Whiton et al. [20] proposed a calibration technique using solar interference, and it
has been widely applied on operational weather radars for monitoring the sensitivity
of the radar receiver and antenna pointing accuracy [21–23]. Wolff et al. [24] also
demonstrated the calibration technique using statistical analysis of the echo power
returns from fixed targets (e.g. high ground). For a radar network, the radar
calibration bias can be monitored by joint observations from two or more radars
[25]. This technique ensures the stability of radar calibration by comparing the radar
reflectivity values of two or more radars in the same area. By using dual-polarization
radar, Gorgucci et al. [26] developed a procedure for radar calibration, based on the
self-consistency between the radar reflectivity at horizontal polarization (Z H ), differential reflectivity (Z dr ) and specific differential phase shift (K dp ). However, the
self-consistency technique is also sensitive to the variations of the drop size
distribution and raindrop shape [27]. Furthermore, the stability of radar calibration
can also be monitored by comparing with raingauge accumulations [28]. However,
these comparisons are more suitable for long-term adjustment, due to the differences
in spatial-temporal samplings of the two sensors and the high variability of the Z – R
relationship [29]. Recently, a new methodology [30] has been developed to match
the precipitation observations from ground-based and space-borne radars for the
determination of calibration biases in ground-based radar systems. It has been
shown that the radar calibration bias can be less than 1.5 dB in well-calibrated
ground-based radars. This technique can be a useful tool for the systematic monitoring of the radar calibration bias.
2.2 Echoes Due to Non-meteorological Origin
The radar usually scans at low elevation angles to obtain measurements close to the
ground surface. Echoes from mountains or buildings can be misinterpreted as heavy
precipitation, which are known as ground clutter. Such echoes are often permanent
under standard beam propagation conditions, and thus techniques using a map of
ground clutter locations are often successful in removing them [31]. However,
echoes from targets under atmospheric super-refraction conditions are unpredictable
in terms of location. This is known as anomalous propagation (AP), where the radar
beam is bent toward the Earth’s surface due to changes in the atmospheric temperature and humidity distributions [7]. AP is an important source of error in radar
rainfall measurements. For instance, the presence of AP echoes may produce
reflectivities reaching 60 dBZ, which is comparable with echoes observed during
severe thunderstorms [32].
Several methods to identify and suppress clutter echoes have been developed. For
Doppler radar systems, filtering of the radial velocity signal can discriminate clutter
and AP echoes from meteorological echoes. The assumption is that ground clutter
echoes can be characterized as having zero-velocity and narrow spectral widths
compared to weather echoes [33]. However, precipitation echoes may also have
near-zero radial velocity and low spectral widths, which is commonly observed in
Precipitation Measurement with Weather Radars
239
has been widely applied on operational weather radars for monitoring the sensitivity
of the radar receiver and antenna pointing accuracy [21–23]. Wolff et al. [24] also
demonstrated the calibration technique using statistical analysis of the echo power
returns from fixed targets (e.g. high ground). For a radar network, the radar
calibration bias can be monitored by joint observations from two or more radars
[25]. This technique ensures the stability of radar calibration by comparing the radar
reflectivity values of two or more radars in the same area. By using dual-polarization
radar, Gorgucci et al. [26] developed a procedure for radar calibration, based on the
self-consistency between the radar reflectivity at horizontal polarization (Z H ), differential reflectivity (Z dr ) and specific differential phase shift (K dp ). However, the
self-consistency technique is also sensitive to the variations of the drop size
distribution and raindrop shape [27]. Furthermore, the stability of radar calibration
can also be monitored by comparing with raingauge accumulations [28]. However,
these comparisons are more suitable for long-term adjustment, due to the differences
in spatial-temporal samplings of the two sensors and the high variability of the Z – R
relationship [29]. Recently, a new methodology [30] has been developed to match
the precipitation observations from ground-based and space-borne radars for the
determination of calibration biases in ground-based radar systems. It has been
shown that the radar calibration bias can be less than 1.5 dB in well-calibrated
ground-based radars. This technique can be a useful tool for the systematic monitoring of the radar calibration bias.
2.2 Echoes Due to Non-meteorological Origin
The radar usually scans at low elevation angles to obtain measurements close to the
ground surface. Echoes from mountains or buildings can be misinterpreted as heavy
precipitation, which are known as ground clutter. Such echoes are often permanent
under standard beam propagation conditions, and thus techniques using a map of
ground clutter locations are often successful in removing them [31]. However,
echoes from targets under atmospheric super-refraction conditions are unpredictable
in terms of location. This is known as anomalous propagation (AP), where the radar
beam is bent toward the Earth’s surface due to changes in the atmospheric temperature and humidity distributions [7]. AP is an important source of error in radar
rainfall measurements. For instance, the presence of AP echoes may produce
reflectivities reaching 60 dBZ, which is comparable with echoes observed during
severe thunderstorms [32].
Several methods to identify and suppress clutter echoes have been developed. For
Doppler radar systems, filtering of the radial velocity signal can discriminate clutter
and AP echoes from meteorological echoes. The assumption is that ground clutter
echoes can be characterized as having zero-velocity and narrow spectral widths
compared to weather echoes [33]. However, precipitation echoes may also have
near-zero radial velocity and low spectral widths, which is commonly observed in
Precipitation Measurement with Weather Radars
239