed and stored into one byte characters.
To preserve as much data precision as possible the difference
between the values at grid points is saved and packed rather than
the actual values.  The grid is then reconstructed by adding the
differences between grid values starting with the first value,
which is stored in unpacked ASCII form in the header record. To
illustrate the process, assume that a grid of real data, R, of
dimensions i,j is given by the below example.

  1,j       2,j        ....    i-1,j      i,j
  1,j-1     2,j-1      ....    i-1,j-1    i,j-1
  ....      ....       ....    ....       ....
  1,2       2,2        ....    i-1,2      i,2
  1,1       2,1        ....    i-1,1      i,1

The packed value, P, is then given by 

     Pi,j = (Ri,j  - Ri-1,j)* (2**(7-N)),

where the scaling exponent

     N = ln dRmax / ln 2 .

The value of dRmax is the maximum difference between any two
adjacent grid points for the entire array.  It is computed from the
differences along each i index holding j constant.  The difference
at index (1,j) is computed from index (1,j-1), and at 1,1 the
difference is always zero. The packed values are one byte unsigned
integers, where values from 0 to 126 represent 
-127 to -1, 127 represents zero, and values of 128 to 254 represent
1 to 127. Each record length is then equal in bytes to the number
of array elements plus 50 bytes for the header label information. The 
50 byte label field precedes each packed data field and contains the
following ASCII data:

Field          Format    Description                             
Year           I2        Greenwich date for which data valid
Month          I2                       "
Day            I2                       "
Hour           I2                       "
Forecast*      I2        Hours forecast, zero for analysis
Level          I2        Level from the surface up (see Table 3)
Grid           I2        Grid identification (see Table 1)
Variable       A4        Variable label (see Table 2)
Exponent       I4        Scaling exponent needed for unpacking
Precision      E14.7     Precision of unpacked data
Value 1,1      E14.7     Unpacked data value at grid point 1,1
*Forecast hour is -1 for missing data.  

Sample Program

for an example of the unpacking program.


Derber J.C., D.F. Parrish, and S.J. Lord, 1991: The new global
operational analysis system at the National Meteorological Center,
Weather and Forecasting, 6, (538-547).

Kanamitsu, M., 1989: Description of the NMC Global Data
Assimilation and Forecast System, Weather and Forecasting, 4 (335-

Parrish, D.F. and J.C. Derber, 1992: The National Meteorological Center's 
spectral statistical interpolation analysis system.  Mon. Wea. Rev. 120

Petersen, R.A. and J.D. Stackpole, 1989: Overview of the NMC
Production Suite, Weather and Forecasting, 4 (313-322).

Sela, J.G., 1980: Spectral modeling at the National Meteorological
Center, Mon. Wea. Rev., 108 (1279-1292).


	Appendix A.  Definition File - Northern Hemisphere

MODEL TYPE:         FNL 
GRID NUMB:          12
VERT COORD:         2
POLE LAT:           90.
POLE LON:           0.
REF LAT:            60.
REF LON:            -080.
REF GRID:           190.5
ORIENTATION:        0.
CONE ANGLE:         90.
SYNC X:             65.
SYNC Y:             65.
SYNC LAT:           90.
SYNC LON:           0. 
SPECIAL:            0.
NUMB X:             129
NUMB Y:             129
NUMB LEVELS:        14
LEVEL 2:            1000.  06 UWND VWND HGTS TEMP WWND RELH 
LEVEL 3:            925.   06 UWND VWND HGTS TEMP WWND RELH
LEVEL 4:            850.   06 UWND VWND HGTS TEMP WWND RELH
LEVEL 5:            700.   06 UWND VWND HGTS TEMP WWND RELH
LEVEL 6:            500.   06 UWND VWND HGTS TEMP WWND RELH
LEVEL 7:            400.   06 UWND VWND HGTS TEMP WWND RELH
LEVEL 8:            300.   06 UWND VWND HGTS TEMP WWND RELH
LEVEL 9:            250.   05 UWND VWND HGTS TEMP WWND 
LEVEL 10:           200.   05 UWND VWND HGTS TEMP WWND 
LEVEL 11:           150.   05 UWND VWND HGTS TEMP WWND 
LEVEL 12:           100.   05 UWND VWND HGTS TEMP WWND
LEVEL 13:           050.   04 UWND VWND HGTS TEMP 
LEVEL 14:           020.   04 UWND VWND HGTS TEMP 

	Appendix B.  Definition File - Southern Hemisphere

MODEL TYPE:         FNL 
GRID NUMB:          13
VERT COORD:         2
POLE LAT:           -90.
POLE LON:           0.
REF LAT:            -60.
REF LON:            -080.
REF GRID:           190.5
ORIENTATION:        0.
CONE ANGLE:         -90.
SYNC X:             65.
SYNC Y:             65.
SYNC LAT:           -90.
SYNC LON:           0. 
SPECIAL:            0.
NUMB X:             129
NUMB Y:             129
NUMB LEVELS:        14
LEVEL 2:            1000.  06 UWND VWND HGTS TEMP WWND RELH 
LEVEL 3:            925.   06 UWND VWND HGTS TEMP WWND RELH
LEVEL 4:            850.   06 UWND VWND HGTS TEMP WWND RELH
LEVEL 5:            700.   06 UWND VWND HGTS TEMP WWND RELH
LEVEL 6:            500.   06 UWND VWND HGTS TEMP WWND RELH
LEVEL 7:            400.   06 UWND VWND HGTS TEMP WWND RELH
LEVEL 8:            300.   06 UWND VWND HGTS TEMP WWND RELH
LEVEL 9:            250.   05 UWND VWND HGTS TEMP WWND 
LEVEL 10:           200.   05 UWND VWND HGTS TEMP WWND 
LEVEL 11:           150.   05 UWND VWND HGTS TEMP WWND 
LEVEL 12:           100.   05 UWND VWND HGTS TEMP WWND
LEVEL 13:           050.   04 UWND VWND HGTS TEMP  
LEVEL 14:           020.   04 UWND VWND HGTS TEMP 


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