Precipitation at the Ground: Radar Techniques
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to extract a mean velocity and spectrum width. The second and more complex system requiring
very fast processing capability uses a Fast Fourier Transform (FFT) algorithm on a time series
of data to produce a full spectrum of the velocities in each sample volume or unit.
Ground clutter appearing in the Doppler spectrum will have zero velocity while weather echoes
have a broad velocity interval (see Fig. 12.2). The ground echo power can then be almost eliminated, even within precipitation echoes, while retaining the precipitation echo power (Passarelli
et al., 1981). This is a major advantage of spectral processing. Digital filtering in the time
domain to notch out the ground clutter will also eliminate weather echo near the zero velocity
band. Stripped of clutter echo, the significant meteorological parameters can be computed.
12.2.5 Polarization diversity radars
Experiments with polarization diversity radars have been performed for many years to determine their potential for enhanced radar observations of the weather. Some promising studies
point towards the possibility of determining better drop size distribution information and subsequently a better measure of rainfall amounts and of differentiating between hydrometeor types.
The basic premise of polarization is to detect the microdifferences amongst the hyrometeors in
a radar volume.
There are two basic radar techniques in current usage. In one case, a circularly polarized wave
is transmitted and the amount of power in the co-polar and cross-polar sense are measured
as well as the correlation between the orthogonal polarizations. In the other case, pulses are
transmitted alternately with horizontal then vertical polarization and the returned signals at
each polarization are measured either simultaneously or in sequence. Most conventional weather
radars are horizontally polarized in order to maximize the backscatter from large raindrops.
Conversion of these radars lead to the popularity of the latter type of polarization system.
Matching the beams, switching polarizations and measurement of small differences in signals
are formidable tasks requiring great care in applying the techniques.
12.2.6 Radar accuracy requirements
The accuracy requirements depend on the most important application( s) of the radar observations. Modern radars appropriately installed, calibrated and maintained are relatively stable and
do not produce significant measurement errors. External factors such as ground clutter effects,
anomalous propagation, attenuation and propagation effects, beam effects, target composition
particularly with variations and changes in the vertical, and rain rate-reflectance relationship
inadequacies contribute most to the inaccuracy. Considering only errors attributable to the radar system, the measurable radar parameters can be determined with an acceptable accuracy
(see Table 12.2).
12.3 Propagation and Scattering of Radar Signals
12.3.1 Scattering and propagation effects
Electromagnetic waves propagate in straight lines in a homogeneous medium with the speed
of light. The Earth's atmosphere is not homogeneous and microwaves undergo refraction,
absorption and scattering along their propagation path. The atmosphere is usually vertically
stratified and the rays change direction depending on the changes in height of the refractive
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to extract a mean velocity and spectrum width. The second and more complex system requiring
very fast processing capability uses a Fast Fourier Transform (FFT) algorithm on a time series
of data to produce a full spectrum of the velocities in each sample volume or unit.
Ground clutter appearing in the Doppler spectrum will have zero velocity while weather echoes
have a broad velocity interval (see Fig. 12.2). The ground echo power can then be almost eliminated, even within precipitation echoes, while retaining the precipitation echo power (Passarelli
et al., 1981). This is a major advantage of spectral processing. Digital filtering in the time
domain to notch out the ground clutter will also eliminate weather echo near the zero velocity
band. Stripped of clutter echo, the significant meteorological parameters can be computed.
12.2.5 Polarization diversity radars
Experiments with polarization diversity radars have been performed for many years to determine their potential for enhanced radar observations of the weather. Some promising studies
point towards the possibility of determining better drop size distribution information and subsequently a better measure of rainfall amounts and of differentiating between hydrometeor types.
The basic premise of polarization is to detect the microdifferences amongst the hyrometeors in
a radar volume.
There are two basic radar techniques in current usage. In one case, a circularly polarized wave
is transmitted and the amount of power in the co-polar and cross-polar sense are measured
as well as the correlation between the orthogonal polarizations. In the other case, pulses are
transmitted alternately with horizontal then vertical polarization and the returned signals at
each polarization are measured either simultaneously or in sequence. Most conventional weather
radars are horizontally polarized in order to maximize the backscatter from large raindrops.
Conversion of these radars lead to the popularity of the latter type of polarization system.
Matching the beams, switching polarizations and measurement of small differences in signals
are formidable tasks requiring great care in applying the techniques.
12.2.6 Radar accuracy requirements
The accuracy requirements depend on the most important application( s) of the radar observations. Modern radars appropriately installed, calibrated and maintained are relatively stable and
do not produce significant measurement errors. External factors such as ground clutter effects,
anomalous propagation, attenuation and propagation effects, beam effects, target composition
particularly with variations and changes in the vertical, and rain rate-reflectance relationship
inadequacies contribute most to the inaccuracy. Considering only errors attributable to the radar system, the measurable radar parameters can be determined with an acceptable accuracy
(see Table 12.2).
12.3 Propagation and Scattering of Radar Signals
12.3.1 Scattering and propagation effects
Electromagnetic waves propagate in straight lines in a homogeneous medium with the speed
of light. The Earth's atmosphere is not homogeneous and microwaves undergo refraction,
absorption and scattering along their propagation path. The atmosphere is usually vertically
stratified and the rays change direction depending on the changes in height of the refractive