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Eta Data Assimilation System (EDAS40) Archive Information

Archive began: January 1, 2004
NOAA-Air Resources Laboratory 1315 East-West Highway Silver Spring, MD 20910 (301 713-0295)

EDAS 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 EDAS (Eta Data Assimilation System), covering the U.S. Addition current information on this model can be found on NCEP's website ( http://www.nco.ncep.noaa.gov/pmb/products/ ). At NOAA's Air Resources Laboratory (ARL), NCEP model data are used for air quality transport and dispersion modeling. ARL archives both EDAS and FNL data 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 EDAS. (Note: information here may not be up to date. Check the NCEP website for current information.) The EDAS was implemented into the operational early Eta model runs during 1995. The EDAS is an intermittent assimilation system consisting of successive 3-h Eta model forecasts and Optimum Interpolation (OI) analyses for a pre-forecast period (12-h for the early Eta) on a 38 level, 48 km grid. A 6-h forecast from the GDAS is used to to start the assimilation at 12-h prior to model start time. The following is a schematic for the 12Z cycle

  6-h GDAS    | 3-h eta | 3-h eta | 3-h eta | 3-h eta |  48-h Eta
------------> |-------->|-------->|-------->|-------->|------------------....-->
Forecast      |  fcst   |  fcst   |   fcst  |   fcst  |  forecast
                 00Z       03Z       06Z       09Z       12Z
where;
|
| = Eta OI analysis
|
The 3-h analysis updates allow for the use of high frequency observations, such as wind profiler, NEXRAD, and aircraft data. ARL saves the successive 3-hour analyses, twice each day to produce a continuous data archive. Some fields such as precipitation and surface fluxes are not available in the analysis files, therefore these are taken from the successive 3-hour forecast files. The 48 km data are interpolated to a 40 km, Lambert Conformal Grid, covering the continental United States.

ARL PROCESSING The ARL archiving program produces a 3 hourly, 40 km dataset on pressure surfaces. In addition, 14 gridpoints on the western end of the model domain and 10 gridpoints on the northern end of the domain are removed to reduce the size of the semi-monthly files (currently at about 627 Mbytes). The data are put into semi-monthly files and made available online at ARL's FTP server (ftp://arlftp.arlhq.noaa.gov/pub/archives/edas40/ ) for easy access via ftp.

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 approximately two weeks: days one through 15, and 16 through the end 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. EDAS data are available in the files called edas.mmmyy.00#, where mmm is the month (e.g. jul) and yy is the year (97) and 00# refer to:

00#=001 - days 1-15 of the month
00#=002 - rest of the month

Data Grid The data are on a 185 by 129 Lambert Conformal grid (Fig. 1- EDAS grid domain). 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 TypeID #X MaxY MaxPole Lat.Pole Lon.Ref. Lat.Ref. Lon.Ref. GridOrientationCone Ang.Sync XSync YSync Lat.Sync Lon.
EDAS2818512990N0W35N95W40025.01.01.012.190N133.459W

Table 2. Meteorological Fields contained in the EDAS Archive.

FieldUnitsLabelData Order
Pressure reduced to mean sea levelhPaMSLPS1
Temperature at surfaceKTMPSS2
Accumulated precipitation (3 h accumulation)mTPP3S3
Accumulated convective precipitation (3 h accumulation)mCPP3S4
Soil temperatureKSOLTS5
Volumetric soil moisture contentfrac.SOLWS6
Temperature at 2m AGLKTO2MS7
Relative Humidity at 2m AGL%RH2MS8
U-component of wind at 10 m AGLm/sU10MS9
V-component of wind at 10 m AGLm/sV10MS10
Potential temperature at 10 m AGLKP10MS11
Pressure at surfacehPaPRSSS12
Water equivalent of snow depthKg/m2WESDS13
Categorial snow (yes=1, no=0)CSNOS14
Categorial ice (yes=1, no=0)CICES15
Categorial freezing rain (yes=1, no=0)CFZRS16
Categorial rain (yes=1, no=0)CRAIS17
Latent heat net flux at surfaceW/m2LHTFS18
Sensible heat net flux at surfaceW/m2SHTFS19
Friction velocitym/sUSTRS20
VisibilitymVSBYS21
Surface roughnessmRGHSS22
Low cloud cover%LCLDS23
Middle cloud cover%MCLDS24
High cloud cover%HCLDS25
Total cloud cover%TCLDS26
Downward short wave radiation fluxW/m2DSWFS27
Water temperatureCWTMPS28
Convective available potential energyJ/KgCAPES29
Convective inhibitionW/m2CINHS30
Standard lifted indexW/m2LISDS31
Best 4-layer lifted indexW/m2LIB4S32
U-component of wind with respect to gridm/sUWNDU1
V-component of wind with respect to gridm/sVWNDU2
Geopotential heightgpm*HGTSU3
TemperatureKTEMPU4
Pressure vertical velocityhPa/sWWNDU5
Relative humidity%RELHU6
Turbulent kinetic energyJoulTKENU7

* geopotential meters

Meteorological Fields and Vertical Structure The archived data files contain only some of the fields normally 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 EDAS data are output on the following 26 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)
26 50
25 100
24 150
23 200
22 250
21 300
20 350
19 400
18 450
17 500
16 550
15 600
14 650
13 700
12 725
11 750
10 775
9 800
8 825
7 850
6 875
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 is defined as "NULL".

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

FormatInformation
A4Data 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:

FieldFormatDescription
YearI2Greenwich date for which data valid
MonthI2"
DayI2"
HourI2"
Forecast* I2Hours forecast, zero for analysis
LevelI2Level from the surface up (see Table 3)
GridI2Grid identification (see Table 1)
VariableA4Variable label (see Table 2)
ExponentI4Scaling exponent needed for unpacking
PrecisionE14.7Precision of unpacked data
Value 1,1E14.7Unpacked 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 - EDAS.CFG

MODEL TYPE:EDAS
GRID NUMB:28
VERT COORD:2
POLE LAT:90.
POLE LON:0.
REF LAT:35.
REF LON:-95.
REF GRID:40.0
ORIENTATION:0.
CONE ANGLE:25.
SYNC X:1.0
SYNC Y:1.0
SYNC LAT:12.190
SYNC LON:-133.459
SPECIAL:0.
NUMB X:185
NUMB Y:129
NUMB LEVELS:27
LEVEL 1: 0. 32 MSLP TMPS TPP3 CPP3 SOLT SOLW T02M RH2M U10M V10M P10M PRSS WESD CSNO CICE CFZR CRAI LHTF SHTF USTR VSBY RGHS LCLD MCLD HCLD TCLD DSWF WTMP CAPE CINH LISD LIB4
LEVEL 2:1000. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 3: 975. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 4:950. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 5:925. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 6:900. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 7:875. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 8:850. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 9:825. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 10:800. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 11:775. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 12: 750. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 13: 725. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 14:700. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 15:650. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 16:600. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 17: 550. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 18: 500. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 19:450. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 20:400. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 21:350. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 22:300. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 23: 250. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 24:200. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 25:150. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 26:100. 07 UWND VWND HGTS TEMP WWND RELH TKEN
LEVEL 27:50. 07 UWND VWND HGTS TEMP WWND RELH TKEN
Modified: October 30, 2008
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