Precipitation at the Ground: Radar Techniques
313
by using an empirical Z - R relation, agree well with gauge measurements close to the radar.
Variations in the DSDs, the inability to measure precipitation close enough to the ground, beam
averaging and filling are the main shortcomings of weather radar.
To make the best possible use of radar, the radar site should be chosen such that precipitation
can be seen by the radar as close as possible to the ground. Seen means that there is no shielding
or clutter echoes, or that the influence of clutter can be eliminated, for instance by Doppler
processing. Wavelength and antenna size should be chosen such that a suitable compromise
between attenuation caused by precipitation and good spatial resolution is achieved. Systems
should be rigorously maintained and quality controlled. Measurements of reflectance should be
corrected for errors originating from the averaging effect of a broadened beamwidth and/or for
the vertic"l profile of reflectance. The three dimensional echo pattern, together with knowledge
about the position of the radar and the orography around it, allows for the correction in real
time for a large fraction of the vertical profile problem, though the optimal technique remains
to be found.
The sample size must be adequate for the application. For hydrological applications, and especially when adjusting radar estimates with gauges, it is desirable to integrate the data over
a number of hours and/or square kilometers. Polarimetric measurements may provide some
further improvement, but it has yet to be demonstrated that the limited operating conditions, additional cost, complexity and risk of misinterpreting polarization measurements can be
justified for operational applications in hydrology.
The principle advantages of radar are its high spatial and temporal resolution, wide area coverage and immediacy (real-time data). Radar also has the capability of measuring over inaccessible areas such as lakes and to follow a floating target or a convective complex in a real-time
sequence, for instance to make a short term forecast. It is unlikely that radar will ever completely replace the raingauge, since gauges can provide additional information and are essential
for adjusting and/or checking the radar indications. Good quantitative information is already
obtained from radar networks in many places.
12.8 Acknowledgments
Portions of this document were extracted from the WMO ClMO Guide, Chapter 9, P. Joe and
C.L. Crozier (Ed.)' 1995. Other contributors to that document were: J. Aoyagi, M. Gilet, J.
Golden, J. Joss, L.P. Merritt, T.M. Puhakka and P.L. Smith.
12.9 References
Abraham J, Macdonald K, Joe P (1991) The interaction of hurricane Hugo with the
Mid-Latitude Westerlies, 19 th Conference on Hurricanes and Tropical Meteorology, May 6-10,
Miami, Florida
AI-Khatib HN, Seliga TA, Bringi VN (1979) Differential reflectance and its use in the
radar measurement of rainfall. Atmos Sci Prog Rep AS-S-106, Ohio State University, Columbus
Aoyagi J (1983) A study of the MTl weather radar system for rejecting ground clutter. Papers
in Meteor and Geophys 33 (4), 187-243
Atlas D (1964) Advances in Radar Meteorology. Advances in Geophysics, Vol. 10, Editors
H.E. Landsberg and J. Van Meighen. New York, Academic Press 317-479
313
by using an empirical Z - R relation, agree well with gauge measurements close to the radar.
Variations in the DSDs, the inability to measure precipitation close enough to the ground, beam
averaging and filling are the main shortcomings of weather radar.
To make the best possible use of radar, the radar site should be chosen such that precipitation
can be seen by the radar as close as possible to the ground. Seen means that there is no shielding
or clutter echoes, or that the influence of clutter can be eliminated, for instance by Doppler
processing. Wavelength and antenna size should be chosen such that a suitable compromise
between attenuation caused by precipitation and good spatial resolution is achieved. Systems
should be rigorously maintained and quality controlled. Measurements of reflectance should be
corrected for errors originating from the averaging effect of a broadened beamwidth and/or for
the vertic"l profile of reflectance. The three dimensional echo pattern, together with knowledge
about the position of the radar and the orography around it, allows for the correction in real
time for a large fraction of the vertical profile problem, though the optimal technique remains
to be found.
The sample size must be adequate for the application. For hydrological applications, and especially when adjusting radar estimates with gauges, it is desirable to integrate the data over
a number of hours and/or square kilometers. Polarimetric measurements may provide some
further improvement, but it has yet to be demonstrated that the limited operating conditions, additional cost, complexity and risk of misinterpreting polarization measurements can be
justified for operational applications in hydrology.
The principle advantages of radar are its high spatial and temporal resolution, wide area coverage and immediacy (real-time data). Radar also has the capability of measuring over inaccessible areas such as lakes and to follow a floating target or a convective complex in a real-time
sequence, for instance to make a short term forecast. It is unlikely that radar will ever completely replace the raingauge, since gauges can provide additional information and are essential
for adjusting and/or checking the radar indications. Good quantitative information is already
obtained from radar networks in many places.
12.8 Acknowledgments
Portions of this document were extracted from the WMO ClMO Guide, Chapter 9, P. Joe and
C.L. Crozier (Ed.)' 1995. Other contributors to that document were: J. Aoyagi, M. Gilet, J.
Golden, J. Joss, L.P. Merritt, T.M. Puhakka and P.L. Smith.
12.9 References
Abraham J, Macdonald K, Joe P (1991) The interaction of hurricane Hugo with the
Mid-Latitude Westerlies, 19 th Conference on Hurricanes and Tropical Meteorology, May 6-10,
Miami, Florida
AI-Khatib HN, Seliga TA, Bringi VN (1979) Differential reflectance and its use in the
radar measurement of rainfall. Atmos Sci Prog Rep AS-S-106, Ohio State University, Columbus
Aoyagi J (1983) A study of the MTl weather radar system for rejecting ground clutter. Papers
in Meteor and Geophys 33 (4), 187-243
Atlas D (1964) Advances in Radar Meteorology. Advances in Geophysics, Vol. 10, Editors
H.E. Landsberg and J. Van Meighen. New York, Academic Press 317-479