Chapter 12
Precipitation at the Ground: Radar
Techniques
Paul Joe
Atmospheric Environment Service of Canada
Meteorological Research Branch
Cloud Physics Research Division
4905 Dufferin St., Downsview, Ontario
Canada, M3H 5T4
Radar can estimate precipitation over a large area (of order 100 km radius) with high spatial
(of order 1 km) and temporal (of order 10 minute) resolution. The standard measurement of
rainfall is by rain gauges which make essentially point measurements. A very dense network of
rain gauges is required to match the radar precipitation fields due to the spatial and temporal
variability. This is costly and not very practical. It is only recently that radar has been able to
provide adequate estimates of precipitation for hydrological applications. There are many choices, issues and assumptions made when using radar for precipitation monitoring. These range
from theoretical, empirical, technological and processing considerations. Basic radar theory
and implementation considerations as it relates to precipitation measurements will be discussed. The basic radar equation will be presented with an emphasis on the assumptions made in
its derivation. Hardware considerations such as beamwidth, wavelength and signal processing
will be discussed. The intelligent use of Doppler signal processing in operational networks can
help in the elimination of many radar echo artifacts resulting from ground targets or anomalous
propagation. The signal processing for polarization has the potential to classify the precipitation targets and produce better estimates of heavy rainfalls. Advanced data processing can
be used to extrapolate the radar echoes measured aloft to the ground, to account for local
enhancement or shadowing effects and to adjust for bright band effects. Real-time adjustment
procedures using telemetered rain gauges have been proposed to reduce the discrepancy between the gauge and radar measurements. While most of the work has been with rain, snowfall
studies have been limited. This is in part due to the difficulty in making accurate snow fall
measurements. There is no accepted standard for the high temporal measurement (of order 10
minute) of snow fall. The limited snowfall studies show a reasonable average relationship valid
for daily accumulations but short term accumulations show that there are various relationships
which are attributed to changes in crystal type. Radar has played a significant role in the study
of processes leading to the evolution of precipitation type and distributions. This is a result of
the sensitivity of the radar backscatter power to drop size. In addition, the Doppler velocity
NATO ASI Series, Vol. 145
Radiation and Water in the Climate System:
Remote Measurements
Edited by Ehrhard Raschke
© Springer-Verlag Berlin Heidelberg 1996
Precipitation at the Ground: Radar
Techniques
Paul Joe
Atmospheric Environment Service of Canada
Meteorological Research Branch
Cloud Physics Research Division
4905 Dufferin St., Downsview, Ontario
Canada, M3H 5T4
Radar can estimate precipitation over a large area (of order 100 km radius) with high spatial
(of order 1 km) and temporal (of order 10 minute) resolution. The standard measurement of
rainfall is by rain gauges which make essentially point measurements. A very dense network of
rain gauges is required to match the radar precipitation fields due to the spatial and temporal
variability. This is costly and not very practical. It is only recently that radar has been able to
provide adequate estimates of precipitation for hydrological applications. There are many choices, issues and assumptions made when using radar for precipitation monitoring. These range
from theoretical, empirical, technological and processing considerations. Basic radar theory
and implementation considerations as it relates to precipitation measurements will be discussed. The basic radar equation will be presented with an emphasis on the assumptions made in
its derivation. Hardware considerations such as beamwidth, wavelength and signal processing
will be discussed. The intelligent use of Doppler signal processing in operational networks can
help in the elimination of many radar echo artifacts resulting from ground targets or anomalous
propagation. The signal processing for polarization has the potential to classify the precipitation targets and produce better estimates of heavy rainfalls. Advanced data processing can
be used to extrapolate the radar echoes measured aloft to the ground, to account for local
enhancement or shadowing effects and to adjust for bright band effects. Real-time adjustment
procedures using telemetered rain gauges have been proposed to reduce the discrepancy between the gauge and radar measurements. While most of the work has been with rain, snowfall
studies have been limited. This is in part due to the difficulty in making accurate snow fall
measurements. There is no accepted standard for the high temporal measurement (of order 10
minute) of snow fall. The limited snowfall studies show a reasonable average relationship valid
for daily accumulations but short term accumulations show that there are various relationships
which are attributed to changes in crystal type. Radar has played a significant role in the study
of processes leading to the evolution of precipitation type and distributions. This is a result of
the sensitivity of the radar backscatter power to drop size. In addition, the Doppler velocity
NATO ASI Series, Vol. 145
Radiation and Water in the Climate System:
Remote Measurements
Edited by Ehrhard Raschke
© Springer-Verlag Berlin Heidelberg 1996