Products of Earth's atmosphere and surface: Overview of the AIRS instrument suite physical retrievals
physical Retrievals from AIRS Data
Physical retrieval data are available within the AIRS Level 2 and Level 3 data products. Because AIRS measures the globe twice daily, two sets of retrievals are available each day with global coverage. All AIRS retrievals are the result of the combined infrared and microwave retrieval except for microwave-only standard products and AIRS-only data products. AIRS data products have been and continue to be made using various combinations of the AIRS, AMSU and HSB instruments (HSB ceased operation on February 5, 2003). Data products are available within 72 hours of acquisition.
AIRS Temperature product is reported on a fixed pressure grid at altitudes from the surface to the mesosphere, and the nominal resolution is approximately 1 kilometer vertically in the troposphere. AIRS temperatures have been validated against measurements from a wide variety of sensors, including those on balloons, aircraft and other satellites, and have been used in over one hundred published research studies.
Precipitable Water Vapor
AIRS specific humidity product is reported from the surface to the upper troposphere (subject to a limiting sensitivity equivalent to a mixing ratios ~10-15 ppmv) at nominal vertical resolution of 2 kilometers. Sensitivity begins to degrade at the 300 hPa pressure level, and at altitudes above the 100 hPa level the reported specific humidity is set to the initial climatology. The AIRS retrieval algorithm usually derives temperature and water vapor information simultaneously. As with temperature, AIRS water vapor has been validated against numerous other sources and has been used in many published research studies.
Water Vapor Saturation Quantities Derived from Temperature
Profiles of water vapor saturation mixing ratios over both ice and water, based on AIRS retrieved temperatures, are reported in the AIRS products. These are used to calculate the profiles of relative humidity.
Profiles of relative humidity calculated relative to liquid water and relative to the stable phase of water (i.e., taking into account the phase change from liquid to ice in a freezing layer) are reported in the AIRS products.
AIRS provides satellite retrieval of mid-tropospheric carbon dioxide day and night under cloudy conditions over ocean and land, without the use of a priori information from models. AIRS retrievals are based on the cloud-cleared thermal IR radiance spectra in the 15 micron band and associated Level 2 geophysical profiles of temperature, water vapor and ozone and achieves a 2 ppm accuracy in the tropics and mid-latitudes. AIRS measures the concentration of carbon dioxide (ppmv) with peak sensitivity at the 400 hP pressure level at a nadir resolution of 90 km x 90 km. The AIRS broad swath makes it able to map the global distribution of carbon dioxide every day.
AIRS carbon monoxide product (ppbv) is reported as a profile on the fixed pressure grid and also as a total column burden. The retrieval uses an a priori based on separate monthly climatologies for the northern and southern hemisphere that blend smoothly across the tropics. The peak sensitivity occurs at 500 hPa. The algorithm is based on optimal estimation and the retrieval is performed for 9 layers.
AIRS operational processing generates core products of fractional cloud cover, cloud top pressure and cloud top temperature. Cloud research products include cloud thermodynamic phase (liquid or ice), ice cloud optical thickness, and ice cloud effective diameter. The core products are well validated against several other information sources, and have been used in many published studies of atmospheric processes. Efforts are underway to compare the AIRS cloud research products against observations by other satellite sensors, and to assess their value in weather and climate studies.
AIRS carbon monoxide product (ppbv) is reported as a profile on the fixed pressure grid and also as a total column burden. The peak sensitivity occurs at 400 hPa. The algorithm is based on optimal estimation and the retrieval is performed for 10 layers. Researchers are to consider the Version 6 methane profile product to be a provisional release.
Outgoing Longwave Radiation (OLR) and Clear Sky OLR Retrievals
OLR is the total longwave radiative flux (in units of W/m²), emitted to space by the earth-atmosphere system, integrated from radiances emitted at all angles and all frequencies. OLR is not directly measured but is calculated from the retrieved state. OLR has been widely used as a proxy for convective activity in the tropics. Susskind et al (2012) used the first nine years of AIRS retrievals to attribute changes in OLR to the effects of El Niño/ La Niña oscillations on the distributions of tropical water vapor and cloud cover.
AIRS monitors the total column and three-dimensional distribution of ozone, allowing observation of ozone transport from the stratosphere to the troposphere during certain atmospheric events. AIRS ozone data can be used for studies of global transport of ozone.
The AIRS standard surface product is the result of the combined infrared-microwave retrieval or of the infrared-only retrieval when AMSU is not used. In addition, there are microwave-only surface products derived solely from the microwave radiance data. Surface properties provide include surface skin temperature, surface air temperature, surface emissivity and reflectance.
Tropopause Derived from Temperature
Retrieved tropopause height is determined by testing the lapse rate of the higher vertical resolution air temperature profile against the WMO (1992) criteria.
AIRS profiles and tropopause height are generally provided on a pressure grid. Geopotential heights provide the information users need to translate between this pressure grid and physical altitude.
Top of Boundary Layer (in pressure)
Pressure at the top of the planetary boundary layer and its associated quality control are reported in the Level 2 Support Product at the resolution of the AMSU FOV. This height is reported in units of pressure (hPa). The top of the boundary layer corresponds to the pressure of the largest absolute value of the gradient of relative humidity (relative to liquid phase of water) layer profile calculated on the support pressure layer grid.
Flagged values from AIRS are available in Level 1B and Level 2 data formats.
The AIRS instrument is very sensitive to atmospheric aerosols, such as dust and ash. AIRS aerosols are not produced in geophysical units (e.g. concentration or optical thickness. Dust is expressed as a flag indicating an identifiable amount of material present in the atmosphere.) The detection of the presence of dust is made by comparison of radiances, and the flags originate in the AIRS L1B Data Product. They are propagated to the Level 2 Standard and Support Products. The dust flag is valid only over ocean scenes and fails if thin cirrus or other clouds are present at altitudes above the dust. Despite being valid only over ocean, the dust detection algorithm should be useful for filtering data contaminated by dust in the Saharan Air Layer. We recommend that users filtering to select high quality data over oceans should avoid Level 2 retrievals for which dust_score >= 380 or dust_flag = 1.
Sulfur Dioxide Flag
The detection of the presence of dust or volcanic SO2 is made by comparison of radiances, and the flags originate in the AIRS L1B Product. They are propagated to the Level 2 Standard and Support Products. Physical retrievals can be seriously compromised if the AIRS field of regard is contaminated by dust and volcanic ash. We recommend that users filtering to select high quality data should avoid Level 2 retrievals for which BT_dif_SO2 <= 6 K. Those retrievals are likely to be contaminated by volcanic ash.
The microwave-only standard products are retrieved by the microwave retrieval stage of the AIRS algorithm using either AMSU radiances alone or AMSU+HSB radiances combined. No infrared data are used to retrieve these products. Standard products include atmosphereic temperature profile, geopotential height, land-type classification, emmissivities, total precipital water and cloud liquid water. AMSU-only microwave-only support products contain useful (at altitudes below the 300 hPa level) retrieved moisture profiles as well as rain rate, cloud liquid water, and cloud/ice flag.
The spatial footprint for all retrievals is the field-of-view (FOV) of the AMSU instrument which is 40.5 km at nadir. Note that microwave-only products are absent from the AIRS-only products.