FNL Archive Information
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Access -
The FNL archive may be accessed by a ftp client or by a web browser:
1) If you want to use a ftp client, Windows PC users may use WinSCP, FileZilla,
etc. that support passive mode. MacOS and Linux users may use the ftp
command. Connect your ftp client to ftp.arl.noaa.gov using anonymous and
your email address for the username and password, respectively and change
directory to /archives/fnl. Note if you are using multiple ftp
connections, limit the number of concurrent connections equal to or
less than two. Otherwise, your access to our ftp server may be blocked off.
2) If you are using a web browser, point it to
https://www.ready.noaa.gov/data/archives/fnl.
It may take a few minutes or longer for the webpage to display.
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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
Email: orders@ncdc.noaa.gov
Phone: 828-271-4800
Fax: 828-271-4876
ftp: ftp.ncdc.noaa.gov
www: https://www.ncdc.noaa.gov
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.
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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
FNL ARCHIVE
OVERVIEW
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.
ORIGIN OF DATA
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 (nic.fb4.noaa.gov) in
GRIB (GRIdded Binary) format.
ARL PROCESSING
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."
DATA DESCRIPTION
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.
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Table 1. Data Grid Specifications
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Model ID X Y Delta True Align X Y
Type # Max Max Km Lat. Lon. Pole Pole
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FNL 12 129 129 190.5 60N 80W 65.0 65.0
FNL 13 129 129 190.5 60S 100E 65.0 65.0
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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.
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Table 2. Meteorological Fields for FNL Archive Data.
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Field Units Label Data
Order
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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***
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@ 6-hour average
* geopotential meters
** Vertical velocity available only through 100 hPa.
*** Relative humidity available only through 300 hPa.
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Table 3. Description of Vertical Levels
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Level Height
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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
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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
level.
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
END LEVEL AND VARIABLE LOOPS
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
REFERENCES
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-
342).
Parrish, D.F. and J.C. Derber, 1992: The National Meteorological Center's
spectral statistical interpolation analysis system. Mon. Wea. Rev. 120
(1747-1766).
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).
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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 1: 0. 15 PRSS MSLP TMPS TPP6 UMOF VMOF SHTF LHTF DSWF T02M RH2M U10M V10M SOLW TCLD
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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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 1: 0. 15 PRSS MSLP TMPS TPP6 UMOF VMOF SHTF LHTF DSWF T02M RH2M U10M V10M SOLW TCLD
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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