Q. What is the definition of a sounder in the context of atmospheric
instruments?
A. The AIRS instrument is a "remote sounder". There are several different
types, depending upon just what the instrument is designed to measure.
AIRS is a passive remote sounder that performs a spectral analysis of
the infrared and microwave radiation escaping from the top of the
atmosphere to determine the vertical structure of the temperature and
water vapor distribution within its field of view (FOV). The
instrument scans the FOV in a direction orthogonal to the flight path
of the satellite, so that it maps a swath approximately 2000 km wide
centered upon the nadir with a spatial resolution of 50 km. As the
spacecraft circles the Earth from pole-to-pole, the swath becomes a
band which progresses in a westerly manner as the Earth rotates. In 12
hours the instrument maps out the entire globe (day and night).
Here is a very simple description of how our instrument works. CO2 is
a strongly absorbing gas in our atmosphere. We have 2378-channel
infrared "radio receiver" on our satellite. One channel is in the core
of the CO2 absorption feature, and there are others that are offset in
wavelength so that they march up the wings of the feature and finally
are completely off it. The channel in the core receives radiation
emitted from very high in the atmosphere, for the atmosphere is opaque
to that frequency below that level. Hence, we can derive the
atmospheric temperature at that height from the amount of infrared
radiation we capture in that channel. The channels in the wings see
progressively deeper into the atmosphere, and we derive the
temperature
at correspondingly deeper levels. We are "remotely sounding" the
atmospheric temperature. Infrared channels at wavelengths chosen where
water vapor has absorption features are used for that derivation.
Another "active sounder", TRMM prcipitation radar, can determine the
amount of water vapor in its footprint and in the form of rain. It
uses
microwave radar from orbit to measure the backscatter of its
transmission, which is an indication of the amount of precipitation in
its FOV. Its FOV is scanned as well, so a picture is also built up as
it orbits the earth.
I believe the etymology of "sounder" as used by our community can be
traced to the following (with a bow to the navies of the world):
Entry: sound
Function: verb
Etymology: Middle English, from Middle French sonder, from sonde
sounding line, probably of Germanic origin; akin to Old English
sundlIne
sounding line, sund sea
transitive senses
1 : to measure the depth of : FATHOM
2 : to try to find out the views or intentions of : PROBE often
used
with out
3 : to explore or examine (a body cavity) with a sound
intransitive senses
1 a : to ascertain the depth of water especially with a sounding line
b
: to look into or investigate the possibility <sent commissioners...
to
sound for peace Thomas Jefferson>
2 : to dive down suddenly used of a fish or whale