Precipitation at the Ground: Radar Techniques

Abstract

Radar can estimate precipitation over a large area (of order 100 kin 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.