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P. Joe
spectrum from a vertically pointing radar can be transformed into drop size distributions if the
air motion is known. The technology for hydrometeor identification has only recently advanced
to a promising stage. Polarization radar for the discrimination of hail, ice and water appear to
be possible. Dual wavelength studies making use of either differential attenuation or diffential
backscatter have had a resurgence for precipitation estimation.
12.1 The Weather Radar
Meteorological radars are capable of detecting precipitation and variations of the refractive
index in the atmosphere. The latter may be generated by local variations of temperature
or humidity. Radar echoes may also be produced from airplanes, dust, birds or insects. The
meteorological radars having characteristics best suited for atmospheric observation and investigation transmit electromagnetic pulses in the 3-10 GHz frequency range (10-3 cm wavelength,
respectively), and are designed for detecting, mapping and measuring precipitation intensity,
their motion and perhaps their type. Higher frequencies (35 and 95 GHz), are used to detect
smaller hydrometeors, such as cloud or even fog droplets. Although this has valuable applications in cloud physics research, these frequencies generally are not used in operational weather
forecasting because of the excessive attenuation of the radar signal by the intervening medium.
At lower frequencies, radars known as strato-tropospheric or wind profiling radars are capable
of detecting variations of the refractive index of clear air. The radars detect precipitation as
well. They are usually fixed beam systems.
The returned signal from the transmitted pulse encountering a weather target, called an echo,
has an amplitude, a phase and a polarization. Most operational radars worldwide are still
limited to analysis of the amplitude which is related to the size distribution and numbers
of particles in the (pulse) volume illuminated by the radar beam. The amplitude is used to
determine a parameter called the reflectance factor (Z) which is used to estimate the mass
of precipitation per unit volume (M) or the intensity of precipitation (R) through the use of
empirical relations. A primary application is thus to detect, map and estimate the precipitation
at ground level instantaneously, nearly continuously, and over large areas.
Some research radars have used reflectance factors measured at two orthogonal polarizations
of the transmitted and received waveform. Research continues to determine the value and potential of polarization systems for precipitation measurement and target state, but operational
systems do not exist at present. Doppler radars have the capability of determining the phase
difference between the transmitted and received pulse. The rate of change of the phase difference is a measure of the mean Doppler velocity of the particles. The mean Doppler velocity
is a reflectance weighted average of the radial components of the displacement velocities of the
hydrometeors in the pulse volume. The Doppler spectrum width is a measure of the spatial
variability of the velocities and may provide some indication of the wind shear and turbulence.
Weather radar can serve many purposes. The focus of this paper will be on the ability of the
radar to estimate precipitation. Its use for other applications will be briefly described. We
proceed by discussing radar technology, basic radar theory, applications and then the various
techniques for the measurement of precipitation. We briefly discuss the use of high frequency
radars for the detection of clouds.
P. Joe
spectrum from a vertically pointing radar can be transformed into drop size distributions if the
air motion is known. The technology for hydrometeor identification has only recently advanced
to a promising stage. Polarization radar for the discrimination of hail, ice and water appear to
be possible. Dual wavelength studies making use of either differential attenuation or diffential
backscatter have had a resurgence for precipitation estimation.
12.1 The Weather Radar
Meteorological radars are capable of detecting precipitation and variations of the refractive
index in the atmosphere. The latter may be generated by local variations of temperature
or humidity. Radar echoes may also be produced from airplanes, dust, birds or insects. The
meteorological radars having characteristics best suited for atmospheric observation and investigation transmit electromagnetic pulses in the 3-10 GHz frequency range (10-3 cm wavelength,
respectively), and are designed for detecting, mapping and measuring precipitation intensity,
their motion and perhaps their type. Higher frequencies (35 and 95 GHz), are used to detect
smaller hydrometeors, such as cloud or even fog droplets. Although this has valuable applications in cloud physics research, these frequencies generally are not used in operational weather
forecasting because of the excessive attenuation of the radar signal by the intervening medium.
At lower frequencies, radars known as strato-tropospheric or wind profiling radars are capable
of detecting variations of the refractive index of clear air. The radars detect precipitation as
well. They are usually fixed beam systems.
The returned signal from the transmitted pulse encountering a weather target, called an echo,
has an amplitude, a phase and a polarization. Most operational radars worldwide are still
limited to analysis of the amplitude which is related to the size distribution and numbers
of particles in the (pulse) volume illuminated by the radar beam. The amplitude is used to
determine a parameter called the reflectance factor (Z) which is used to estimate the mass
of precipitation per unit volume (M) or the intensity of precipitation (R) through the use of
empirical relations. A primary application is thus to detect, map and estimate the precipitation
at ground level instantaneously, nearly continuously, and over large areas.
Some research radars have used reflectance factors measured at two orthogonal polarizations
of the transmitted and received waveform. Research continues to determine the value and potential of polarization systems for precipitation measurement and target state, but operational
systems do not exist at present. Doppler radars have the capability of determining the phase
difference between the transmitted and received pulse. The rate of change of the phase difference is a measure of the mean Doppler velocity of the particles. The mean Doppler velocity
is a reflectance weighted average of the radial components of the displacement velocities of the
hydrometeors in the pulse volume. The Doppler spectrum width is a measure of the spatial
variability of the velocities and may provide some indication of the wind shear and turbulence.
Weather radar can serve many purposes. The focus of this paper will be on the ability of the
radar to estimate precipitation. Its use for other applications will be briefly described. We
proceed by discussing radar technology, basic radar theory, applications and then the various
techniques for the measurement of precipitation. We briefly discuss the use of high frequency
radars for the detection of clouds.