124
Y. Fouquart and M. Vesperini
and accounting for absorbers, the total transmittance (for downwarding and up warding paths)
over bandwidth t!.v,
Tt!.V, (ZT' ZT, ()o, ()) = iv, exp {-~ f~T k~bS(ZI)p(ZI)dz'} exp {-t f~T k~bs(ZI)p(ZI)dz'}dv
(5.108)
Note that Tt!.V, (ZT' ZT, ()o, ()) =I Tt!.V, (ZT' 0, ()o) Tt!.V, (0, ZT, ()).
The retrieval of water vapour content in the shortwave can be understood through the concept
of equivalent absorber amount explained in section 5.5.4. The equivalent absorber amount
(here water vapour) accounts for pressure and temperature dependence of the absorption line
In the atmosphere, temperature, pressure and absorbers have a near exponential profile, with
their associated scale heights Hp, HT , and Hp. As mentioned by Frouin et al. (1990), the
equivalent absorber amount can be expressed statistically as function of the absorber amount
itself
Using a band model to parameterize the spectral integration of the transmittance over band
width t!.v, for example the one of Goody (1952)
Su
Tt!.v, = exp {- J
}
d 1 + SU/1fU
which in case of strong absorption Su » 1fU, the average transmittance reduces to
_ ,;s;;:;;;
- f3 J.l + lv'll
T6V 1 ::::::::: e
d
= e
/-to
Iithat can be simplified defining j3 as a spectral constant which can be calculated for the corresponding channel.
The ratio between the two channels measurements is
from which u can be inferred directly. An example showing the implementation of this method
is given by Frouin et al. (1990).
5.7 Active Remote Sensing
Active remote sensing methods are also based on radiation measurements but in that case, the
radiation is generated by the instrument itself, which means that its intensity is known and
its wavelength is chosen. Active remote sensing involves the same radiative transfer theory as
passive radiometry. Retrieval methods are based on extinction of radiation along the optical
path or backscattering by atmospheric constituents. Radar (RAdiation Detection And Ranging)
and LIDAR (LIght Detection And Ranging) are systems which contain transmitter and receiver.
Y. Fouquart and M. Vesperini
and accounting for absorbers, the total transmittance (for downwarding and up warding paths)
over bandwidth t!.v,
Tt!.V, (ZT' ZT, ()o, ()) = iv, exp {-~ f~T k~bS(ZI)p(ZI)dz'} exp {-t f~T k~bs(ZI)p(ZI)dz'}dv
(5.108)
Note that Tt!.V, (ZT' ZT, ()o, ()) =I Tt!.V, (ZT' 0, ()o) Tt!.V, (0, ZT, ()).
The retrieval of water vapour content in the shortwave can be understood through the concept
of equivalent absorber amount explained in section 5.5.4. The equivalent absorber amount
(here water vapour) accounts for pressure and temperature dependence of the absorption line
In the atmosphere, temperature, pressure and absorbers have a near exponential profile, with
their associated scale heights Hp, HT , and Hp. As mentioned by Frouin et al. (1990), the
equivalent absorber amount can be expressed statistically as function of the absorber amount
itself
Using a band model to parameterize the spectral integration of the transmittance over band
width t!.v, for example the one of Goody (1952)
Su
Tt!.v, = exp {- J
}
d 1 + SU/1fU
which in case of strong absorption Su » 1fU, the average transmittance reduces to
_ ,;s;;:;;;
- f3 J.l + lv'll
T6V 1 ::::::::: e
d
= e
/-to
Iithat can be simplified defining j3 as a spectral constant which can be calculated for the corresponding channel.
The ratio between the two channels measurements is
from which u can be inferred directly. An example showing the implementation of this method
is given by Frouin et al. (1990).
5.7 Active Remote Sensing
Active remote sensing methods are also based on radiation measurements but in that case, the
radiation is generated by the instrument itself, which means that its intensity is known and
its wavelength is chosen. Active remote sensing involves the same radiative transfer theory as
passive radiometry. Retrieval methods are based on extinction of radiation along the optical
path or backscattering by atmospheric constituents. Radar (RAdiation Detection And Ranging)
and LIDAR (LIght Detection And Ranging) are systems which contain transmitter and receiver.