A sounding is a vertical profile of atmospheric temperature, moisture or some other parameter. Soundings, including measurements of total water vapour content, total ozone, cloud height, cloud amount, surface temperature, etc., can be obtained from remote measurements of spectral radiance through carefully selected channels, often in the infrared (IR) or microwave regions of the electromagnetic spectrum.
Remote sensing of the vertical structure of the atmosphere or of its constituents is possible because of the wavelength dependence of outgoing radiation emitted from the earth's surface, clouds and the atmosphere. The emitted radiation is governed by a combination of Planck's law and by pronounced absorption over narrow wavelength intervals due to absorbing species such as carbon dioxide, water vapour and ozone.
The existence of absorption bands is exploited in remote sounding. Radiation measured by a satellite in the centre of an absorption band, where absorption is very strong (i.e. the atmosphere is opaque), arises from the upper atmosphere because the atmospheric radiation emitted at lower levels is absorbed. In the band wings, where the absorption is weak, a large path of atmosphere is required to absorb the incident radiation, so the greatest contribution to outgoing radiation is from lower levels of the atmosphere where the density of emitting molecules is largest. In a moderately absorbing part of the spectrum the upper part of the atmosphere is relatively transparent, but the outgoing radiation does not come from the surface, rather from molecules in the middle troposphere. These molecules have absorbed the energy emitted from the surface and re-emit at their characteristic temperature (middle tropospheric temperatures) in accordance with Planck's law. Figure 23 shows the 15 micron carbon dioxide absorption band and the location of the seven channels of the IRIS (InfraRed Interferometer Spectrometer) instrument which was carried on board the Nimbus-4 satellite.
Figure 23: NIMBUS-4 IRIS spectrum of the 15 micron carbon dioxide band. Spectral channels for IRIS are indicated. Oordinate is equivalent blackbody temperature, abscissa is wavenumber in cm-1. From Smith (1972), p1076.
Sounding of atmospheric parameters from spectral radiance measurements is therefore based on the highly selective absorption (and re-emission at characteristic temperature) of gases, and the amount of absorbing gas. Given a gas uniformly mixed through the atmosphere (e.g. carbon dioxide for which changes in measured radiance may be assumed to be caused by changes in the temperature of the absorber) then a set of spectral radiance measurements gives information about the temperature for various layers in the atmosphere. For example, consider the NIMBUS-4 IRIS spectrum (shown below) in the vicinity of the 15 micron CO2 band. In the far wing of the band (~800 cm-1) the upwelling radiance corresponds to a surface temperature of 320K. Toward the center of the band, the radiation temperature decreases because of the decrease of temperature with altitude in the lower atmosphere. The minimum radiation temperature is around 210K, corresponding to emissions from the tropopause region. The radiation temperature increases to almost 240K for the sharp peak at the band center because the atmospheric temperature increases with altitude in the stratosphere.
Figure 24: NIMBUS-4 InfraRed Interferometer Spectrometer (IRIS) spectrum of upwelling atmospheric radiance from about 7 to 20 microns. Various bands are marked including the 15 micron carbon dioxide band. Planck function curves for various blackbody temperatures are shown as dashed lines. From Smith (1985), p416.
The figure below summarises spectral regions which are commonly used in remote sounding and imaging.
Figure 25: Summary of spectral regions commonly used in atmospheric remote sounding and imaging. From Smith (1985), p415.
The regions may be sub-divided as follows:
Dr D C Griersmith
