## Gridded Meteorological Data Archives

Overview The National Weather Service's National Centers for Envi ronmental Prediction (NCEP) runs a series of computer an alyses and forecasts operationally. NOAA's Air Resources Laboratory (ARL) routinely uses NCEP model data for use in air quality transport and dispersion modeling calculations. In 1989 ARL began to archive some of these dataset s for future research studies. ARL has in the past, or is present ly archiving the following NCEP datasets, which can be ret rieved via ftp by clicking on the name of the dataset. For further information on model changes see the follo wing web sites: http://www.nco.ncep.noaa.gov/pmb/ http://www.nco.ncep.noaa.gov/pmb/changes/ http://www.emc.ncep.noaa.gov/mmb/research/eta.log.html http://www.emc.ncep.noaa.gov/modelinfo/index.html 

### Currently Available Data NAM (Eta) Data Assimilation System (EDAS 40km, 2004 -) FTP Data NOAA ARL FTP Server Readme file Latitude/longitude pairs Recent changes Missing Data 2014 2013 2012 2011 2011 Alternate archive 2010 2009 2008 2007 Grid domain map NCEP NAM(Eta)/EDAS and the GCIP Project GDAS one-degree archive (Dec 2004- ) FTP Data NOAA NCEP Server (recent files only) NOAA ARL FTP Server University of Alaska Fairbanks FTP Server Details on the NCEP Global Data Assimilation System (GD AS) model Details on the NCEP Global Forecast System (GFS), the forecast component of the GDAS Readme file Missing data listing Grid domain map GDAS half-degree archive (Sep. 1, 2007- ) FTP Data NOAA ARL FTP Server Details on the NCEP Global Data Assimilation System (GD AS) model Details on the NCEP Global Forecast System (GFS), the forecast component of the GDAS Readme file Missing data listing 

# Air Resources Laboratory - Gridded Data Archives https://ready.arl.noaa.gov/archives.php

1 of 3 10/27/2015 2:01 PMGrid domain map

NCEP/NCAR Reanalysis (1948 - present)- Note: the data are usually only updated at the begi nning of the year for the previous year. FTP Data NOAA ARL FTP Server Readme file NCEP Reanalysis Information Page NCAR Reanalysis Home Page NAM 12 km (31-day rotating archive)... FTP Data NOAA ARL FTP Server Readme file Grid domain map WRF ARW (Weather Research & Forecasting Model for C APTEX Tracer Experiment) FTP Data NOAA ARL FTP Server Readme file CAPTEX grid domain map(every 5th point shown) North American Regional Reanalysis (NARR, 1979-2014 ) FTP Data NOAA ARL FTP Server Readme file NARR grid domain map (Every 5th grid point shown) MM5 (MM5 Community Model for the METREX Tracer Expe riment) FTP Data NOAA ARL FTP Server Readme file Grid domain map CFSR (The Climate Forecast System Reanalysis) FTP Data NOAA ARL FTP Server Readme file Grid domain map(every 10th point shown)


### Data No Longer Updated FNL archive (1997-2006) of NCEP Global Data Assimilation System (GDAS) model outp ut ***** FNL datasets are no longer online; use Reanalysis d ata as an alternate source of data. Readme file NGM (Jan. 1991 - Apr. 1997) Readme file Missing data listing Latitude/longitude pairs Grid domain map NAM(Eta) Data Assimilation System (EDAS 80km, 1997-2004 ) Readme file Changes 2004 missing data listing 2003 missing data listing 2002 missing data listing 2001 missing data listing 2000 missing data listing 1999 missing data listing 1998 missing data listing 1997 missing data listing 

# Air Resources Laboratory - Gridded Data Archives https://ready.arl.noaa.gov/archives.php

2 of 3 10/27/2015 2:01 PMModified: January 30, 2015

US Dept. of Commerce | NOAA | NOAA Research | ARL Privacy | Disclaimer | Information Quality Accessibility | webmaster Latitude/longitude pairs Grid domain map EDAS Presentation by Eric Rogers of NOAA NCEP NCEP NAM(Eta)/EDAS and the GCIP Project All of theses datasets contain basic fields such as the u- an d v-wind components, temperature, and humidity. However, the archives differ from each other because of the hori zontal and vertical resolution, as well as in the specific fields p rovided by NCEP. All fields were selected by ARL accordin g to what is most relevant for transport and dispersion st udies and disk space limitations. 

### Data Packing Format NCEP typically saves their model output in GRIB format. H owever, at ARL the data are reprocessed and stored in a 1-byte packing algorithm. This 1-byte packing is a bit more compact than GRIB 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 characte rs. To preserve as much data precision as possible the differ ence between the values at grid points is saved and packed rather than the actual values. The grid is then reconstru cted by adding the differences between grid values starti ng with the first value, which is stored in unpacked ASCII form in the header record. To illustrate the process, assume that a g rid 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 an y two adjacent grid points for the entire array. It is computed from the differences along each i index holdin g 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 val ues are one byte unsigned integers, where values from 0 t o 126 represent -127 to -1, 127 represents zero, and values of128 to 254 represent 1 to 127. Each record length is t hen equal in bytes to the number of array elements plus 50 bytes for the header label information. The 50 byte label field pr ecedes each packed data field and contains the following A SCII data:

Field Format Description Year I2 Greenwich date for which da ta valid Month I2 " Day I2 " Hour I2 " Forecast* I2 Hours forecast, zero for a nalysis Level I2 Level from the surface up ( see Table 3) Grid I2 Grid identification (see Ta ble 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.
The following Fortran90 program can be used to to unp ack and read the first few elements of the data array fo r each record of an ARL packed meteorological file.
CHK_DATA.F 

# Air Resources Laboratory - Gridded Data Archives https://ready.arl.noaa.gov/archives.php

3 of 3 10/27/2015 2:01 PM