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- Info
AIRS Level 2 Standard Retrieval Data Set
Page Contents:
The AIRS Standard Retrieval Product consists of retrieved estimates of cloud and surface properties, plus profiles of retrieved temperature, water vapor, ozone and a flag indicating the presence of cloud ice or water. Estimates of the errors associated with these quantities are also part of the Standard Product. The profile vertical resolution is 28 points total between 1000 mb and .02 mb. The dataset contains quality assessment flags in addition to retrieved quantities and are generated at all locations atmospheric soundings were taken.
Retrievals from AIRS data requires the use of co-located temperature and humidity data from passive microwave sounders; Advanced Microwave Sounding Unit-A (AMSU-A) and Humidity Sounder for Brazil (HSB). The AMSU-A aids in the atmospheric temperature profiling, especially for the upper atmosphere. AMSU-A provides temperature measurements up to an altitude of 40 km, plus a cloud-filtering capability for observations in the troposphere, the portion of the atmosphere from the surface to an altitude of approximately 12 km, a height varying with location and time. The HSB obtains humidity sounding through the atmosphere, for determining cloud liquid water, precipitation, and integrated precipitable water. The HSB is particulary important for allowing the AIRS/AMSU-A/HSB complement to obtain accurate humidity profiles under overcast conditions. An AIRS granule has been set as 6 minutes of data. This will normally correspond to approximately 1/15 of an orbit but exactly 45 scanlines of AMSU-A data or 135 scanlines of AIRS and HSB data.
AIRS
The AIRS infrared spectrometer acquires 2378 spectral samples at resolutions, l/Dl, ranging from 1086 to 1570, in three bands: 3.74 µm to 4.61 µm, 6.20 µm to 8.22 µm, and 8.8 µm to 15.4 µm. A 360 degree rotation of the scan mirror generates a cross-track Earth-scene scan line of IR data every 2.667 seconds. The spatial resolution at nadir is 13.5 km. This instrument provides fine vertical scale resolution soundings of atmospheric temperature and water vapor, and integrated column burden for trace gases. Cold space-views for calibration of data are taken at the beginning and end of the Earth-scene when the mirror sweeps through space-view scenes.
The IR focal plane is cooled to 60 K by a Stirling/pulse tube cryocooler. The scan mirror operates at approximately 265 K due to radiative coupling to the Earth and space and to the 150 K IR spectrometer. Cooling of the IR optics and detectors is necessary to achieve the required instrument sensitivity.
AIRS VIS/NIR
The Visible/Near-IR (VIS/NIR)
photometer contains four spectral bands, each with nine pixels along track,
with a 0.185 degree instantaneous field-of-view (FOV). It is boresighted to the IR spectrometer to allow simultaneous measurements of the visible and infrared scene. The VIS/NIR photometer uses optical filters to define four spectral bands in the 400 nm to 1000 nm region. The VIS/NIR detectors are not cooled and operate in the 293 K to 300 K ambient temperature range of the instrument housing. The spatial resolution at nadir is 2.3 km. The primary function of the AIRS VIS/NIR channels is to provide diagnostic support to the infrared retrievals: setting flags that warn of the presence of low-clouds or highly variable surface features within the infrared FOV.
AMSU-A
The AMSU-A microwave multichannel radiometer consists of two physically separate units, AMSU-A1 and AMSU-A2. Together they have 15 channels, measuring radiation in the frequency span of 23 GHz to 90 GHz. Twelve channels (between 50 GHz and 60 GHz) are predominantly used for atmospheric temperature sounding. The remaining three channels (24 GHz, 31 GHz and 89 GHz) are predominantly used for atmospheric water vapor sounding. The rotating scanning mirror generates a cross-track scan line every 8 seconds. The spatial resolution at nadir is 40.5 km.
HSB
The HSB microwave multichannel radiometer has 4 channels. One channel measures radiation at 150 GHz and the other three are centered on 183.31 GHz.All channels are used for atmospheric water vapor sounding. The rotating scanning mirror generates a cross-track scan line every 2.667 seconds. The spatial resolution at nadir is 13.5 km.
HSB ceased operation on February 5, 2003. We are continuing investigation with the hope of recovering the instrument at some future date. The impact on AIRS core products
(temperature profile, water vapor profile, ozone burden) is negligible. Some future research products (cloud liquid water and precipitation) are adversely effected.
For more detail information, see the following instrument guide below:
The AIRS Level2 Support Retrieval data is in Hierarchical Data Format-Earth Observing System (HDF-EOS)
swath format. The swath concept for HDF-EOS is based on a typical satellite swath, where an instrument takes a series of scans perpendicular to the ground track of the satellite as it moves along that ground track (
Diagram). As the AIRS is profiling instrument that scans across the ground track, the data would be a three dimensional array of measurements where two of the dimensions correspond to the standard scanning dimensions (along the ground track and across the ground track),
and the third dimension represents a range from the sensor. The "horizontal" dimensions can be handled as normal geographic dimensions,
while the third dimensions can be handled as a special "vertical" dimension.
An AIRS Level 1B data granule contains data fields, geolocation fields,
dimension, and swath attributes for a single swath. A detailed description of each attribute can be found in
AIRS Processing Files Descriptions.
Each file contains a single 6-minute swath data and files are named using the following convention:
6-minute granule:
AIRS.{Year}.{Mon}.{Day}.{Gran}.{Level}.{FileType}.{VerID}.{PGenFac}{Cycle#}.hdf
where:
Year is 4 digit year of data; e.g., 2003.
Month is 2 digit month (1-based); e.g., 08
Day is 2 digit day of month (1-based); e.g.,18
Gran is 3 digit granule of day (001-240) for standard (45 scanset) granules.
Level is product level; e.g., L2 is for Level 2
FileType is a string defining the product file type; for example,Sup.
VerID is the PGEVERSION.
PGenFac is the PRODUCTGENERATIONFACILITY char. "G" for GSFC DAAC.
Cycle# is set during production using the "Times Processed" input field
(Cycle# = Times Processed - 1) and is used by the data creator to assure
uniqueness of the LOCALGRANULEID; Cycle# is 3 digits and 0-based; e.g., 002.
hdf is the format of the file
Examples: AIRS.2003.08.18.240.L2.Ret.v3.0.12.0.G03087153709.hdf
Volume average of data files:
- AIRS Level2 Standard Retrieval Product: 4.7 MB
The geolocation data fields of immediate interest to the user are:
- Latitude FOV boresight geodetic latitude
(degrees North, -90->+90), dimension (30,45)
- Longitude FOV boresight geodetic longitude
(degrees East, -180->+180), dimension (30,45)
The attribute of immediate interest to the user are
- pressStd standard pressure (mb) for each of 28 levels in atmosphere associated with temperature, moisture and ozone profiles. The array order is from the surface upward, in conformance with WMO standard. Note that topography may place some of these levels below the surface, dimension (28)
The swath data fields of immediate interest to the user are:
- RetQAFlag always check this, dimension (30,45)
- PsurfStd surface pressure, interpolated from forecast and mean topography of FOV (mb), dimension (30,45)
- nSurfStd index of last physically meaningful profile entries. Retrieved profile entries beyond this index are filled with –9999. It may be level just above or just below the surface, dimension (30,45)
- TSurfStd retrieved surface skin temperature (K), dimension (30,45)
- TSurfAir retrieved surface air temperature (K), dimension (30,45)
- totH2Ostd total precipitable water vapor in air column (kg/m2), dimension (30,45)
- totO3Std total ozone burden in air column (Dobson Units), dimension (30,45)
- TAirStd retrieved atmospheric temperature profile (K) at the pressStd pressures. Array values below the surface (index < nSurfStd) may not always be filled with –9999.0. Always check nSurfStd to avoid invalid TAirStd elements below the surface, dimension (28,30,45)
- H2OMMRStd retrieved water vapor mass mixing ratio (g/kg_dry_air). Array values below the surface (index < nSurfStd) may not always be filled with –9999.0. Always check nSurfStd to avoid invalid H2OMMRStd elements below the surface, dimension (28,30,45)
- H2OMMRSat retrieved water vapor saturation mass mixing ratio (g/kg_dry_air). Array values below the surface (index < nSurfStd) may not always be filled with –9999.0. Always check nSurfStd to avoid invalid H2OMMRSat elements below the surface, dimension (28,30,45)
- O3VMRStd retrieved ozone volume mixing ratio (ppm). Array values below the surface (index < nSurfStd) may not always be filled with –9999.0. Always check nSurfStd to avoid invalid O3VMRStd elements below the surface, dimension (28,30,45)
- numHingeSurf number of IR hinge points for retrieved surface emissivity and reflectivity. It can be as large as 100. Usually 7, 49 or 51 depending upon the retrieval outcome. A MW-only retrieval results in 7. A full MW/IR retrieval results in 49. A partial MW/IR retrieval results in 51. dimension (30,45)
- freqEmis Frequencies (cm-1) for retrieved surface emissivity and reflectivity in order of increasing emissivity. Only the first numHingeSurf are valid, dimension (100,30,45)
- emisIRStd The retrieved spectral IR surface emissivity in order of increasing frequency. Only the first numHingeSurf are valid, dimension (100,30,45)
- rhoIRStd The retrieved spectral IR bi-directional surface reflectivity in order of increasing frequency. Only the first numHingeSurf are valid, dimension (100,30,45)
- NumCloud number of retrieved cloud layers (= 0, 1 or 2), dimension (30,45)
- TcldTopStd retrieved cloud top temperature (K) for each of up to two retrieved cloud layers, uppermost layer first. Use with caution, under development, dimension (2,30,45)
- PcldTopStd retrieved cloud top pressure (mb) for each of up to two retrieved cloud layers, uppermost layer first. Use with caution, under development, dimension (2,30,45)
- CldFrcStd retrieved cloud fraction (0->1) for each AIRS footprint associated with the retrieval FOV, for each layer. Ignore CldFrcStd values exactly equal to zero. The current cloud clearing code sometimes yields a false zero result. Values very near zero, i.e., 0.0001 or greater, are correct. Use with caution, under development, dimension (2,3,3,30,45)
- clear_flag_4um a flag indicating clear FOV valid over ocean only at night, which is based on the agreement of the predicted SST using observed AIRS radiance at 2616 cm-1 and 2707 cm-1 and the coherency among the 9 AIRS spots in the FOV at 2616 cm-1,. A value of 1 indicates possibly clear. Ignore, under development, dimension (3,3,30,45)
- clear_flag_11um a flag indicating clear FOV valid over ocean day and night, which is based on the agreement of the predicted SST using AIRS 11 um split window test and the coherency among the 9 AIRS spots in the FOV. A value of 1 indicates possibly clear. Ignore, under development, dimension (3,3,30,45)
- clear_flag a flag indicating clear FOV derived in the final retrieval step. Ignore, under development, dimension (30,45).
See V3.0_Release_ProcFileDesc.pdf for complete list of variables.
Contacts for Archive/Data Access Information:
Atmospheric Dynamic Data Support Team
Goddard Distributed Active Archive Center
NASA/Goddard Space Flight Center
Code 610.2
Greenbelt, MD 20771
Phone: (301) 614-5323
Fax: (301) 614-5268
Email: atmdyn-dst@disc.gsfc.nasa.gov
You may access the AIRS data from:
Search and Order
1. AIRS Algorithm Theoretical Basis Document, AIRS Team Unified Retrieval for Core Products (Level 2 ATBD) JPL D-17006, Version 2.1 15 December 1999
2. AIRS Version 3.0 L2 Data Release Documentation 11 August 2003
3. AIRS Version 3.0 Processing Files Description
Contact for Data Center Access Information:
The Goddard Earth Sciences Data and Information Services Center
Distributed Active Archive Center (GES DISC DAAC)
Phone: (301) 614-5224
Fax: (301) 614-5268
E-mail: help-disc@listserv.gsfc.nasa.gov
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Atmos Composition
