AIRS Observations

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$Id: AIRS.html 6340 2013-07-31 14:15:20Z nancy $



The AIRS instrument is an Atmospheric Infrared Sounder flying on the Aqua spacecraft. Aqua is one of a group of satellites flying close together in a polar orbit, collectively known as the "A-train". The programs in this directory help to extract the data from the distribution files and put them into DART observation sequence (obs_seq) file format.

AIRS data includes atmospheric temperature in the troposphere, derived moisture profiles, land and ocean surface temperatures, surface emmissivity, cloud fraction, cloud top height, and ozone burden in the atmosphere.


Access to the web pages where the AIRS data are stored is available by registering as a data user.

There are two products this converter can be used on: AIRX2RET, which is the L2 standard retrieval product using AIRS IR and AMSU (without-HSB); and AIRS2RET, which is the L2 standard retrieval product using AIRS IR-only. More detailed information on the AIRS2RET data product and the AIRX2RET data product is available from the nasa web pages.

The data is distributed in HDF-4 format, using some additional conventions for metadata called HDF-EOS. There is a basic library for accessing data in hdf files, and a variety of generic tools that work with hdf files. The specific libraries we use are the HDF-EOS2 library built on HDF4. The web page has a link to specific build instructions. Also, see below on this web page for very specific instructions for getting the required software and building it. If you find more recent instructions online, use those. But in the absence of anything else, it's someplace to start.

Besides the programs in this directory, a variety of specific tools targeted at AIRS data are available to help read and browse the data. General information on using hdf in the earth sciences is available here.

Several types of AIRS data, with varying levels of processing, are available. The following descriptions are taken from the V5_Data_Release_UG document:

The L1B data product includes geolocated, calibrated observed microwave, infrared and visible/near infrared radiances, as well as Quality Assessment (QA) data. The radiances are well calibrated; however, not all QA data have been validated. Each product granule contains 6 minutes of data. Thus there are 240 granules of each L1B product produced every day.

The L2 data product includes geolocated, calibrated cloud-cleared radiances and 2-dimensional and 3-dimensional retrieved physical quantities (e.g., surface properties and temperature, moisture, ozone, carbon monoxide and methane profiles throughout the atmosphere). Each product granule contains 6 minutes of data. Thus there are 240 granules of each L2 product produced every day.

The L3 data are created from the L2 data product by binning them in 1°x1° grids. There are three products: daily, 8-day and monthly. Each product provides separate ascending (daytime) and descending (nighttime) binned data sets.

The converter in this directory processes level 2 (L2) data files, using data set AIRS_DP and data product AIRX2RET or AIRS2RET without HSB (the instrument measuring humidity which failed).

The Atmospheric Infrared Sounder (AIRS) is a facility instrument aboard the second Earth Observing System (EOS) polar-orbiting platform, EOS Aqua. In combination with the Advanced Microwave Sounding Unit (AMSU) and the Humidity Sounder for Brazil (HSB), AIRS constitutes an innovative atmospheric sounding group of visible, infrared, and microwave sensors. AIRS data will be generated continuously. Global coverage will be obtained twice daily (day and night) on a 1:30pm sun synchronous orbit from a 705-km altitude.

The AIRS Standard Retrieval Product consists of retrieved estimates of cloud and surface properties, plus profiles of retrieved temperature, water vapor, ozone, carbon monoxide and methane. Estimates of the errors associated with these quantities will also be part of the Standard Product. The temperature profile vertical resolution is 28 levels total between 1100 mb and 0.1 mb, while moisture profile is reported at 14 atmospheric layers between 1100 mb and 50 mb. The horizontal resolution is 50 km. An AIRS granule has been set as 6 minutes of data, 30 footprints cross track by 45 lines along track. The Shortname for this product is AIRX2RET. (AIRS2RET is the same product but without the AMSU data.)

The converter outputs temperature observations at the corresponding vertical pressure levels. However, the moisture obs are the mean for the layer, so the location in the vertical is the midpoint, in log space, of the current layer and the layer above it. There is an alternative computation for the moisture across the layer which may be more accurate, but requires a forward operator subroutine to be written and for the observation to contain metadata. The observation could be defined with a layer top, in pressure, and a number of points to use for the integration across the layer. Then the forward operator would query the model at each of the N points in the vertical for a given horizontal location, and compute the mean moisture value. This code has not been implemented yet, and would require a different KIND_xxx to distinguish it from the simple location/value moisture obs. See the GPS non-local operator code for an example of how this would need to be implemented.

Getting the data currently means putting in a start/stop time at this web page. The keyword is AIRX2RET and put in the time range of interest and optionally a geographic region. Each file contains 6 minutes of data, is about 2.3 Megabytes, and globally there are 240 files/day (about 550 Megabytes/day). There are additional options for getting only particular variables of interest, but the current reader expects whole files to be present. Depending on your connection to the internet, there are various options for downloading. We have chosen to download a wget script which is created by the web page after adding the selected files to a 'cart' and 'checking out'. The script has a series of wget commands which downloads each file, one at a time, which is run on the machine where you want the data to end up.


The temperature observations are located on standard levels; there is a single array of heights in each file and all temperature data is located on one of these levels. The moisture observations, however, are an integrated quantity for the space between the levels; in their terminology the fixed heights are 'levels' and the space between them are 'layers'. The current converter locates the moisture obs at the midpoint, in log space, between the levels.

The hdf files need to be downloaded from the data server, in any manner you choose. The converter program reads each hdf granule and outputs a DART obs_seq file containing up to 56700 observations. Only those with a quality control of 0 (Best) are kept. The resulting obs_seq files can be merged with the obs_sequence_tool into larger time periods.

It is possible to restrict the output observation sequence to contain data from a region of interest throught the use of the namelist parameters. If you need a region that spans the Prime Meridian lon1 can be a larger number than lon2, for example, a region from 300 E to 40 E and 60 S to 30 S (some of the South Atlantic), would be lon1 = 300, lon2 = 40, lat1 = -60, lat2 = -30.

The scripts directory here includes some shell scripts that make use of the fact that the AIRS data is also archived on the NCAR HPSS (tape library) in daily tar files. The script has options to download a day of granule files, convert them, merge them into daily files, and remove the original data files and repeat the process for any specified time period. (See

Here is a very specific script I used to build the required libraries on a Linux cluster. If you find more up-to-date instructions, use those. But in the absence of anything else, here's a place to start:

# (log in as 'anonymous' and your email as the password)
cd /edhs/hdfeos/latest_release
mget *
# mar 2013, the dir contents:
# hdf-4.2.6.tar.gz
# HDF-EOS2.18v1.00.tar.Z
# HDF-EOS2.18v1.00_TestDriver.tar.Z
# HDF_EOS_UG.pdf
# jpegsrc.v6b.tar.gz
# zlib-1.2.5.tar.gz
# (i skipped a 'windows' dir).

for i in *.tar.gz
  tar -zxvf $i

# start with smaller libs, work up to HDF-EOS.

echo zlib:

cd zlib-1.2.5
./configure --prefix=/glade/p/work/nancy
make test 
make install

echo jpeg:

cd jpeg-6b
./configure --prefix=/glade/p/work/nancy
make test 
mkdir /glade/p/work/nancy/{bin,man,man/man1} 
make install

# (make install wouldn't create the dirs if they didn't exist.
# lib was there from the zlib install, the others weren't.)

echo hdf:

cd hdf-4.2.6
./configure --prefix=/glade/p/work/nancy
# (it found zlib and jpeg, from the install prefix i guess)
# (there is apparently no 'make test')
make install

echo hdf-eos:

cd hdfeos
./configure CC='/glade/p/work/nancy/bin/h4cc -Df2cFortran' --prefix=/glade/p/work/nancy
# (the CC= is crucial)
# (i didn't build the test drivers so i didn't do make test)
make install

echo AIRS converter:

cd $DART/observations/AIRS/work

echo edit mkmf_convert_airs_L2 to have all the base paths
echo be /glade/p/work/nancy instead of whatever.  make it look like:
echo ' '
echo 'set JPGDIR = /glade/p/work/nancy'
echo 'set JPGLIB = ${JPGDIR}/lib'
echo 'set JPGINC = ${JPGDIR}/include'
echo ' '
echo 'set HDFDIR = /glade/p/work/nancy'
echo 'set HDFLIB = ${HDFDIR}/lib'
echo 'set HDFINC = ${HDFDIR}/include'
echo ' '
echo 'set EOSDIR = /glade/p/work/nancy'
echo 'set EOSLIB = ${EOSDIR}/lib'
echo 'set EOSINC = ${EOSDIR}/include'
echo ' '


exit 0


This namelist is read in a file called input.nml. We adhere to the F90 standard of starting a namelist with an ampersand '&' and terminating with a slash '/' for all our namelist input. Character strings that contain a '/' must be enclosed in quotes to prevent them from prematurely terminating the namelist.

   l2_files           = 'input.hdf',
   l2_file_list       = '',
   datadir            = '.',
   outputdir          = '.',
   lon1               =   0.0,
   lon2               = 360.0,
   lat1               = -90.0,
   lat2               =  90.0,
   min_MMR_threshold  = 1.0e-30,
   top_pressure_level = 0.0001,
   cross_track_thin   = 0,
   along_track_thin   = 0,

Contents Type Description Default
l2_files character(len=128) (:) A list of one or more names of the HDF file(s) to read, NOT including the directory. If multiple files are listed, each will be read and the results will be placed in a separate file with an output filename constructed based on the input filename.
l2_file_list character(len=128) The name of an ascii text file which contains one filename per line, NOT including the directory. Each file will be read and the observations converted into an output file where the output filename is based on the input filename. Only one of 'l2_files' and 'l2_file_list' can be specified. The other must be ' ' (empty).
datadir character(len=128) The directory containing the HDF files
outputdir character(len=128) The directory for the output observation sequence files.
lon1 real(r4) the West-most longitude of interest in degrees. [0.0, 360]
lon2 real(r4) the East-most longitude of interest in degrees. [0.0, 360]
lat1 real(r4) the South-most latitude of interest in degrees. [-90.0, 90.0]
lat2 real(r8) the North-most latitude of interest in degrees. [-90.0, 90.0]
min_MMR_threshold real(r8) The data files contains 'Retrieved Water Vapor Mass Mixing Ratio'. This is the minimum threshold, in gm/kg, that will be converted into a specific humidity observation.
cross_track_thin integer provides ability to thin the data by keeping only every Nth data value in a particular row. e.g. 3 == keep every third value.
along_track_thin integer provides ability to thin the data by keeping only every Nth row. e.g. 4 == keep only every 4th row.


Earlier versions of this converter mistakenly put the moisture obs at level heights, in the same location as the temperature observations. The moisture observations are in fact an integrated value across the distance between two levels. This means the location was shifted 1/2 level in the vertical from the center of the layer. The fixed converter outputs the location at the center, in log space, of each layer.


If a more accurate moisture observation was needed, the observation value could be computed by actually integrating multiple values between the levels. At this point it doesn't seem necessary.

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This open source software is provided by UCAR, "as is",
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Contact: nancy collins
Revision: $Revision: 6340 $
Source: $URL: $
Change Date: $Date: 2013-07-31 08:15:20 -0600 (Wed, 31 Jul 2013) $
Change history:  try "svn log" or "svn diff"