Global Positioning System Reference
In-Depth Information
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lines. When gas molecules interact the potential energy changes, due to changed rel-
ative positions and orientations of the molecules. As a result, the gas is able to absorb
photons at frequencies well removed from the resonance lines. Pressure broadening
converts the line spectrum into a continuous absorption spectrum, called the line pro-
file. The interactions and thus the broadening increase with pressure. Given the struc-
ture of molecules it is possible to derive mathematical functions for the line profiles.
Because of the complexities of these computations and the presence of collisions,
these functions typically require refinement with laboratory observations. The results
are line profile models.
Figures 6.7 and 6.8 show line profiles for water vapor, oxygen, and liquid water
computed with Fortran routines provided by Rosenkranz. (See also Rosenkranz,
1998). All computations refer to a temperature of 15°C. The top three lines in Figure
6.7 show the line profiles for water vapor for pressures of 700 mbar, 850 mbar,
and 1013 mbar, and a water vapor density of 10 g/m
3
. The maximum absorption
occurs at the resonance frequency of 22.235 GHz. The effect of pressure broadening
on the absorption curve is readily visible. Between about 20.4 GHz and 23.8 GHz
the absorption is less, the higher the pressure. The reverse is true in wings of the
line profile. In the vicinity of these two particular frequencies, the absorption is
relatively independent of pressure. Most WVRs use at least one of these frequencies to
minimize the sensitivity of brightness temperature to the vertical distribution of water
vapor. The water vapor absorption is fairly stable in regard to changes in frequency
around 31.4 GHz. Dual-frequency WVRs for ground-based sensing of water vapor
typically also use the 31.4 GHz frequency to separate the effects of water vapor from
cloud liquid. The 31.4 GHz channel is approximately twice as sensitive to cloud liquid
emissions as the channel near 20.4 GHz. The opposite is true for water vapor, allowing
separate retrievals of the two most variable atmospheric constituents. The absorption
line of oxygen in Figure 6.7 refers to a water vapor density of 10 g/m
3
and a pressure
[20
Lin
—
4.0
——
No
PgE
[20
Fi
gure 6.7
Absorption of water vapor, liquid water, and oxygen between 10 and 40 GHz.
