Main Science Objectives
Understanding the dynamics of climate, the transport of chemical agents in the atmosphere and their distribution over the surface of the Earth, and the rainfall and evaporation that control the growth of vegetation requires a precise knowledge of the global atmospheric circulation, temperature profiles, and water vapor content.
AIRS/AMSU/HSB will observe and characterize the entire atmospheric column from the surface to the top of the atmosphere in terms of surface emissivity and temperature, atmospheric temperature and humidity profiles, cloud amount and height, and the spectral outgoing infrared radiation. These data and scientific investigations will answer long-standing questions about the exchange and transformation of energy and radiation in the atmosphere and at the Earth's surface.
1. Determination of the factors that control the global energy and water cycles
The study of the global hydrologic cycle and its coupling to the energy cycle is a key to understanding the major driving forces of the Earth’s climate system. AIRS/AMSU/HSB will measure the major components of these driving forces including the thermal structure of the surface and the atmosphere, the outgoing longwave infrared radiation, and the atmospheric water vapor content.
2. Investigation of atmosphere-surface interactions
The high spectral resolution of AIRS will provide several spectrally transparent window channels that will observe the surface with minimal contamination by the atmosphere and will allow the determination of accurate surface temperature and infrared spectral emissivity. In addition, the narrow spectral channels in the short-wavelength infrared region will observe the atmospheric layers near the Earth’s surface with the highest vertical resolution possible by passive remote sensing. The observations will enable investigations of the fluxes of energy and water vapor between the atmosphere and the surface, along with their effect on climate.
3. Improving numerical weather prediction
Improving the world weather observing system is an essential objective of Earth system science and applications, because the phenomena that govern long term climate are the same as those that manifest themselves in transient weather perturbations. AIRS/AMSU/HSB observations are, therefore, equally applicable to both climate and weather studies and appropriated to be provided to the US National Centers for Environmental Prediction (NCEP) and the European Center for Medium-Range Weather Forecasts (ECMWF) for assimilation into the operational forecast General Circulation Models. Through such assimilation, the AIRS/AMSU/HSB observations will lead to substantial increases in the mid- and long-range weather forecast skill.
Numerical weather prediction models have now progressed to the point where they can predict atmospheric temperature profiles to an accuracy of 2 K, which is equivalent to the accuracy of current satellite data. Further improvement in our knowledge of temperature profiles is essential in order to improve forecasting accuracy. AIRS/AMSU/HSB temperature profiles with radiosonde accuracy of 1 K in 1 km-thick layers are key to improving the accuracy and extending the range of weather forecasts.
4. Detection of the effects of increased greenhouse gases
AIRS will map the concentration of carbon dioxide and methane globally. In addition, the ability to provide simultaneous observations of the Earth's atmospheric temperature, ocean surface temperature, and land surface temperature and infrared spectral emissivity, as well as humidity, clouds and the distribution of greenhouse gases, makes AIRS/AMSU/HSB a primary space instrument to observe and study the response of the atmosphere to increased greenhouse gases.
5. Assessing climate variations and feedbacks
The accuracy and high spectral resolution of AIRS provide a powerful new tool for climate studies. AIRS’ high resolution infrared coverage from 3.74 to 15 µm will give researchers the ability to validate numerical models and to study different climate processes as needed. For example, emission to space by strong and weak water vapor lines is a critical climate feedback mechanism in the middle and lower troposphere. Numerical models must reproduce such lines as an indication of their ability to describe the climate system.
6. Atmospheric Sounding
Atmospheric sounding for information about temperature and abundance of gases is based on the fact that thermal radiation received by a radiometer originates at wavelength-dependent depths in the atmosphere. This is caused by a non-uniform absorption spectrum, particularly by molecular absorption lines. (Note that in an atmosphere in thermal and radiative equilibrium, emission equals absorption. If that were not the case, the atmosphere would either cool down or heat up until balance is reached.) At wavelengths near the peak of such a line, absorption may be so strong that most of the underlying atmosphere is opaque, and only the top of the atmosphere is seen.” Conversely, at wavelengths away from the lines, often called a window” region, the atmosphere may be nearly transparent, and the surface or the bottom of the atmosphere is seen. Through spectral sampling, i.e., by measuring narrow spectral bands or channels,” it is then possible to probe into different depths of the atmosphere.
It is possible to separate the effects of different atmospheric gases by using channels in different spectral regions where one gas has absorption features while the others do not. To measure temperature profiles, AIRS uses a large number of CO2 absorption lines in the infrared spectral region, while AMSU-A uses a few O2 absorption lines at microwave wavelengths. To measure water vapor profiles, AIRS uses many H2O absorption lines throughout its spectral range, and HSB uses a single H2O absorption line in the microwave region. Since the vertical distribution of CO2 and O2 are both stable and well known, the CO2 and O2 channels allow the temperature distribution to be determined. With that known, the H2O channels allow the vertical distribution of water vapor density to be determined.
The infrared spectral range covered by AIRS also features absorption lines of other molecular species, such as O3 and CH4. This makes it possible to deduce ozone and methane profiles. Finally, while liquid water makes most clouds completely opaque in the infrared region, in the microwave region they are partially transparent. The microwave spectral absorption features of liquid water therefore make it possible to determine the vertical distribution of liquid water in clouds from AMSU-A and HSB measurements. This information is used to make the derived AIRS temperature and water vapor profiles more accurate.