Sounding is a method used in remote sensing as opposed to surface observation. The focus of the remote observer is shifted from the (earth) surface to the medium that is in between, the atmosphere. Without entering in mathematical detail, the radiation measured at some receiver (e.g. satellite) results from the contribution of both the radiation emitted from the surface and that emitted from the atmosphere itself as a result of its above-zero temperature.
Each of the gases that comprise the atmosphere has a different ability to absorb the radiation, or let it through, which varies with the radiation wavelength. Wavelength regions where absorption for the whole atmosphere is very low are called window regions and are obviously suitable for earth-surface observation. Regions where a significant absorption occurs for some gas are called absorption bands for that particular gas and may be used to get information about it.
It is easy to understand that absorption increases with the concentration of the gas and the thickness of the gas layer. This two
variables determine a parameter called transmittance. As the thickness of the gas layer keeps increasing, going
from to the top of the atmosphere to the surface, increases, the transmittance is expected to decrease accordingly.
It will, however, decrease more steeply in some regions than others, depending on the wavelength. This concept of steepness
is expressed mathematically with the use of a derivative. The figure below shows the derivative of the transmittance for several wavelengths
, where the vertical axis represents atmospheric pressure as a useful substitute for height (1000 is ground level).
The real meaning of this function is to show from where most of the radiation is coming from and it's therefore called
weighting function. As this has a bell shape it follows that, for each wavelength, the radiation received is coming from
a different altitude. This is exactly the purpose of sounding. As the wavelength employed gets closer to a window region, the atmosphere becomes more and more transparent and it's possible to obtain information from lower layers.
Provided the temperature at each level is known, it is then possible to retrieve the concentration of a gas of interest for various heights. There are, however, some constraints. To begin with, the absorption band must be selected very carefully to avoid interference from other gases; this may limit the choice to regions that are less than optimal for many reasons. Second, the number of observable layers is limited by the number of wavelength used, as well as the precision with which the temperature profile is known. Finally, as the wighting function moves closer to the surface, emissions from the latter cannot be considered negligible any more.
The Geostationary Operational Environmental Satellite (GOES) program of NASA and NOAA was started in 1975 and provided for the first time the possibilty to get weather system animations, while the preceding polar-orbitiong satellites only provided two snapshots a day. The latest generation of GOES satellites was introduced in 1994 with GOES-8 that should be operational until April 1999. This satellite was equipped for complete temperature and moisture sounding of the atmosphere. For complete and detailed information about the GOES-8 satellite, follow the link to the NASA site, from which this material was taken.
The GOES-8 sounder uses four of its eighteeen bands for water vapor sounding. They are in the thermal IR region, centered at 11.0 micrometer (band 8), 7.43 micrometer (band 10), 7.0 micrometer (band 11) and 6.5 micrometer (band 12). Band 8 is the most transparent, providing information near the surface, while the others give information about increasingly higher levels.
The figure above shows the continental USA at 1146 UTC on 7 October 1994. Dry and moist tongues become obvious in the band-12 image of the upper troposphere.
So far, passive sounding applications have been presented. Active limb sounding in the radio-wave region is being experimented using GPS satellites. Limb sounding is accomplished by looking through the atmosphere tangently to the earth surface, as shown in the picture below.
For an interesting non-technical description of the applications of limb GPS sounding visit this site of the University Corporation for Atmospheric Research.
A detailed technical overview is also available.
Send your comments or report improper links to Vito Ilacqua
Water vapor sounding/ HMW 4/6/98/ Remote Sensing '98/ Environmental Science Department/ Rutgers University - New Brunswick