Precipitation at the Ground: Radar Techniques
293
12.5 Meteorological Application
12.5.1 Meteorological requirements
The radar characteristics of anyone radar will not be ideal for all applications. Radars observations have been found most useful for:
• severe weather detection, tracking and warning,
• surveillance of synoptic and mesoscale weather systems, and
• estimation of precipitation amounts.
The selection criteria of a radar system is usually optimized to meet several applications but
can also be specified to best meet a specific application of major importance. The choice of
wavelength, beamwidth, pulse length and PRF have particular consequences.
12.5.2 Operational severe weather detection and warning
Radar is the only realistic means of monitoring severe weather over a wide area. Radar echo
intensities, area and patterns can be used to identify areas of severe weather. These storms
include thunderstorms with probable hail and damaging winds. Doppler radar which can
identify and provide a measure of intense winds associated with gust fronts, downbursts and
tornadoes adds a new dimension. Nominal range of coverage is about 200 km, which is sufficient
for local short range forecasting and warning. Radar networks are used to extend the coverage.
Effective interpretation requires alert and well trained personnel to provide effective warnings
at present and until automated algorithms and storm models have been developed for the local
areas.
Reflectance signatures for some severe storms are well known. The most commonly used criterion for conventional radar detection of potentially-severe thunderstorms today is reflectance
intensity. Operational forecasters are advised to look for regions of high reflectivities (50 dBZ
or greater), hook echoes, overhangs and other echo shapes to warn of tornadoes or of severe
thunderstorms, but the false alarm rate is high (see Fig. 12.6; Lemon et al., 1978). These signatures extend to the spiral-bands and eyewall structure for hurricanes and hook or finger-like
echoes for tornadoes and hail (Donaldson, 1970).
In Doppler data, the existence of the meso cyclone has led to a useful severe storm detection
technique. A meso cyclone is a vertical column of rising cyclonically rotating air typically 10-20
km across. The mesocyclone signature (or velocity couplet) is observed to form in mid-levels
of a storm and descend to cloud base, coincident with tornado development (Burgess, 1976;
Burgess and Lemon, 1990). This behavior has led to improved tornado-warning lead times of
20 min or longer, during quasi-operational experiments in Oklahoma (JDOP, 1979). Most of
the Doppler observations on this relationship have been made in the U.S. and it is not known
if it can be generalized yet (Houze et al., 1993; Crozier et al., 1991). During experiments in
Oklahoma, roughly 50% of all mesocyclones produced verified tornadoes; as well, all storms with
violent tornadoes formed in environments with strong shear and possessed strong mesocyclones
(Burgess and Lemon, 1990).
The second signature - Tornado Vortex Signature - (TVS) is produced by the tornado
itself and is the location of a very tight circulation in a region much smaller than that of the
293
12.5 Meteorological Application
12.5.1 Meteorological requirements
The radar characteristics of anyone radar will not be ideal for all applications. Radars observations have been found most useful for:
• severe weather detection, tracking and warning,
• surveillance of synoptic and mesoscale weather systems, and
• estimation of precipitation amounts.
The selection criteria of a radar system is usually optimized to meet several applications but
can also be specified to best meet a specific application of major importance. The choice of
wavelength, beamwidth, pulse length and PRF have particular consequences.
12.5.2 Operational severe weather detection and warning
Radar is the only realistic means of monitoring severe weather over a wide area. Radar echo
intensities, area and patterns can be used to identify areas of severe weather. These storms
include thunderstorms with probable hail and damaging winds. Doppler radar which can
identify and provide a measure of intense winds associated with gust fronts, downbursts and
tornadoes adds a new dimension. Nominal range of coverage is about 200 km, which is sufficient
for local short range forecasting and warning. Radar networks are used to extend the coverage.
Effective interpretation requires alert and well trained personnel to provide effective warnings
at present and until automated algorithms and storm models have been developed for the local
areas.
Reflectance signatures for some severe storms are well known. The most commonly used criterion for conventional radar detection of potentially-severe thunderstorms today is reflectance
intensity. Operational forecasters are advised to look for regions of high reflectivities (50 dBZ
or greater), hook echoes, overhangs and other echo shapes to warn of tornadoes or of severe
thunderstorms, but the false alarm rate is high (see Fig. 12.6; Lemon et al., 1978). These signatures extend to the spiral-bands and eyewall structure for hurricanes and hook or finger-like
echoes for tornadoes and hail (Donaldson, 1970).
In Doppler data, the existence of the meso cyclone has led to a useful severe storm detection
technique. A meso cyclone is a vertical column of rising cyclonically rotating air typically 10-20
km across. The mesocyclone signature (or velocity couplet) is observed to form in mid-levels
of a storm and descend to cloud base, coincident with tornado development (Burgess, 1976;
Burgess and Lemon, 1990). This behavior has led to improved tornado-warning lead times of
20 min or longer, during quasi-operational experiments in Oklahoma (JDOP, 1979). Most of
the Doppler observations on this relationship have been made in the U.S. and it is not known
if it can be generalized yet (Houze et al., 1993; Crozier et al., 1991). During experiments in
Oklahoma, roughly 50% of all mesocyclones produced verified tornadoes; as well, all storms with
violent tornadoes formed in environments with strong shear and possessed strong mesocyclones
(Burgess and Lemon, 1990).
The second signature - Tornado Vortex Signature - (TVS) is produced by the tornado
itself and is the location of a very tight circulation in a region much smaller than that of the