AIRS is a high-resolution infrared (IR) sounder selected to fly on the EOS Aqua platform with two operational microwave sounders, AMSU and HSB. Measurements from the three instruments will be analyzed jointly to filter out the effects of clouds from the IR data in order to derive clear-column air-temperature profiles and surface temperatures with high vertical resolution and accuracy. Together, these three instruments constitute an advanced operational sounding system whose data will:
- improve global modeling efforts and numerical weather prediction;
- enhance studies of the global energy and water cycles, the effects of greenhouse gases, and atmosphere-surface interactions; and
- facilitate monitoring of climate variations and trends.
AIRS measures upwelling radiances in 2378 spectral channels covering the IR spectral band, 3.74 to 15.4 µm. A set of four channels in the Visible/Near-IR (VIS) observes wavelengths from 0.4 to 1.0 µm to provide cloud cover and spatial-variability characterization. The microwave sounders provide sea ice concentration, snow cover, and additional temperature-profile information as well as precipitable water and cloud liquid-water content. If cloud cover is too great for IR retrievals, the microwave (MW) measurements alone will provide a coarse, low-precision atmospheric-temperature profile and surface characterization.
The Standard Products Flow Chart provides an overview of the data processing architecture and the products which originate at the various stages.
Three independent Product Generation Executives (PGEs), one each for AIRS/VIS, AMSU, and HSB, execute at the DAAC to ingest Level 0 data to produce Level 1A geolocated science data counts and engineering parameters in Hierarchical Data Format (HDF)swath format.
Four independent PGEs, one for each instrument, execute at the DAAC to ingest Level 1A data to produce a Level 1B calibrated IR radiance product, a microwave brightness temperature product, and associated calibration coefficients. The AIRS L1B PGE also produces a subset data file of selected cloudy IR radiances for ingest by the AIRS L1B daily browse PGE. The AIRS and AMSU-A/HSB L1B daily browse PGEs subsequently produce browse products from this subset file and the L1B brightness temperature product data.
A combined AIRS/AMSU/HSB/VIS Level 2 PGE executes at the DAAC to ingest the Level 1B data products and ancillary data and to produce Level 2 standard products and support products, all in HDF swath format. A Level 2 subset data file is also produced for ingest by the Level 2 daily browse PGE. This daily browse PGE produces browse products from this subset file as well as summary data to be ingested by follow-on Level 2 pentad and Level 2 monthly browse PGEs. All browse PGEs are executed at the DAAC.
Quality Assurance (QA) Support Products are produced at the Team Leader Science Computing Facility (TLSCF) in HDF swath format for a fraction of the total data set and are used to monitor the operation of the instrument and validate the retrieval algorithm and products.
The microwave Level 1B data sets and Level 2 standard products are ingested at the TLSCF along with the ancillary radiosonde observations (RAOBS) to produce a daily statistic by comparison of profiles observed by radiosondes with spatially and temporally matched retrieval profiles.
The combined Level 2 PGE consists of three major retrieval stages:
- Microwave-Only Retrieval
- Cloud Clearing
- Combined MW/IR/VIS Retrieval
The Microwave-Only Retrieval employs only the AMSU and HSB data in combination with an ancillary general circulation model (GCM) (providing general climatology and accurate surface pressure) and a digital elevation model (DEM) (providing topography and land/water fractions) to retrieve atmospheric temperature, water vapor, and liquid-water profiles as well as surface skin temperature and microwave spectral surface emissivity. A rain flag is also produced. These profiles are passed to the combined MW/IR/VIS retrieval as an initial guess. In the event of rain or overwhelming cloud cover, the combined retrieval is not attempted and the MW-Only Retrieval is passed through to the standard product.
The Cloud Clearing Stage employs MW and selected IR channels to estimate the clear-column IR radiances for all IR channels. It also employs IR window channels to retrieve the surface skin temperature, emissivity, and reflectivity. The temperature and water vapor profiles are corrected for the effects of the clouds. This rapid stage produces reasonably accurate AIRS/AMSU-A products that are suitable for assimilation into operational numerical weather prediction models.
The Combined MW/IR/VIS Retrieval Stage recursively refines the temperature and water vapor profiles and surface parameters, while retrieving ozone and multiple cloud-layer parameters and calculating uncertainties for all retrieved parameters. If a profile is rejected, a new retrieval is produced, starting from the Microwave-Only product and utilizing the AMSU-A/HSB channels together with those AIRS channels which are insensitive to cloud effects. The final profiles and improved clear-column radiances for all IR channels are passed to the standard product.
SOURCE: EOS Data Products Handbook, Volume 2, 2000, pp. 21-22. |
