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Polarimetric Doppler Radar
IDENTIFYING PROBLEMS AND FINDING SOLUTIONS:
Improving Data Quality
PROBLEM: Conventional radar is plagued with contamination of its echoes
SOLUTION: Polarimetric radar is immune to many contamination issues and offers improvements in data quality
SUCCESSES:
- Anomalous Propagation (AP): A problem that frequently plagues radar measurements is the presence of anomalous propagation. AP refers to a "ground return contamination" that sometimes occurs in the radar data when a warm layer of air forms above a cold layer of air. This phenomena, which is called an inversion layer, essentially bends the radar beam back towards the ground resulting in a ground return contamination that makes it very difficult at times to distinguish the location and intensity of clouds and precipitation. Polarimetric radar helps eliminate AP.
- Filtering non-meteorological echoes: The polarimetric classification algorithm identifies and filters out about 99% of non-meteorological echos if the signal-to-noise ratio exceeds 10 dB.
- Polarimetric radar reduces biases and errors due to calibration errors, partial beam blockage, and attenuation in heavy rain and hail.
- Eliminating non-meteorological targets: Birds, insects, airplanes, buildings and trees are all detected by radar. We can use dual-polarization measurements to "clean-up" the data and identify where it is actually raining/snowing.
- Discrimination of birds and insects mixed in a resolution volume: Polarimetric radar variables exhibit unique signatures for many non-meteorological scatterers, such as birds and insects. The motion of birds and insects may affect the interpretation of measured Doppler winds. NSSL researchers have figured out how to tell the difference between birds and insects using the spectral densities of dual-polarization variables. This information is important because insects carried by the wind represent wind velocity. Also, migrating birds contaminate radar data by causing echoes that falsely indicate winds that are stronger and from a different direction than reality. By being able to discriminate between insects and birds, NSSL researchers not only discovered how to remove the birds from the data, but also how to calculate both wind and mean bird flow velocities and directions.
- Other commercial and scientific disciplines may find radar measurements of birds and insects useful: the aviation industry who are concerned about the hazards birds pose to aircraft, and entomologists who study crop damage from insect migrations. For weather radars, of course, birds and bugs are generally thought of as a data contamination. Fortunately, their unique polarimetric signatures generally make them easily identifiable in the data.
