Priciples of Active and Passive Remote Measurements ...
117
the 9.6 11m 0 3 bands and the weak CO2 bands, there is an important water vapour continuum,
the origin of which is not well established. Although water vapour presents such a continuum
absorption over the whole IR spectral interval, its major impacts is the IR window since it is
there the only active absorption. Several parameterizations of the water vapour continuum absorption have been made (Bignell, 1970; Roberts et al., 1976; Clough et al., 1989) but questions
remain about their validity over the whole spectrum.
5.6 Application to Remote Sensing of Water Vapour
In the previous pages, the general form of radiative transfer equation resulting from contributions between the boundary (10) and a given position x has been derived
(5.95)
with the source function accounting for emission and scattering at position I
Jv(l,S) = ~+,~[JLp(e)Lv(l,s/)dwll,
emission
scattering
Emission is well described by the Planck function. Shortwave are only emitted at very high
temperatures thus by the sun whereas the earth and atmosphere emit radiation in the longwave
and microwave.
Scattering is caused by atmospheric constituents such as molecules and aerosols and cloud
particles. Its magnitude depends mostly on the ratio between wavelength and the size of
scattering particles. Problems in computing the scattering term of radiative transfer are twofold:
• it involves very expensive computation to account for multiple scattering, and interactions
between scattering and absorption .
• the actual distribution of cloud and aerosols particles IS unknown, due to their high
variability in space and time.
It is thus difficult to calculate accurately how much radiation is scattered so that scattering is
often a limitation to the retrieval of atmospheric parameters. In the visible and infra-red, where
wavelength is small with respect to the size of scattering particles, the retrieval of atmospheric
parameters is only possible in clear sky conditions. In the microwave, scattering is significant
for very large particles only, those found in convective clouds and precipitation. Therefore,
microwave instruments are considered to a certain extent as cloud penetrating sensors. In the
following, we consider only cases in which scattering can be neglected, and the radiative transfer
is computed for non scattering atmospheres.
5.6.1 Passive radiometry in the longwave and mIcrowave
Molecular scattering is negligible at these wavelengths. Aerosols and cloud particles affect
strongly infrared radiation and microwave in the case of large drops. These cases are not
considered hereafter.
117
the 9.6 11m 0 3 bands and the weak CO2 bands, there is an important water vapour continuum,
the origin of which is not well established. Although water vapour presents such a continuum
absorption over the whole IR spectral interval, its major impacts is the IR window since it is
there the only active absorption. Several parameterizations of the water vapour continuum absorption have been made (Bignell, 1970; Roberts et al., 1976; Clough et al., 1989) but questions
remain about their validity over the whole spectrum.
5.6 Application to Remote Sensing of Water Vapour
In the previous pages, the general form of radiative transfer equation resulting from contributions between the boundary (10) and a given position x has been derived
(5.95)
with the source function accounting for emission and scattering at position I
Jv(l,S) = ~+,~[JLp(e)Lv(l,s/)dwll,
emission
scattering
Emission is well described by the Planck function. Shortwave are only emitted at very high
temperatures thus by the sun whereas the earth and atmosphere emit radiation in the longwave
and microwave.
Scattering is caused by atmospheric constituents such as molecules and aerosols and cloud
particles. Its magnitude depends mostly on the ratio between wavelength and the size of
scattering particles. Problems in computing the scattering term of radiative transfer are twofold:
• it involves very expensive computation to account for multiple scattering, and interactions
between scattering and absorption .
• the actual distribution of cloud and aerosols particles IS unknown, due to their high
variability in space and time.
It is thus difficult to calculate accurately how much radiation is scattered so that scattering is
often a limitation to the retrieval of atmospheric parameters. In the visible and infra-red, where
wavelength is small with respect to the size of scattering particles, the retrieval of atmospheric
parameters is only possible in clear sky conditions. In the microwave, scattering is significant
for very large particles only, those found in convective clouds and precipitation. Therefore,
microwave instruments are considered to a certain extent as cloud penetrating sensors. In the
following, we consider only cases in which scattering can be neglected, and the radiative transfer
is computed for non scattering atmospheres.
5.6.1 Passive radiometry in the longwave and mIcrowave
Molecular scattering is negligible at these wavelengths. Aerosols and cloud particles affect
strongly infrared radiation and microwave in the case of large drops. These cases are not
considered hereafter.