The GEOS-3 met fields are saved in 3 different timescales:
I6 = instantaneous value (some 3-D fields), every 6 hours at hour hh (in nt2,
     see "Data format" below)
A6 = 6-hour average (some 3-D fields), centered at hour hh (in nt2, see
     "Data format" below)
A3 = 3-hour average (2-D fields), ended at hour hh (in nt2, see "Data format"
     below)

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File name: 
----------------------
terra.s30.byyyymm01.eyyyymm01x.field

where byyyymm = beginning year and month (e.g., b200101)
      eyyyymm = ending year and month (e.g., b200102)
      field   = GEOS-3 met field (e.g., cldfrc, pbl, sphu)

Each file contains every timestep within the beginning month plus the first
timestep of the next month. For example, total timesteps in
terra.s30.b20010101.e20010201x.pbl is 31(days) x 8(timestep/day) + 1(first
timestep in 20010201) = 249.
      
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Data format:
----------------------
These are direct access binary files, 32 bits, big endian.

parameter (imx=144, jmx=91, lmx=30)   ! nl is version dependent,30 in GEOS-3
dimension Q(imx,jmx,lmx)  !for 3-D or Q(imx,jmx) for 2-D
do k=1,nstep
 read(unit) nt1,nt2,Q
enddo

Where imx = total number of longitudinal grid (144),
      jmx = total number of latitudinal grid (91),
      lmx = total number of vertical layers (version dependent),
      nt1 = yyyymmdd after 2000 (year-month-day, e.g., 20010201), or
            yymmdd before 2000 (e.g., 970101)
      nt2 = hhmmss (hour-minute-second, e.g., 120000)
      Q   = 3 or 2-dimensional aerosol optical thickness
      nstep = total timesteps
Note: nt1 and nt2 are long integers.

To read using IDL:

imx=144 & jmx=91 & lmx=30
Q=fltarr(imx,jmx,lmx)  ; or Q=fltarr(imx,jmx) for 2-D
head=lonarr(2)
for k=0,nstep-1 do begin
 readu,1,head,Q
endfor

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!!!! IMPORTANT !!!!
Since there are some difference in default format of data saving, Fortran
compiler on your workstation may not be the same as that on the Cray where
these files are saved.  Please print out the nt1, nt2 for every timestep to 
be sure that the files are read correctly.

If you use fortran on a SGI machine, please use the following statement when
read the binary file:

open(unit,file='xyz',status='old',access='sequential',form='system')

If you use linux or PC, you have to convert the data to little endian.