290
P. Joe
• Variations in Z - R Relationship: The variations in types and size distribution of
hydrometeors cannot be estimated in operational measurements and are sometimes an
important source of error. The basic assumption is that the target is rain but this is not
always the case.
• Attenuation By Intervening Precipitation: Attenuation by rain may be important,
especially at the shorter radar wavelengths (5 cm and 3 cm). Attenuation by snow,
although less than for rain, may be important over long path lengths. Precipitation
intensities are underestimated due to this effect.
• Beam Blocking: Depending on the radar installation, the radar beam may be partly or
completely occulted by topography or obstacles located between the radar and the target.
This results in underestimations of reflectance, and hence of rainfall rate.
• Attenuation Due To A Wet Radome: Most radars antennas are protected from wind
and rain by a radome, usually made of fiberglass. Under intense rainfall, the surface of
the radome may become coated with a thin film of water or ice, resulting in a strong
azimuth dependent attenuation (Donaldson, 1990).
• Electromagnetic Interference: Electromagnetic interference from other radars or devices such as microwave links or the sun may be an important factor of error in some
cases. This type of problem is easily recognized by observation.
• Ground and Other Clutter: Contamination of rain echoes by ground clutter may
bring very large errors in precipitation estimation. The ground clutter should first be
minimized by good antenna engineering and a good choice of the radar location. This
effect may be greatly reduced by a combination of hardware ground clutter suppression
devices (Aoyagi, 1983), signal and data processing.
Chaff, birds, refractive inhomogenieties and insects may also produce echoes detectable
by the radar.
• Anomalous Propagation: Anomalous propagation distorts the radar beam path, and
has the effect of increasing ground clutter by refracting the beam towards the ground.
Anomalous propagation is frequent in some regions, when the atmosphere is subject to
strong decreases in humidity and/or increases in temperature with height. Clutter returns
due to anomalous propagation may be very misleading to untrained human observers.
• Antenna Accuracy: The antenna position may be known within .1 degrees with a well
engineered system. Errors may also be produced by the excessive width of the radar
beam or by the presence of sidelobes, in the presence of clutter or of strong precipitation
echoes.
• Electronics Stability: Modern electronic systems are subject to small variations with
time. This may be controlled by using a well engineered monitoring system, which will
keep the variations of the electronics within less than 1 dB, or activate an alarm when a
fault is detected.
• Processing Accuracy: The signal processing must be designed to take the best advantage of the sampling capacities of the rest of the system. It must keep to a minimum, the
variances of the estimation of reflectance, Doppler velocity and spectrum width. Range
and velocity aliasing may be important sources of error.
P. Joe
• Variations in Z - R Relationship: The variations in types and size distribution of
hydrometeors cannot be estimated in operational measurements and are sometimes an
important source of error. The basic assumption is that the target is rain but this is not
always the case.
• Attenuation By Intervening Precipitation: Attenuation by rain may be important,
especially at the shorter radar wavelengths (5 cm and 3 cm). Attenuation by snow,
although less than for rain, may be important over long path lengths. Precipitation
intensities are underestimated due to this effect.
• Beam Blocking: Depending on the radar installation, the radar beam may be partly or
completely occulted by topography or obstacles located between the radar and the target.
This results in underestimations of reflectance, and hence of rainfall rate.
• Attenuation Due To A Wet Radome: Most radars antennas are protected from wind
and rain by a radome, usually made of fiberglass. Under intense rainfall, the surface of
the radome may become coated with a thin film of water or ice, resulting in a strong
azimuth dependent attenuation (Donaldson, 1990).
• Electromagnetic Interference: Electromagnetic interference from other radars or devices such as microwave links or the sun may be an important factor of error in some
cases. This type of problem is easily recognized by observation.
• Ground and Other Clutter: Contamination of rain echoes by ground clutter may
bring very large errors in precipitation estimation. The ground clutter should first be
minimized by good antenna engineering and a good choice of the radar location. This
effect may be greatly reduced by a combination of hardware ground clutter suppression
devices (Aoyagi, 1983), signal and data processing.
Chaff, birds, refractive inhomogenieties and insects may also produce echoes detectable
by the radar.
• Anomalous Propagation: Anomalous propagation distorts the radar beam path, and
has the effect of increasing ground clutter by refracting the beam towards the ground.
Anomalous propagation is frequent in some regions, when the atmosphere is subject to
strong decreases in humidity and/or increases in temperature with height. Clutter returns
due to anomalous propagation may be very misleading to untrained human observers.
• Antenna Accuracy: The antenna position may be known within .1 degrees with a well
engineered system. Errors may also be produced by the excessive width of the radar
beam or by the presence of sidelobes, in the presence of clutter or of strong precipitation
echoes.
• Electronics Stability: Modern electronic systems are subject to small variations with
time. This may be controlled by using a well engineered monitoring system, which will
keep the variations of the electronics within less than 1 dB, or activate an alarm when a
fault is detected.
• Processing Accuracy: The signal processing must be designed to take the best advantage of the sampling capacities of the rest of the system. It must keep to a minimum, the
variances of the estimation of reflectance, Doppler velocity and spectrum width. Range
and velocity aliasing may be important sources of error.