Precipitation at the Ground: Radar Techniques
283
Volumetric observations of the atmosphere are normally made by scanning the antenna pseudohorizontally and elevating by steps in the vertical at each revolution. An important consideration is the resolution of the targets. Parabolic reflector antennas are used to focus the waves
into a conical shaped beam. Larger reflectors create narrower beams, greater resolution and
sensitivity at increasing costs. The beamwidth (lib), the points where the power is one-half that
of the axis, is dependent also on the wavelength and can be approximated by:
IIb=~
Dantenna
(12.5)
The units of lib are degrees. Dantenna is the diameter of the parabolic antenna dish. Good
weather-radars have beamwidths of 0.5 to 1 degree.
The useful range of weather radars, except for long range detection of thunderstorms, is of
the order of 200 km since at that range, the 1 0 beam at an elevation angle of 0.5
0
is already
at a height of 4 km above the curved earth surface with a beamwidth of the order of 1.5 km
or greater. For good quantitative precipitation measurements, the maximum effective range
is less than 130 km, since the beam is too high for good ground estimates, beam spreading
reduces resolution and the measurement can be affected by underfilling with target. Technically,
there is a maximum unambiguous range determined by the pulse repetition frequency since the
range must be measured during the listening period between pulses. At usual pulse repetition
frequencies (PRF), this is not a problem. For example, for a PRF equal to 250 pulses per
second, the maximum range is 600 km. At higher PRFs required for good Doppler systems,
the range will be greatly reduced, although new developments in progress will ameliorate this
situation (Joe et al., 1994).
12.2.4 Doppler radar
The development and introduction of Doppler weather radars to weather surveillance provides
a new dimension to the observations. Doppler radar provides a measure ofthe target(s) velocity
along a radial from the radar in a direction either towards or away from the radar. A further
advantage of the Doppler technique is the greater effective sensitivity to low reflectance targets
close to the radar noise level when velocity fields can still be distinguished in a noisy Z field.
At the normal speeds of meteorological targets, the frequency shift is relatively small compared
to the radar frequency and very difficult to measure. An easier task is to retain the phase of
the transmitted pulse, compare it with the phase of the received pulse and then determine the
change in phase between successive pulses. The time rate of change of the phase is directly
related to the frequency shift which in turn is directly related to the target velocity - the Doppler
effect. If the phase changes by greater than ± 180 0 the velocity is not determined unambiguously
but will be biased by some multiple of the Nyquist interval (the highest unambiguous velocity
that can be measured by a Doppler radar). An additional processing step is required to retrieve
the correct velocity.
The maximum unambiguous Doppler velocity depends on the radar wavelength (>.) and the
pulse repetition frequency (PRF) and can be expressed as:
PRF>.
Vmax = ±--4(12.6)
The maximum unambiguous range can be expressed as:
c
(12.7)
7'max = PRF x 2
283
Volumetric observations of the atmosphere are normally made by scanning the antenna pseudohorizontally and elevating by steps in the vertical at each revolution. An important consideration is the resolution of the targets. Parabolic reflector antennas are used to focus the waves
into a conical shaped beam. Larger reflectors create narrower beams, greater resolution and
sensitivity at increasing costs. The beamwidth (lib), the points where the power is one-half that
of the axis, is dependent also on the wavelength and can be approximated by:
IIb=~
Dantenna
(12.5)
The units of lib are degrees. Dantenna is the diameter of the parabolic antenna dish. Good
weather-radars have beamwidths of 0.5 to 1 degree.
The useful range of weather radars, except for long range detection of thunderstorms, is of
the order of 200 km since at that range, the 1 0 beam at an elevation angle of 0.5
0
is already
at a height of 4 km above the curved earth surface with a beamwidth of the order of 1.5 km
or greater. For good quantitative precipitation measurements, the maximum effective range
is less than 130 km, since the beam is too high for good ground estimates, beam spreading
reduces resolution and the measurement can be affected by underfilling with target. Technically,
there is a maximum unambiguous range determined by the pulse repetition frequency since the
range must be measured during the listening period between pulses. At usual pulse repetition
frequencies (PRF), this is not a problem. For example, for a PRF equal to 250 pulses per
second, the maximum range is 600 km. At higher PRFs required for good Doppler systems,
the range will be greatly reduced, although new developments in progress will ameliorate this
situation (Joe et al., 1994).
12.2.4 Doppler radar
The development and introduction of Doppler weather radars to weather surveillance provides
a new dimension to the observations. Doppler radar provides a measure ofthe target(s) velocity
along a radial from the radar in a direction either towards or away from the radar. A further
advantage of the Doppler technique is the greater effective sensitivity to low reflectance targets
close to the radar noise level when velocity fields can still be distinguished in a noisy Z field.
At the normal speeds of meteorological targets, the frequency shift is relatively small compared
to the radar frequency and very difficult to measure. An easier task is to retain the phase of
the transmitted pulse, compare it with the phase of the received pulse and then determine the
change in phase between successive pulses. The time rate of change of the phase is directly
related to the frequency shift which in turn is directly related to the target velocity - the Doppler
effect. If the phase changes by greater than ± 180 0 the velocity is not determined unambiguously
but will be biased by some multiple of the Nyquist interval (the highest unambiguous velocity
that can be measured by a Doppler radar). An additional processing step is required to retrieve
the correct velocity.
The maximum unambiguous Doppler velocity depends on the radar wavelength (>.) and the
pulse repetition frequency (PRF) and can be expressed as:
PRF>.
Vmax = ±--4(12.6)
The maximum unambiguous range can be expressed as:
c
(12.7)
7'max = PRF x 2