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Eta Data Assimilation System (EDAS) Archive Information


              NCEP Model Output -- EDAS ARCHIVE

TD-6141






Archive began: January 1, 1997

Archive ended: April 30, 2004







Prepared for the

National Climatic Data Center (NCDC)









by

Glenn D. Rolph

NOAA-Air Resources Laboratory

1315 East-West Highway

Silver Spring, MD 20910

(301 713-0295)







March 2002





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 extracts every other grid point of the 3 hourly, 40 km data to produce a 3 hourly, 80 km dataset on pressure surfaces (note that we do NOT interpolate the data before or after data extraction). 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 84 Mbytes). The data are put into semi-monthly files and sent to NCDC. In addition, all of the EDAS data is available online at ARL’s web site (archives.php) 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.


Cartridge Specifications


     Originally, two files containing data for one month, were archived on a 3480 cartridge and sent to NCDC.. The files were copied to a cartridge with the UNIX "dd" command.


     TYPE 3480 cartridge, ASCII*

     RECORD LENGTH 30475

     BLOCK SIZE 30475


* 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.


EDAS data are available in the files called edas.subgrd.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


NCDC can be reached at:

                             

      National Climatic Data Center

       Federal Building

      151 Patton Avenue

      Asheville, NC 28801-5001

   

      Email: questions@nvds.noaa.gov

      Phone: 828-271-4800

      Fax: 828-271-4876

      www: http://www.ncdc.noaa.gov



 

Data Grid


     The data are on a 79 by 55 Lambert Conformal grid (Fig. 1). 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).

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                                        Table 1. Data Grid Specifications

------------------------------------------------------------------------------------------------------

Model ID X Y Pole Pole Ref. Ref. Ref. Orient Cone Sync Sync Sync Sync

Type # Max Max Lat. Lon. Lat. Lon. Grid Ang. X Y Lat. Lon.

------------------------------------------------------------------------------------------------------

EDAS 38 79 55 90N 0W 35N 95W 80 0 25 39.5 25.5 35N 95W

------------------------------------------------------------------------------------------------------



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      Table 2. Meteorological Fields contained in the EDAS Archive.

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Field Units Label Data

                                                                                                    Order

-------------------------------------------------------------------------------------------------

Pressure reduced to mean sea level                                       hPa      MSLP             S1

Temperature at surface                                                           K         TMPS             S2

Accumulated precipitation (3 h accumulation)                      m        TPP3               S3

Accumulated convective precipitation (3 h accumulation)    m        CPP3              S4

Volumetric soil moisture content                                           frac.    SOLW            S5

Temperature at 2m AGL                                                        K         T02M              S6

Relative Humidity at 2m AGL                                               %        RH2M            S7

U-component of wind at 10 m AGL                                      m/s      U10M             S8

V-component of wind at 10 m AGL                                      m/s      V10M             S9

Pressure at surface                                                                 hPa      PRSS              S10

Categorial snow (yes=1, no=0)                                              -          CSNO             S11

Categorial rain (yes=1, no=0)                                                 -          CRAI              S12

Latent heat net flux at surface                                                W/m2  LHTF              S13

Sensible heat net flux at surface                                             W/m2  SHTF              S14

Friction velocity                                                                     m/s      USTR             S15

Visibility                                                                                 m        VSBY             S16

Low cloud cover                                                                     %        LCLD             S17

Medium cloud cover                                                              %        MCLD            S18

High cloud cover                                                                    %        HCLD             S19

Total cloud cover                                                                    %        TCLD             S20

Downward short wave radiation flux                                     W/m2  DSWF            S21

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

-----------------------------------------------------------------------------------------------------

* 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 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 22 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

-------------------------------------------------------------------

                        22 50 hPa

                        21 100 hPa

                        20 150 hPa

                        19 200 hPa

                        18 250 hPa

                        17 300 hPa

                        16 400 hPa

                        15 500 hPa

                        14 550 hPa

                        13 600 hPa

                        12 650 hPa

                        11 700 hPa

                        10 750 hPa

                         9 800 hPa

                         8 825 hPa

                         7 850 hPa

                         6 875 hPa

                         5 900 hPa

                         4 925 hPa

                         3 950 hPa

                         2 975 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. 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


     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 - EDAS.CFG


MODEL TYPE: EDAS

GRID NUMB: 38

VERT COORD: 2

POLE LAT: 90.

POLE LON: 0.

REF LAT: 35.

REF LON: -95.

REF GRID: 80.0

ORIENTATION: 0.

CONE ANGLE: 25.

SYNC X: 39.0

SYNC Y: 25.0

SYNC LAT: 35.

SYNC LON: -95.

SPECIAL: 0.

NUMB X: 79

NUMB Y: 55

NUMB LEVELS: 23

LEVEL 1: 0. 21 MSLP TMPS TPP3 CPP3 SOLW T02M RH2M U10M V10M PRSS CSNO CRAI LHTF SHTF USTR VSBY LCLD MCLD HCLD TCLD DSWF

LEVEL 2: 1000. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 3: 975. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 4: 950. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 5: 925. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 6: 900. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 7: 875. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 8: 850. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 9: 825. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 10: 800. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 11: 750. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 12: 700. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 13: 650. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 14: 600. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 15: 550. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 16: 500. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 17: 400. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 18: 300. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 19: 250. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 20: 200. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 21: 150. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 22: 100. 06 UWND VWND HGTS TEMP WWND RELH

LEVEL 23: 50. 06 UWND VWND HGTS TEMP WWND RELH

Modified: October 30, 2008
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