AIRS Level 2 Cloud-Cleared IR Radiance Data Set

Page Contents:

• 1 Data Set Information
• 2 Data Source Information
• 3 Data Set Organization
• 4 Data Access Information
• 5 References
• 6 Point of Contact
• 7 Sample Read Programs

1. Data Set Information:

This is a global Cloud-Cleared IR radiance data which contains radiances calibrated, geolocated channel-by-channel AIRS infrared spectra that would have been observed within each AMSU footprint if there were no clouds in the FOV (milliWatts/m2/cm-1/steradian) and produced along with the AIRS Standard Product, as they are the radiances used to retrieve the Standard Product. Nevertheless, they are an order of magnitude larger in data volume than the remainder of the Standard Products and, many Standard Product users are expected to have little interest in the Cloud Cleared Radiance. For these reasons they are a separate output file, but like the Standard Product, are generated at all locations.

2. Data Source Information:

AIRS is a complex instrument, especially with regard to the downlink telemetry of science and engineering data. There are 2378 spectral channels, i e., one channel refers to a spectral resolution element, with two samples per channel. There are 4482 total detectors on the IR Focal Plane Assembly (FPA), with hybrid PV/PC:HgCdTe detector types. There are 2,104 IR spectral channels, with dual PV detectors, and 274 channels in the critical 13.6-15.4 µm region, with single PC detectors. The dual detectors are summed (or selected) on board prior to downlink. After digitizing and formatting, each AIRS science data packet contains 2666 detector samples that include both IR and VIS/NIR.

The heart of the AIRS system is the multi-aperture, echelle grating spectrometer and corresponding multiple detector arrays. The IR Spectrometer Assembly includes a pupil imaging (i.e. detectors are located at a pupil stop of the spectrometer optics as opposed to the detectors being at a field stop) telescope which views the Earth and calibrator assemblies through a rotating scan mirror in the scan head assembly. While in-flight calibration measurements are made once per scan line (every 2.667 seconds), data from ten or more scan lines are combined by the ground calibration software to update calibration coefficients.

For more detail information, see AIRS instrument guide.

3. Data Set Organization:

Format:

The AIRS Infrared level 1B 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.

Data Granularity:

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.CC.v3.0.12.0.G03087153709.hdf

Volume average of data files:

• AIRS Level2 Cloud-Cleared Radiances: 25.8 MB

Parameter Description:

The fields of immediate interest to user are:

• RetQAFlag always check this, dimension (30,45)
• radiances calibrated, geolocated channel-by-channel AIRS infrared spectra that would have been observed within each AMSU footprint if there were no clouds in the FOV (milliWatts/m2/cm-1/steradian), dimension (2378,30,45)
• radiance_err estimate of the radiance error, channel-by-channel (milliWatts/m2/cm-1/steradian), dimension (2378,30,45)
• solzen solar zenith angle (degrees, 0->180; daytime if < 85), dimension (30,45)
• clear_flag do not use; still under development and not yet validated, dimension (30,45)

Where the combined microwave/infrared retrieval is complete (bits 0, 1, 2, 3 and 4 of RetQAFlag are all not set), the radiances data field contains the final product calibrated, geolocated AIRS infrared spectra.

Where the combined retrieval is not complete (one or more of RetQAFlag bits 0, 1, 2, 3 and 4 are set), the radiances data field contains the last AIRS spectrum computed by the retrieval algorithm. This is an aid to the developers of the algorithm, providing insight into the stage of the retrieval that rejected the FOV. Users should ignore radiances from incomplete retrievals.

See V3.0_Release_ProcFileDesc.pdf for complete list of variables.

4. Data Access Information:

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

• DAAC Data Pool
• EOS Data Gateway
• Mirador

5. References:

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

6. Point of Contact:

Contact for Data Center Access Information:

The Goddard Earth Sciences Data and Information Services Center

Distributed Active Archive Center (GES DISC DAAC)
Phone: (877)-422-1222
Fax: (301)614-5304
E-mail: help-disc@listserv.gsfc.nasa.gov

7. Sample Read Programs:

• read_swath_l1_l2cc_airs.pro Sample data reader for AIRS L2 Cloud-Cleared Radiance data in IDL.