Retrieval of Precipitation from Satellites
259
parameters RR is inferred by tatistical relations between the convective activity of the cloud
(described by the above parameters) and RR derived from in-situ data or ground-based radar.
11.4.2 Passive high-frequency microwaves
At frequencies above 60 GHz the interaction intensity between radiation and precipitation has
its maximum (Fig. 11.3). Raining clouds are almost opaque and in addition to absorption
scattering by both liquid and frozen precipitation-size particles is important. Accordingly
raining clouds appear cold compared to the environment. The radiance depression is dependent
on the amount of scatterers in the upper part of the cloud. Since ice particles playa key role
in precipitation generation we can estimate RR from the observed radiance depression. The
microwave reflection and emission of water surfaces is highly polarized in contrast to emission
and scattering by atmospheric constituents. So, in addition to the radiance depression the
polarization reduction serves as another piece of information over ocean areas. The main
advantage of the high-frequency method is its relative independence of the signal of the surface
below the cloud. Thus the signal can be observed over both land and ocean surfaces.
11.4.3 Passive mid- to low-frequency microwaves
Below 40 GHz raining clouds become increasingly transparent for microwave radiation, below
10 GHz emission dominates and the emitted radiation is almost proportional to the rain water
content in the atmospheric column. If the height of the water column and its profile structure
is known RR can be retrieved. As above, additional information is provided by the polarization
reduction. Over land the polarization of the background radiation is low and the signal of the
rain cloud can barely be distinguished from its surroundings. This is the main disadvantage
of the low-frequency approach. At higher frequencies absorption/emission and scattering by
precipitation particles increase. This leads to a strongly non-linear signal dependence on RR and
to saturation at decreasing RR with increasing frequency. This non-linear relation is a serious
problem, because rain cells are typically smaller than the resolution of satellite radiometers
(e.g. 30 to 50 km for SSM/I). Compared to the methods described above the low-frequency
approach is based on a more direct relation between signal and RR. This method is, however,
restricted to oceanic areas.
11.4.4 Rain radar
Rain radar measure the backscattering of microwave radiation at even lower frequeucies. Atmosphere and cloud drops interfere only weakly with the signal, and under certain assumptions
about the drop-size distribution RR can be inferred from the backscatter intensity. This method is based on the most direct relation between signal and RR of all the methods discussed.
The ranging capability will also allow to determine rain water profiles. There have been no
rain radars in satellite orbits up to now. On the TRMM-satellite a rain radar is planned to be
used together with passive instruments in the microwave, infrared, and visible spectrum. Thus
TRMM will allow for the testing and/or calibration of the passive techniques. Rain radars are
the topic in other parts of the book and will not be discussed further here.
259
parameters RR is inferred by tatistical relations between the convective activity of the cloud
(described by the above parameters) and RR derived from in-situ data or ground-based radar.
11.4.2 Passive high-frequency microwaves
At frequencies above 60 GHz the interaction intensity between radiation and precipitation has
its maximum (Fig. 11.3). Raining clouds are almost opaque and in addition to absorption
scattering by both liquid and frozen precipitation-size particles is important. Accordingly
raining clouds appear cold compared to the environment. The radiance depression is dependent
on the amount of scatterers in the upper part of the cloud. Since ice particles playa key role
in precipitation generation we can estimate RR from the observed radiance depression. The
microwave reflection and emission of water surfaces is highly polarized in contrast to emission
and scattering by atmospheric constituents. So, in addition to the radiance depression the
polarization reduction serves as another piece of information over ocean areas. The main
advantage of the high-frequency method is its relative independence of the signal of the surface
below the cloud. Thus the signal can be observed over both land and ocean surfaces.
11.4.3 Passive mid- to low-frequency microwaves
Below 40 GHz raining clouds become increasingly transparent for microwave radiation, below
10 GHz emission dominates and the emitted radiation is almost proportional to the rain water
content in the atmospheric column. If the height of the water column and its profile structure
is known RR can be retrieved. As above, additional information is provided by the polarization
reduction. Over land the polarization of the background radiation is low and the signal of the
rain cloud can barely be distinguished from its surroundings. This is the main disadvantage
of the low-frequency approach. At higher frequencies absorption/emission and scattering by
precipitation particles increase. This leads to a strongly non-linear signal dependence on RR and
to saturation at decreasing RR with increasing frequency. This non-linear relation is a serious
problem, because rain cells are typically smaller than the resolution of satellite radiometers
(e.g. 30 to 50 km for SSM/I). Compared to the methods described above the low-frequency
approach is based on a more direct relation between signal and RR. This method is, however,
restricted to oceanic areas.
11.4.4 Rain radar
Rain radar measure the backscattering of microwave radiation at even lower frequeucies. Atmosphere and cloud drops interfere only weakly with the signal, and under certain assumptions
about the drop-size distribution RR can be inferred from the backscatter intensity. This method is based on the most direct relation between signal and RR of all the methods discussed.
The ranging capability will also allow to determine rain water profiles. There have been no
rain radars in satellite orbits up to now. On the TRMM-satellite a rain radar is planned to be
used together with passive instruments in the microwave, infrared, and visible spectrum. Thus
TRMM will allow for the testing and/or calibration of the passive techniques. Rain radars are
the topic in other parts of the book and will not be discussed further here.