AIRS

Frequently Asked Questions


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Q.  Since there is an increase in surface "brightening" and greenhouse gases
may be adding to the infra-red at the surface, have you been able to
subtract out the infra-red?  

A.  I believe from your question phrasing that you have a slight confusion
about "brightening". I suggest that you use Google to search out terms
and phrases like "Keeling Curve" and "greenhouse gas" and "global
warming" and "blackbody radiation" and follow the links. Wikipedia is
also a good source. All bodies above absolute zero emit radiation.
However, the distribution of the emitted energy changes with
temperature and composition, and can be modified by the intervening
medium. The emissions from a solid, opaque object are much simpler to
model than those from a semi-transparent medium like our atmosphere.
In the former you see the radiation emitted from the surface. In the
latter, the radiation source is distributed throughout the medium and
the structure oftemperature and composition makes the spectrum very
complex to interpret (BUT very informative).

The radiation extends throughout the entire frequency spectrum, from
ultraviolet through visible to infrared and then microwave. If you
plot the intensity, the "brightness", as a function of wavelength or
frequency you will see something that approximates the black-body
curve. If the temperature of the object being observed increases,
the amount of radiation emitted at all wavelengths increases, but the
peak of the curve moves toward the shorter wavelengths. The
temperature of the photosphere of the sun is approximately 5,500 F.
This is the solar disk you see when you look at the sun just as it is
rising or setting (the only time it is dim enough due to atmospheric
absorption for you to glance at the sun unprotected with much reduced
danger of damage to your eyes). The peak of the emission curve is in
the yellow portion of the visible spectrum. It is no surprise that our
eyes are most efficient in that portion of the spectrum. The Earth
global average temperature is approximately 295 K, which corresponds
to 72 F. The Earth spectrum is far less bright than that of the sun,
and its peak is shifted into the infrared. Greenhouse gases in the
atmosphere preferentially allow visible wavelength radiation to
penetrate all the way to the surface of the Earth. However, they
preferentially absorb and reradiate infrared radiation that
is being emitted from the surface back toward space. Thus the surface
warms, increasing the brightness of the Earth's emissions at all
wavelengths, until sufficient energy leaks through to space to achieve
stability once more. This is the very simple view. The modeling of the
response of the Earth to greenhouse gases, aerosols, soot and
dust, etc is a much more challenging undertaking. It requires accurate
observations over long periods to have any hope of success. Thus
enters AIRS and other satellites.

Let me give you a background about AIRS.

In brief, AIRS is a suite of infrared and microwave instruments on
board the Aqua spacecraft. The spacecraft is in a near-polar orbit,
sun-synchronous. By that I mean that the trace of the track of the
spacecraft on the surface of the Earth goes from pole to pole rather
than around the equator. The orbital period is 90 minutes, so the
spacecraft circles the Earth approximately 15 times per day. The sun-
synchronous part means that if you were to run Westward along the
equator fast enough to see the spacecraft pass directly overhead, you
would find that it does so at the same apparent solar time (about 1:15
PM local standard time).

Our microwave and infrared instruments are passive. That is, they do
not broadcast energy at the Earth (as radar does). They simply act as
cameras equiped with unique filters to capture the radiation
(electromatic waves, called "light" by the general public when its
wavelenghts fall within the regime to which our own eyes are
sensitive).

The radiation is emitted from all matter above absolute zero
temperature. Thus our instruments "see" radiation emitted by the
Earth's surface, the atmosphere, clouds, etc. The wavelength at which
we observe controls how deep we see in the atmosphere. The different
molecues in the atmosphere (carbon dioxide, oxygen, water, nitrogen,
argon, ozone, methane, etc) all preferentially absorb/emit
radiation at specific wavelenghts. We make use of that property to
choose the instrument "filters" so that at some wavelenghts we see
radiation emitted very high in the atmosphere, at others from lower
levels, at others ("window frequencies") we see radiation from the
surface.

Our instrument field of view is 50 kilometers (about 30 miles). We
collect all the radiation from that field of view to derive the
average temperature of the atmosphere throughout its vertical extent,
the distribution of water vapor throughout its vertical extent, and
the surface temperature. In effect, we sample the physical state of
the atmosphere in that field of view just like a balloon borne
radiosonde, without requiring the instrument to be physically
present in the atmosphere.

Radiosondes are the way the weather forecasters get samples of the
physical state of the atmosphere around the world, which are then fed
into their (enormous) computer modelling programs to predict how the
weather will change in the future. Unfortunately, people who launch
radiosondes are usually stationed on the land masses (most of which
are in the Northern Hemisphere) and only 25% of the Earth's surface is
land. They also require civilized support, so there are not many
radiosondes being launched in the jungles of South America or the
tundras of Alaska and Siberia.

Our instruments are equivalent to launching radiosondes all over the
Earth, land and ocean, equator to pole, on a grid separated by 30
miles, every day! As the weather forecasters begin to use our data,
their models improve in accuracy of forecasting future weather because
all the empty areas in their grids now have observational data
included. The models are updated every day with new observations. This
is required because the models do not include all the subtle
feedback loops that drive our weather. Thus their forecasts will
diverge with time from reality unless they are constantly updated with
real observations. This is the so-called "Butterfly Effect". Small
errors tend to grow until you miss the development of a hurricane.

I have tried to give you a brief synopsis of what we are doing. To get
the full picture, you should first look through all the information on
our web page http://airs.jpl.nasa.gov and then you should begin to
"Google" the various terms you encounter in those writeups. You will
quickly find that we have learned a lot about this incredible world we
inhabit in the last 60 years, and that we have so much more yet to
learn. It is humbling...

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