6 Precipitation
115
ture (through emission) over the cold background. Hence, a 2 km thick precipitating
cumulus cloud will be detected over the cold surface background at 18 Ghz, and the
observed TBB will increase in proportion to the amount of 'vertically integrated'
raindrops contained in the cloud.
Scattering by raindrops increases with drop size. The effect of scattering is to
reduce the apparent TBB and is also frequency-dependent. The presence of ice in a
precipitating cloud results in a decrease of TBB through scattering (ice particles
scatter but do not absorb microwave radiation). This effect increases with frequency
and particle size, and is used to indirectly delineate rain over the earth's surface.
Measurements of rainfall using passive microwaves falls into one or two classes
depending on the particular effect used to detect precipitation (i.e. absorption or
scattering). To exploit absorption a cold background is necessary. Thus techniques
based on this effect can be applied only over the ocean using frequencies below 10
Ghz.
6.2.4 Space-borne radar
Measuring rain from a space-borne radar is very attractive since it can discriminate
in range (altitude when operated from space), and radar reflectivity is directly related
to rainfall rate. A number of approaches are currently being investigated. A particularly promising approach, known as the surface reference technique (Meneghini et ai.
1983), exploits the fact that the scattering properties of the ocean surface vary on a
much longer horizontal scale than does the rain rate field. Hence, the level of the
signal received from the ocean surface through a rain cell relative to that outside the
rain cell provides a means of monitoring the two way attenuation along a ray path
which may then be related to the rainfall rate using known relationships.
A rain radar, operating at 13.786 and 13.802 Ghz is part of the i.."1Strument package
on the joint US-Japan Tropical Rainfall Measurement Mission (TRMM) satellite
which was launched on 28 November 1997 in a co-latitude orbit (see Theon et aI.,
1992). Since radiometers having visible (0.63, 1.611l11), infrared (3.75, 10.8, 1211l11)
and passive microwave (10.7, 19.4,21.3,37, 85.56 Ghz) channels also fly on this
satellite, it is providing an ideal opportunity to combine a number of the techniques
discussed in the next section. An example of the radar data being recorded is shown
in Colour Plate 6.B.
6.3 Current Techniques
6.3.1 Single polarisation radar measurements of rainfall
In trying to relate radar measurements of reflectivity to rainfall rate, a number of
problems arise from both the characteristics of the radar equipment and the radar site,
and from the characteristics of the precipitation observed by the radar beam. These
problems have been discussed extensively in the literature (see for example, Collier,
1996b).
115
ture (through emission) over the cold background. Hence, a 2 km thick precipitating
cumulus cloud will be detected over the cold surface background at 18 Ghz, and the
observed TBB will increase in proportion to the amount of 'vertically integrated'
raindrops contained in the cloud.
Scattering by raindrops increases with drop size. The effect of scattering is to
reduce the apparent TBB and is also frequency-dependent. The presence of ice in a
precipitating cloud results in a decrease of TBB through scattering (ice particles
scatter but do not absorb microwave radiation). This effect increases with frequency
and particle size, and is used to indirectly delineate rain over the earth's surface.
Measurements of rainfall using passive microwaves falls into one or two classes
depending on the particular effect used to detect precipitation (i.e. absorption or
scattering). To exploit absorption a cold background is necessary. Thus techniques
based on this effect can be applied only over the ocean using frequencies below 10
Ghz.
6.2.4 Space-borne radar
Measuring rain from a space-borne radar is very attractive since it can discriminate
in range (altitude when operated from space), and radar reflectivity is directly related
to rainfall rate. A number of approaches are currently being investigated. A particularly promising approach, known as the surface reference technique (Meneghini et ai.
1983), exploits the fact that the scattering properties of the ocean surface vary on a
much longer horizontal scale than does the rain rate field. Hence, the level of the
signal received from the ocean surface through a rain cell relative to that outside the
rain cell provides a means of monitoring the two way attenuation along a ray path
which may then be related to the rainfall rate using known relationships.
A rain radar, operating at 13.786 and 13.802 Ghz is part of the i.."1Strument package
on the joint US-Japan Tropical Rainfall Measurement Mission (TRMM) satellite
which was launched on 28 November 1997 in a co-latitude orbit (see Theon et aI.,
1992). Since radiometers having visible (0.63, 1.611l11), infrared (3.75, 10.8, 1211l11)
and passive microwave (10.7, 19.4,21.3,37, 85.56 Ghz) channels also fly on this
satellite, it is providing an ideal opportunity to combine a number of the techniques
discussed in the next section. An example of the radar data being recorded is shown
in Colour Plate 6.B.
6.3 Current Techniques
6.3.1 Single polarisation radar measurements of rainfall
In trying to relate radar measurements of reflectivity to rainfall rate, a number of
problems arise from both the characteristics of the radar equipment and the radar site,
and from the characteristics of the precipitation observed by the radar beam. These
problems have been discussed extensively in the literature (see for example, Collier,
1996b).