AIRS

Overview

The Atmospheric Infrared Sounder, AIRS, flying on a NASA weather and climate research satellite called Aqua, is the first spaceborne instrument designed specifically to measure the most critical global climate change indicators. AIRS' cutting edge technology allows it to measure the amounts of water vapor and greenhouse gases with remarkable precision and accuracy. As a result, we are now beginning to gather data from space of the quality that will allow us to address many of the scientific questions related to Earth's climate and global change in the atmosphere.

Even though AIRS has only been in operation since August of 2002, not long enough by far to address many of the climate questions, it is already becoming apparent that data coming from the AIRS system are of unprecedented accuracy. Although still under evaluation, the AIRS data appears to be better than the current "gold standard" weather balloons. It is as if more than 300,000 greatly improved weather balloons were released every day all over the globe - even from uninhabited or totally inaccessible areas. For the first time we are forming a precise picture of the three-dimensional global distribution of water vapor the primary greenhouse gas.

Climate Processes and Model Validation

The largest uncertainties in global climate models are associated with cloud and water vapor feedback processes. Until now, measurements of water vapor from the atmosphere's upper troposphere have been limited, and as a result accurate modeling of water vapor feedbacks with increasing surface temperature a problem for global climate models. Since its launch in 2002, the AIRS instrument has been able to provide data which has shown a positive correlation between sea surface temperature and water vapor at 250 millibar, indicating a "positive" upper tropospheric water vapor feedback with increased surface warming. Other studies using AIRS data have confirmed that water vapor feedback is positive with increased global warming.

Climate models require validation of their "climatology"--the rules they use to define how climate processes operate. These models usually use reanalysis from forecast centers. Reanalysis is a method of constructing a high-quality climate data record that combines past observations from different sources to produce a picture of how the Earth's climate has evolved over time. Like the weather models, the water vapor fields in the reanalysis have been limited in their accuracy. AIRS data are able to provide a highly accurate daily source of global three dimensional water vapor fields.

AIRS data have been used in studies to evaluate the climatology of water vapor fields in the major climate models. Results have shown that the models have considerable errors in the vertical and horizontal distribution of water vapor on an annual climatology. It is believed this is a consequence of compensating errors in the vertical distribution of the water vapor climatology. Drier lower troposphere air is compensated by a wetter upper troposphere in the models to produce an overall correct outgoing longwave radiation (OLR).

Intraseasonal oscillations of intense rainfall and drought are regular annual processes in Indonesia and the Tropical Western Pacific. Called the Madden Julian Oscillation (MJO), the cycle has a period of about 45 days and occurs in the Northern Hemisphere winter, November - April. The AIRS high resolution vertical and horizontal data of temperature, water vapor and ozone amounts have improved scientist's ability to validate theories governing the MJO. For example, AIRS observations show a warm/moist pre-conditioning of the mid troposphere prior to intense rainfall and a cold/dry pre-conditioning prior to drought; temporal and spatial distributions better match expectations than model predictions.

The AIRS enables observation of temperature waves at other spatial frequencies including gravity waves and mountain waves. Gravity waves are oscillations in the vertical temperature profile over a horizontal distance. AIRS provides good coverage and horizontal resolution enabling observations in and above the troposphere. Each year Polar Stratospheric Clouds (PSCs) form when the temperature in the stratosphere drops below 195K. Certain types of PSCs convert reservoirs of chlorine to an activated form that destroys ozone. Scientists used AIRS data to identify a case whereby formation of PSC's were caused by of an Antarctic Peninsula mountain wave event.

For more results, see our page Significant Findings.

1. Studies have used the AIRS data to show that surface warming leads to an increase in water vapor. This water vapor acts as a greenhouse gas and amplifies the surface warming.
   

2. AIRS moisture fields differ from 6 major climate models such that the models are too dry below 800 mb in the tropics compared with AIRS, and too moist between 300 mb and 600 mb especially in the extra-tropics. This affects model predictions of future climate warming.
   

3. AIRS data revealed a repeatable annual cycle in relative humidity over the polar continent and super saturation with respect to ice, particularly in winter, where it might occur almost half the time in the troposphere. This may affect the quantity and isotopic composition of ice over Antarctica.
   

4. Discovery of a trimodal temperature vertical structure and a low-level moisture and temperature preconditioning associated with the was found using AIRS data.
   

5. AIRS high-resolution spectra provides the first global view of small-particle-dominated cirrus clouds.

Water Vapor and Climate

Feedbacks amplify warming

Most of the warming caused by carbon dioxide does not come directly from carbon dioxide, but from amplifying feedbacks. Water vapor is a greenhouse gas whose amount in the atmosphere increases with temperature. This makes its feedback particularly important.

Scientists using AIRS data have removed most of the uncertainty about the future role of water vapor. Temperature and water vapor observations have corroborated climate model predictions that rising carbon dioxide levels will lead to warming and increased water vapor. The increased water vapor greenhouse effect will roughly double the warming effect of carbon dioxide alone. The AIRS data are the strongest observational evidence to date showing the response of water vapor to a warming climate. These studies demonstrate that if greenhouse gas emissions continue to increase the earth's climate is virtually certain to warm by several degrees Celsius in the next century -- unless a strong, negative, and as yet unknown feedback mechanism emerges.

Variations in the three dimensional distribution of atmospheric water vapor, where higher altitudes appear brighter. Made with AIRS data retrieved during summer and fall, 2005.

Image credit: Dr. Vincent J. Realmuto, Earth Surface Science Group, JPL

Cloud and Polar Processes

Clouds also play a very important role in climate science. Solar reflective sensing instruments such as MODIS, GOES and MISR provide vital information on the cloud shortwave response to the climate system. Cloudsat, CALIPSO and CALIOP provide exceptional cloud profiles and phase information. Infrared also has an important role in understanding clouds. In particular, the hyperspectral infrared is particularly good at detecting and characterizing cirrus clouds. These clouds have a tendency of providing a negative feedback (warming effect) since they radiate at a lower temperature and are not as effective in shielding solar radiation.

AIRS provides this information simultaneously and co-located with temperature and water vapor allowing scientists to better understand the processes affecting cloud formation. Scientists have used AIRS data to provide better identification of cloud thermodynamic phase (liquid or ice). Recently, it was found that tropical mean precipitation is well correlated with cloud properties and radiative fields. In particular, the tropical mean precipitation anomaly is positively correlated with the top of the atmosphere reflected shortwave anomaly and negatively correlated with the emitted longwave anomaly. Recently, the Laboratoire de M´et´eorologie Dynamique (LMD) created a cloud climatology using data from several instruments including AIRS. Their findings indicate that high clouds in the tropics have slightly more diffusive cloud tops than at higher latitudes.

Isotherms of surface temperature at 273K retrieved by AIRS in June, July and August, for 2003-2008 overlaid on image of surface temperature for 2008.

AIRS data are used to better understand the unusual melting of ice in the Arctic during 2007. AIRS data clearly see the unusual change in surface and air temperatures (see Figure 3). Data from AIRS, CALIPSO, CALIOP, CloudSat, and MODIS are used to identify an increase in downwelling shortwave radiation by 32W/m2 in the Arctic region in 2007 leading to warming in excess of 2.4K. Increased air temperatures and decreased humidity as measured by AIRS explain the reduction in cloudiness. In an independent study with similar results, AIRS data are used to show that also in 2007, the number and strength of the temperature inversions is higher than in other years . Both studies confirm the anomaly was driven by atmospheric conditions.

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Information on this page taken largely from the paper:

The Atmospheric Infrared Sounder (AIRS) on the NASA Aqua Spacecraft: a general remote sensing tool for understanding atmospheric structure, dynamics and composition

Written by: Thomas S. Pagano, Moustafa T. Chahine, Eric J. Fetzer

California Institute of Technology, Jet Propulsion Laboratory, 4800 Oak Grove Dr. Pasadena, CA 91109