SCIENCE OBJECTIVES
The SAGE III mission is an important part of NASA's Earth Observation System and is designed to fulfill the primary scientific objective of obtaining high quality, global measurements of key components of atmospheric composition (see table below) and their long-term variability.
These measurements are vital inputs to the global scientific community for improved understanding of climate, climate change, and human-induced ozone trends.
AEROSOL EXTINCTION
Aerosols play an essential role in the radiative and chemical processes that govern the Earth's climate. Since stratospheric aerosol loading has varied by a factor of 30 since 1979, long-term monitoring of tropospheric and stratospheric aerosol is crucial. SAGE III aerosol measurements will provide important contributions in the following areas:
Research demonstrates that the long-term variability of aerosol abundance strongly modulates the rate of ozone destruction in the lower stratosphere [Solomon et al., 1997].
The impact of aircraft exhaust on ozone is strongly dependent on the abundance and properties of the ambient aerosol. The future of high speed, stratospheric air travel is, in part, dependent on improved understanding of aerosol properties [Stolarski and Weosky, 1993].
Aerosol radiative forcing is the largest unknown in current climate models and, as a result, in predicting future climate. Accurate vertically-resolved measurements of aerosol optical properties are an important element of improved climate prediction [IPCC, 1996].
Observations from space of many geophysical properties like sea surface temperature, vegetation, and atmospheric trace gas species can be adversely affected by the presence of aerosol. Other EOS sensors like MODIS and MISR require SAGE III measurements of stratospheric aerosols to achieve optimal performance.
CLOUDS
Clouds play a major role in determining the planet's solar and longwave energy balance and, thus, are important in governing the Earth's climate. SAGE III will provide measurements of mid and high level clouds including thin or "sub-visual" clouds that are not detectable by nadir-viewing passive remote sensors. These observations are important because:
While low clouds primarily reflect incoming solar radiation back into space (acting to cool the planet), mid and high level clouds enhance the "greenhouse" effect by trapping infrared radiation (acting to warm the planet).
The presence of thin cloud near the tropopause may play a significant role in heterogeneous chemical processes that lead to ozone destruction in mid-latitudes.
Clouds play an important role in feedback processes (such as changes in altitude or amount of cloud) associated with climate change. Currently neither the sign nor magnitude is known for the climate impact of cloud response to changes in radiative processes brought about by increasing levels of human-derived greenhouse gases (like carbon dioxide) and aerosols.
WATER VAPOR
On a molecule-by-molecule basis, water vapor is the predominant greenhouse gas and plays a crucial role in regulating the global climate system. An improved understanding of the global water vapor distribution can enhance our ability to understand water's role in climate processes. SAGE III water vapor measurements will provide important contributions in the following areas:
Uncertainties in the moisture field impact weather forecast model initialization, especially in the tropics.
The long-term stability and self-calibration capabilities of SAGE III may permit the detection of a trend in water vapor in the upper troposphere and lower stratosphere as a diagnostic of global warming.
SAGE III water vapor observations will provide valuable information to help EOS observation facilities evaluate, and correct for, water vapor induced variations in remotely measured parameters.
High vertical resolution water vapor profiles will help illuminate hydrological processes in the atmosphere such as the tropical "tape-recorder."
OZONE
Ozone research has remained at the forefront of atmospheric science for many years because stratospheric ozone shields the Earth's surface (and its inhabitants) from harmful ultraviolet radiation. Since recent declines in stratospheric ozone have been linked to human activity, accurate long-term measurements of ozone remain crucial. SAGE III ozone measurements will provide important contributions in the following areas:
It is important to monitor ozone levels in the lower stratosphere and upper troposphere since observed trends are the largest and most poorly understood at those altitudes. SAGE III's high vertical resolution and long-term stability make it uniquely well suited to make these measurements.
The relationship between aerosol, cloud, and chemical processes affecting ozone argue for simultaneous measurements of these atmospheric constituents (such as those made by SAGE III).
Ozone is a strong greenhouse gas that modifies the macroscopic structure of the atmosphere. Changes in the amount of stratospheric ozone can lead to changes in sensible weather at the Earth's surface. Accurate simulations of climate change require accurate assessments of changes in ozone levels.
Changing levels of ozone can also cause changes in other chemically and radiatively important gases including methane and the hydroxyl radical.
Ozone can also be used as tracer of dynamic processes in the stratosphere which are difficult to directly observe. These include the transport of air from the tropics to mid-latitudes.
PRESSURE AND TEMPERATURE
SAGE III temperature measurements will provide a unique data set for monitoring and understanding atmospheric temperature changes. In particular, the long-term stability and self-calibration capabilities of SAGE III may permit the detection of trends in stratospheric and mesospheric temperature that would be important diagnostics of climate change. SAGE III temperature measurements will provide important contributions in the following areas:
SAGE III temperature measurements in the upper stratosphere and mesosphere will be the only source of long-term temperature measurements in this region of the atmosphere.
Short-term temperature variations are important in understanding the photochemical interactions and feedback in the formation and destruction of stratospheric ozone.
SAGE III temperature measurements will allow the monitoring of periodic temperature changes, such as those associated with the solar cycle and quasi-biennial oscillation, and the effects of radiative forcing by aerosols.
An improved temperature climatology will lead to better radiative-dynamical-chemical general circulation models and enhance the understanding of the interactions between these processes and the state of the climate system.
Temperature and pressure measurements are essential to SAGE III for computing molecular scattering and the retrieval of mass mixing ratios of gaseous species on pressure surfaces.
NITROGEN DIOXIDE, NITROGEN TRIXOIDE, AND CHLORINE DIOXIDE
Nitrogen dioxide (NO2), nitrogen trixoide (NO3), and chlorine dioxide (OClO) play crucial roles in stratospheric chemistry and the catalytic cycles that destroy stratospheric ozone. SAGE III measurements will provide important contributions in the following areas:
SAGE III NO2 measurements are important because the processes that occur in the Antarctic winter and spring and give rise to the ozone hole effectively convert NO2 to nitric acid (HNO3). Thus NO2 is an important diagnostic of ozone hole chemistry.
Since it is measured during both solar and lunar occultation events, SAGE III observations of NO2 will improve our understanding of the strong diurnal (daily) cycles in stratospheric processes.
SAGE III will make virtually unique measurements of nitrogen trixoide (NO3). Although it is short lived in the presence of sunlight, NO3 plays an active role in the chemistry of other reactive nitrogen species such as NO2 and di-nitrogen pentoxide (N2O5) and, thus, indirectly in ozone chemistry.
Since few other measurements of NO3 are available, SAGE III measurements, which are made during lunar occultation (nighttime) events, will provide crucial validation for our current understanding of reactive nitrogen chemistry.
The unique SAGE III observations of chlorine dioxide (OClO) will provide insight into nocturnal polar chemical processes that set the stage for the ozone loss observed at high latitudes in the spring.
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