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Global Data Assimilation System (GDAS1) Archive Information

Archive began: December 1, 2004

NOAA-Air Resources Laboratory 1315 East-West Highway Silver Spring, MD 20910 (301-713-0295)

GDAS ARCHIVE OVERVIEW

The National Weather Service's National Centers for Environmental Prediction (NCEP) runs a series of computer analyses and forecasts operationally. One of the operational systems is the GDAS (Global Data Assimilation System). Information on this model can be found on the NCEP website. At NOAA's Air Resources Laboratory (ARL), NCEP model output are used for air quality transport and dispersion modeling. ARL archives both EDAS (Eta Data Assimilation System) and GDAS output using a 1-byte packing routine. 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 THE DATA The 3-hourly archive data come from NCEP's GDAS. The GDAS is run 4 times a day, ie, at 00, 06, 12, and 18 UTC. Model output is for the analysis time and 3, 6, and 9-hour forecasts. 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. Model output is in GRIB format. See the web link above for current information on the GDAS. ARL saves the successive analyses and 3-hour forecast, four times each day to produce a continuous data archive. Some fields such as precipitation and surface fluxes are not available at the analysis time, therefore these are taken from the 6-hour forecast files.

ARL PROCESSING ARL processing converts NCEP's 1-degree GRIB output using a one-byte character packing method described below. Then, the ARL archiving program produces a 3 hourly, global, 1 degree latitude longitude dataset on pressure surfaces. The data are put into weekly files and made available online at the ARL server for access via ftp. 7-day archive file size is about 600 MB. If processing for a cycle fails, the data from the 6-h and 9-h forecast will fill in the missings.

DATA DESCRIPTION The archive data file contains the data in synoptic time sequence, without any missing records (missing data is represented by nulls and the forecast hour is set to negative 1). Therefore it is possible to position randomly to any point within a data file. Each file contains data for one week except for files containing data past the 28th of the month. At each time period, an index record is always the first record, followed by surface data, and then all data in each pressure level from the ground up. GDAS1 data are available in the files called gdas1.mmmyy.w#, where mmm is the month (e.g. jul) and yy is the year (05) and # refer to:

#=1 - days 1-7
#=2 - days 8-14
#=3 - days 15-21
#=4 - days 22-28
#=5 - days 29 - rest of the month

Data Grid The data are on a 360 by 181 latitude-longitude (grid). The lower-left corner (1,1) is (0W,90S). The upper-right corner (360,181) is (1W, 90N). In Table 1, the data grid is identified by the model that produced the data, a grid identification number, the number of X and Y grid points, the Pole position (latitude and longitude) of the grid projection, a reference latitude and longitude, the grid spacing (km) which is true at the reference point, the orientation with respect to the reference longitude, the angle between the axis and the cone, and a point on the grid in grid units and latitude and longitude . The given pole position results in the lowest left grid point to have a value of (1,1).

Table 1. Data Grid Specifications

Model Type ID # X Max Y Max Pole Lat. Pole Lon. Ref. Lat. Ref. Lon. Ref. Grid Orientation Cone Ang. Sync X Sync Y Sync Lat. Sync Lon.
GDAS 99 360 181 90N 359E 1N 1E 111.1984 0 0 1.0 1.0 90.0S 0.0

Table 2. Meteorological Fields contained in the GDAS Archive. For accumulation/average fields, 6-h acc/avg at 00, 06, 12, 18 UTC, 3-h acc/avg at 03, 09, 15, 21 UTC.

Field Units Label Data Order
Pressure at surface hPa PRSS S1
Pressure reduced to mean sea level hPa MSLP S2
Accumulated precipitation (6 h accumulation) m TPP6 S3
u-component of momentum flux (3- or 6-h average) N/m2 UMOF S4
v-component of momentum flux (3- or 6-h average) N/m2 VMOF S5
Sensible heat net flux at surface (3- or 6-h average) W/m2 SHTF S6
Downward short wave radiation flux (3- or 6-h average) W/m2 DSWF S7
Relative Humidity at 2m AGL % RH2M S8
U-component of wind at 10 m AGL m/s U10M S9
V-component of wind at 10 m AGL m/s V10M S10
Temperature at 2m AGL K TO2M S11
Total cloud cover (3- or 6-h average) % TCLD S12
Geopotential height gpm* SHGT S13
Convective available potential energy J/Kg CAPE S14
Convective inhibition J/kg CINH S15
Standard lifted index K LISD S16
Best 4-layer lifted index K LIB4 S17
Planetary boundary layer height m PBLH S18
Temperature at surface K TMPS S19
Accumulated convective precipitation (6 h accumulation) m CPP6 S20
Volumetric soil moisture content frac. SOLM S21
Categorial snow (yes=1, no=0) (3- or 6-h average) CSNO S22
Categorial ice (yes=1, no=0) (3- or 6-h average) CICE S23
Categorial freezing rain (yes=1, no=0) (3- or 6-h average) CFZR S24
Categorial rain (yes=1, no=0) (3- or 6-h average) CRAI S25
Latent heat net flux at surface (3- or 6-h average) W/m2 LHTF S26
Low cloud cover (3- or 6-h average) % LCLD S27
Middle cloud cover (3- or 6-h average) % MCLD S28
High cloud cover (3- or 6-h average) % HCLD S29
Geopotential height gpm* HGTS U1
Temperature K TEMP U2
U-component of wind with respect to grid m/s UWND U3
V-component of wind with respect to grid m/s VWND U4
Pressure vertical velocity hPa/s WWND U5
Relative humidity % RELH U6

* geopotential meters

Meteorological Fields and Vertical Structure The archived data files contain only some of the fields produced by the model at NCEP. These fields 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 GDAS data are output on the following 23 pressure surfaces. Table 3 gives the level number corresponding to each data level, which is also written to each header label.

Table 3. Description of Vertical Levels

Level Height (hPa)
23 20
22 50
21 100
20 150
19 200
18 250
17 300
16 350
15 400
14 450
13 500
12 550
11 600
10 650
9 700
8 750
7 800
6 850
5 900
4 925
3 950
2 975
1 1000

Missing Data Missing data are written as an array of nulls with a forecast hour of -1 in the header label. The associated field label may be either "NULL" or the label given in Table 2.

Definition File The definition file given in Appendix A summarizes the grid specifications and data fields. 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. 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 (INDX) record - first record of each time period

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

Format Information
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, at ARL the data are stored in a more compact form and 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 adjacent grid point=s values 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 at grid point (1,1). 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 A sample FORTRAN90 program is available from the ARL ftp server ( ftp://arlftp.arlhq.noaa.gov/pub/archives/utility/chk_data.f ) that can be used to unpack and read the first few elements of the data array for each record of an ARL packed meteorological file.

Appendix A. Definition File - GDAS.CFG

MODEL TYPE: GDAS
GRID NUMB: 99
VERT COORD: 2
POLE LAT: 90.
POLE LON: 359.
REF LAT: 1.
REF LON: 1.
REF GRID: 0.
ORIENTATION: 0.
CONE ANGLE: 0.
SYNC X: 1.0
SYNC Y: 1.0
SYNC LAT: -90.0
SYNC LON: 0.
SPECIAL: 0.
NUMB X: 360
NUMB Y: 181
NUMB LEVELS: 24
LEVEL 1: 0. 29 PRSS MSLP TPP6 UMOF VMOF SHTF DSWF RH2M U10M V10M T02M TCLD SHGT CAPE CINH LISD LIB4 PBLH TMPS CPP6 SOLM CSNO CICE CFZR CRAI LHTF LCLD MCLD HCLD
LEVEL 2: 1000.0 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 3: 975.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 4: 950.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 5: 925.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 6: 900.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 7: 850.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 8: 800.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 9: 750.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 10: 700.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 11: 650.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 12: 600.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 13: 550.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 14: 500.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 15: 450.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 16: 400.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 17: 350.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 18: 300.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 19: 250.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 20: 200.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 21: 150.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 22: 100.00 6 HGTS TEMP UWND VWND WWND RELH
LEVEL 23: 50.000 4 HGTS TEMP UWND VWND
LEVEL 24: 20.000 4 HGTS TEMP UWND VWND
Modified: January 8, 2013
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