Atmospheric Soundings

An important application of satellite measurements is their ability to make soundings of the atmosphere- for example retrieving vertical profiles of  water vapor and ozone (important trace gases) as well as profiles of temperature.  

Two types of passive sounding techniques:

1. Vertical systems that sense the radiation coming both the Earth's surface and the atmosphere
   
2. Limb sounding systems which sees radiation from only the atmosphere, by looking "sideways" (instead of down) through the atmosphere and measuring infrared emission based on temperature and trace gas concentrations in the mesosphere, stratosphere and the upper troposphere.  For species which absorb in the solar spectrum (for example ozone, water vapor and nitrogen dioxide, as well as aerosols) solar occultation instruments can be used to measure the extinction of sunlight through the atmospheric limb during satellite sunrise and sunset.

As we already observed in our review of data from the GOES sounder, sounding theory is based on the integrated form of Schwarzchild's equation , which can be simplified with the following statement:

Radiance detected by the satellite sensor comes from two sources

1) the surface term, which is a function of the radiance leaving the surface times the transmittance of the entire atmosphere along the slant path from surface to satellite.  

2) the contribution of the atmosphere, the  amount of radiance from discrete atmospheric layers which reach the satellite, integrated over all layers.  

as we know radiation, that is not transmitted is absorbed and may interact with gases in the atmosphere in the following manner

• ionization-dissociation interactions (an electron is stripped from an atom or molecule, or a molecule is torn into atoms)
• electronic transitions (orbital electrons quantum leap between energy levels)
• vibrational transitions (changes in molecular vibrational energy states)
• rotational transitions (changes in molecular rotational energy states)

Vertical temperature soundings (and trace gas concentrations) are based on vibrational transitions of atmospheric gases.  

• CO2 bands (at 15um and 4.3 um) are used to retrieve temperatures.  
• The 6.3 um water vapor band is used to retrieve moisture profiles.  
• The 9.6 um band of ozone absorption is used to measure total column ozone.  

POES Sensors: Review

HIRS/2, high resolution infrared radiation sounder, operates like the AVHRR, but the HIRS/2 has more channels (20 versus 4 or 5).  

Here is the main problem with retrieving profiles, we want to use the remotely sensed radiance and relate it to a unique temperature/trace gas profile.   This is called the inversion problem.  The opposite approach, the forward approach, is when temperatures and trace gas profiles are used to calculate outgoing radiance (obviously, this only works in regions which are not contaminated with clouds).

Review of Sounding Tutorial concepts presented in the GOES Tutorial (http://cimss.ssec.wisc.edu/sounder/profinfo.html)

Radiative Transfer Equation Basic Principles

The upwelling radiation sensed by a satellite sensor is governed by a) emission from the earth's surface transmitted through the atmosphere and b) emission from the atmospheric layers transmitted through the outer layers of the atmosphere.

The surface contribution to the upwelling radiation sensed at the top of the atmosphere for wavelength lambda is a product of the surface emissivity, the Planck function at the surface temperature, and the vertical transmittance through the depth of the atmosphere. This is written as:

The transmittance of upwelling radiation through the atmosphere is diminished exponentially with increasing path length,

where k is the absorption coefficient of the atmosphere and u is the density weighted path length.

The formulation for a discrete atmospheric layer is similar to that for the surface contribution except that the appropriate layer emissivity, Planck function at layer temperature , and vertical transmittance would be used. The contribution for all atmospheric layers is then,

The radiative transfer equation combines the surface and atmospheric contributions and for an infinitely layered atmosphere can be written:

Weighting functions and spectral bands

The term

in the radiative transfer equation is known as a weighting function. The weighting function, the derivative of transmittance with respect to height (pressure), specifies the relative contribution each atmospheric layer makes to the radiation emitted to space and thereby determines those regions of the atmosphere which are sensed from space at this wavelength. Sounding is accomplished with a group of spectral bands selected to detect radiation emitted from successively lower layers of the atmosphere.

POES Quantitative Products

The TOVS systems includes four modules

• the preprocessor
• the atmospheric radiance module
• the stratospheric mapper
• the retrieval module

The preprocessor process digital count data and converts it to radiances, and each scan spot is located on the earths surface.   The preprocessor also calculates the solar zenith angle, terrain elevations, and first guess of the skin temperature and surface albedo.  

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