Geophysical Science with AIRS Data
Science
Carbon Dioxide
The most important trace gas retrieved by AIRS for the study of anthropogenic effects on climate is carbon dioxide. AIRS CO2 retrievals use an analytical method for the determination of carbon dioxide and other minor gases in the troposphere from AIRS spectra.
The AIRS data have been shown to be accurate to within 1.20 ppmv of simultaneous measurements by aircraft.
Global monthly maps of CO2 have been generated and identify global transport patterns in the mid-troposphere. These results will aid climate modelers in parameterization of mid-tropospheric transport processes of CO2 and other gases. AIRS CO2 provides a mid-tropospheric measurement that when combined with a total measurement from the Orbiting Carbon Observatory (OCO) to be launched in 2009 will identify surface sources and sinks of CO2.
There are over 13 referred publications on the use of AIRS data to determine CO2 concentrations and the use of the data for understanding global CO2 circulation. Although other instruments measure CO2, their observations are improved with a good understanding of the 'background' CO2 levels measured by AIRS. The AIRS will also provide the 'context' of observations with the wide swath and global coverage capability.
Carbon Monoxide
Tropospheric CO abundances are retrieved from the 4.67 m region of AIRS spectra as one of the last steps of the AIRS team algorithm. AIRS' 1600 km cross-track swath and cloud-clearing retrieval capabilities provide daily global CO maps over approximately 70% of the Earth.
Validation indicates AIRS CO retrievals are approaching the 15% accuracy target set by pre-launch simulations. AIRS/AMSU data have provided the most detailed, daily global observation of transport of mid-tropospheric CO from biomass burning emissions to date.
CO from Mexico city has been observed from AIRS, but it should be understood that AIRS has best sensitivity in the mid to lower troposphere. AIRS CO retrievals have been found to validate the plume rise mechanism in simulations of the transport of CO in the mid-troposphere.
Clouds
The AIRS standard products include cloud height, cloud fraction and cloud temperature. These products are just beginning to find their value as scientists attempt to understand the relationship between changing climate and cloud radiative forcing.
To date, the products have proved valuable through intercomparison with MODIS and Cloudsat, but there is considerable new information in the AIRS spectra about cloud microphysical properties and thin cirrus. This information has been used to detect Polar Stratospheric Clouds, ice clouds in the Antarctic polar stratosphere.
Dust
Infrared instruments are uniquely positioned to detect the presence of the coarser scattering material in the atmosphere, such as dust or cirrus. As dust impacts the thermal infrared window region, instruments such as AIRS are uniquely positioned to measure the radiative effects of dust in the OLR, and hence contribute to studying the overall radiative effect of aerosols. Including the effects of dust should improve the Level 2 retrievals, which should be important during hurricane season.
Comparisons of retrieved optical depths show that AIRS is in very good agreement with dedicated instruments such as MODIS and PARASOL. Unlike these instruments, we can detect dust both day and night,over ocean or land. Unlike many other instruments, AIRS can also retrieve dust heights, and possibly infer dust composition and effective particle size. AIRS data are used to study the evolution of the Saharan Air Layer. Longwave forcing from dust is one of the major uncertainties in climate modeling today. Retrieval of dust properties and longwave forcing should be possible from CrIS and IASI, and the science should be continued with these instruments.
Methane
The accuracy of AIRS CH4 is about 1.2-1.5% depending on different altitudes, which should be able to map seasonal variation of CH4 and can provide valuable information of atmosphere in mid-upper troposphere. AIRS Scientists have observed a significant summer enhancement of CH4 in high northern hemisphere which correlates well with soil temperature and is most likely due to northern wetland emission in summer. They have also observed a significant plume of CH4 over the Tibetan Plateau, formed by Monsoon and deep convection over the region and a high concentration of CH4 from Southeast of China and Alaska Fire in 2004, and some unusual features in Siberia in the winter.
There is still a lot of work to do with the AIRS methane product. There is little validation data available in the mid troposphere and the distribution still shows some anomalous behavior. It is a difficult gas to retrieve due to the contamination by water vapor lines, however, the AIRS Science Team does believe the AIRS to have skill in retrieving this gas.
Water Vapor
Temperature and Water Vapor Profiles are the primary standard products from AIRS. They are widely used for weather forecast improvement operations and research, climate model validation, and climate process studies. Like all the AIRS products, they are provided globally, daily, over land and ocean under clear and cloudy conditions. The wide swath and high yield make the AIRS a desired product for global modelers.
Traditionally, forecast centers have assimilated radiances, however, recent work has shown that assimilation of temperature profiles offers considerable improvement that was not realized before. Modelers at NASA's Short-term Prediction Research and Transition (SPoRT) Center have found that assimilation of temperature and water vapor into the regional models can improve prediction of pressure anomalies and rainfall. Additionally, AIRS temperature and moisture profiles are being used operationally by NWS weather forecast offices in their Advanced Weather Information Processing System (AWIPS) as a supplement to the coarsely spaced twice daily weather balloon observations. These asynoptic profiles provide mesoscale spatial resolution information of changing moisture and stability fields important for convective weather development over the continental United States.
The AIRS Temperature and Water Vapor Profiles are well validated. That makes them useful for validating climate models. Results have shown that several major climate models have considerable errors in the vertical and horizontal distribution of water vapor on an annual climatology. Scientists have improved the understanding of the process of supersaturation in the stratosphere using AIRS data; an important result for understanding the role of clouds on global warming. Most recently, scientists have related reduced cloudiness and downwelling radiation to the associated ice loss in the arctic region.
Other climate applications include hydration of the upper troposphere by tropical cyclones, boundary-layer free-troposphere temperature correlations, processes driving the Madden-Julian Oscillation, gravity waves and deep convection.There is no question that the AIRS atmospheric Temperature and Water Vapor products have found their way into the mainstream weather and climate research and operational communities.
Outgoing Longwave Radiation
The AIRS Outgoing Longwave Radiation (OLR) product is calculated from the AIRS retrieved profile and radiance spectrum. It is useful for understanding the radiative properties associated with the AIRS cloud, water and composition products and for comparison with CERES to ensure the AIRS retrievals obey the total radiative properties to support climate modeling.
The OLR products has not officially been released as it is still in the development stages.
Retrieval of OLR from CrIS and IASI appears to be possible at this time.
Ozone
AIRS monitors the three dimensional distribution of ozone allowing observation of the transport of ozone from the stratosphere to the troposphere.
AIRS provides the total column and profile of ozone with approximately two pieces of information in the boundary between the tropopause and the stratosphere. This makes the AIRS ozone ideal for studies of stratospheric-tropospheric exchange during severe convection events and the global transport of ozone through the Brewer-Dobson circulation. AIRS ozone data have undergone rigorous validation using aircraft data and ozonesondes.
Sulfur Dioxide
Sulfur dioxide (SO2), and aerosols also play a part in global climate. Rather than absorbing sunlight they reflect it, which tends to cool the earth, compensating the effects of global warming. For years scientists have observed an increase in both aerosols and SO2 and the deleterious effects they have on weather, including acid rain. Scientists have speculated that the global warming effect of the greenhouse gases, carbon dioxide and methane have been compensated by the increased aerosols. It is only recently that CO2 levels have risen above the compensating effects of aerosols.
AIRS sensitivity to SO2 is low and is primarily visible in volcanic activity. However the AIRS instrument is very sensitive to atmospheric aerosols, such as dust and ash. AIRS SO2 and aerosols are not produced in geophysical units (e.g. concentration or optical thickness), but due to absorption the SO2 is expressed in terms of a temperature difference. Dust, on the other hand, is expressed as a flag indicating an identifiable amount of material present in the atmosphere.
Surface Properties
The AIRS Sea Surface Temperature (SST), and Land Surface Temperature (LST) and Surface Emissivity have been met with limited success until recently. Under clear ocean conditions, the products all look very good, however other sources of data (MODIS, AMSR-E) have achieved better than AIRS under these conditions with higher spatial resolution.
The value in AIRS comes in the ability to produce a spectrum of the surface leaving radiance and from this a temperature and spectral emissivity. The spectral emissivity over land is highly desirable quantity for weather forecast data assimilation and should provide considerable forecast improvement.
Until recently, the high degree of variability in the scene and the effects of clouds in the boundary layer have limited the retrieval accuracy in this region. Recent efforts use the shortwave channels for temperature retrieval and the longwave channels for cloud clearing resulting in a major improvement in the surface products.
Temperature
Temperature and Water Vapor Profiles are the primary standard products from AIRS. They are widely used for weather forecast improvement operations and research, climate model validation, and climate process studies. Like all the AIRS products, they are provided globally, daily, over land and ocean under clear and cloudy conditions. The wide swath and high yield make the AIRS a desired product for global modelers.
Traditionally, forecast centers have assimilated radiances, however, recent work has shown that assimilation of temperature profiles offers considerable improvement that was not realized before. Modelers at NASA's Short-term Prediction Research and Transition (SPoRT) Center have found that assimilation of temperature and water vapor into the regional models can improve prediction of pressure anomalies and rainfall. Additionally, AIRS temperature and moisture profiles are being used operationally by NWS weather forecast offices in their Advanced Weather Information Processing System (AWIPS) as a supplement to the coarsely spaced twice daily weather balloon observations. These asynoptic profiles provide mesoscale spatial resolution information of changing moisture and stability fields important for convective weather development over the continental United States.
The AIRS Temperature and Water Vapor Profiles are well validated. That makes them useful for validating climate models. Results have shown that several major climate models have considerable errors in the vertical and horizontal distribution of water vapor on an annual climatology.
Scientists have improved the understanding of the process of supersaturation in the stratosphere using AIRS data; an important result for understanding the role of clouds on global warming. Most recently, scientists have related reduced cloudiness and downwelling radiation to the associated ice loss in the arctic region. Other climate applications include hydration of the upper troposphere by tropical cyclones, boundary-layer free-troposphere temperature correlations, processes driving the Madden-Julian Oscillation, gravity waves and deep convection.
There is no question that the AIRS atmospheric Temperature and Water Vapor products have found their way into the mainstream weather and climate research and operational communities.
