The AIRS Project, in conjunction with the AIRS Science Team and NASA GES DISC, have now made Version 6 Level 2 data available to the public. These data represent a significant improvement over AIRS Version 5 Level 2 products in terms of greater stability, yield, and quality. Several algorithmic improvements have been made by the AIRS Project and the AIRS Science Team. Many of the improvements are listed below:
• Improved soundings of Tropospheric and Sea Surface Temperatures, day and night.
• Improvements are larger with increasing cloud cover.
• Improved retrievals of surface spectral emissivity, day and night.
• Most of the spurious temperature bias trend seen in earlier versions has been removed.
• Substantially improved retrieval yield (number of soundings accepted for output) for climate studies.
• AIRS-Only retrievals with comparable accuracy to AIRS+AMSU (Advanced Microwave Sounding Unit) retrievals.
• More realistic hemispheric seasonal variability and global distribution of carbon monoxide (CO).
The Version 6 algorithm determines surface skin temperatures using only the shortwave channels, 2358 cm-1 – 2664 cm-1. The regression start-up of the atmospheric state has been replaced by a Neural-Net algorithm, which degrades much more slowly with increasing cloud cover. The improved first guess for the CO data product is now hemisphere- and season-dependent. In order to avoid processing discontinuities, the AIRS+AMSU retrieval algorithm does not use the failed AMSU channel 4 and 5 radiances, even when those channels were operating nominally.
New additions to Version 6
• All quantities now have associated QC (Quality Control) information. Thus, there are profiles of QC information for each retrieved profile quantity.
• In a response to user needs, the familiar mean layer quantities for water vapor, ozone, CO and methane (CH4) of previous versions have been augmented in Version 6 by the addition of level values at the specific pressure levels upon which they are reported. Thus, the profiles of these quantities are reported in the same manner as the temperature profile.
• Version 6 also adds retrievals of Outgoing Longwave Radiation (OLR) and cloud top characteristics at the infrared footprint resolution, and a preliminary thermodynamic cloud phase discriminant (cloud_phase_3x3) at the infrared footprint resolution in the support product. cloud_phase_3x3 is a flag indicating whether clouds are ice or liquid water.
Version 6 changes from Version 5 are detailed in the AIRS documentation (PDF).
Improvements in V6 vs. V5 are demonstrated in Figure 1 (temperature soundings), Figure 2 (column CO), and the headline image at top.
Production Plans for Version 6
Both AIRS and Microwave (AIRS+AMSU) and AIRS-only (Infrared-only) retrievals are being processed in the forward stream. Version 6 processing of previous years (i.e., reprocessing) of AIRS+AMSU data will proceed year-by-year, in reverse order, until the Version 6 AIRS+AMSU Level 2 data set is up-to-date from the start of the mission to the present. AIRS-Only and AIRS+AMSU+HSB Version 6 processing of data for the period when the Humidity Sounder for Brazil (HSB) was functional will begin after the rest of Version 6 Level 2 data are complete. Version 6 processing of the entire mission in all three modes (AIRS+AMSU, AIRS-Only, and AIRS+AMSU+HSB) is expected to be finished by the end of 2013.
Due to the failure of AMSU channel 5 the production of Version 5 of AIRS+AMSU Level 2 and Level 3 data has ceased with data produced through February 2013. Thereafter, only Version 6 Level 2 and Level 3 products are being produced (Note: AMSU channel 5 is not used in Version 6). Although Version 5 data will continue to be archived and available from the GES DISC, users are encouraged to use the much improved Version 6.
Carbon dioxide (CO2 ) processing has been discontinued at 2012 DOY 60 due to drop-off in yield resulting from the degradation of AMSU-A . Version 6 CO2 products are still under development, and will be released later this year.
There are no changes to calibrated radiances Version 5, and thus they will remain as Version 5.
Version 6 documentation is available from GES DISC AIRS Support.
Figure 1. Comparison of the yield from Version 5 and Version 6 temperature retrievals (Figure 8 from the AIRS/AMSU/HSB Version 6 Level 2 Performance and Test Report).
Figure 2. Five-day average of best and good (QC=0,1) quality of AIRS total column Carbon Monoxide (CO) of Version 5 and Version 6.
Article and web formatting by Andrey Savtchenko and Thomas Hearty. Edited by Steve Friedman and Ed Olsen.
AIRS is managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif., under contract to NASA. JPL is a division of the California Institute of Technology in Pasadena.
The GES DISC is a NASA earth science data center, part of the NASA Earth Science Data and Information System (ESDIS) Project.