What's Next in Radar Technology?
Courtesy of NOAA's National Severe Storms Lab

Polarmetric Doppler Radars

Polarimetric radars, also called dual-polarization radars, transmit radio wave pulses that have both horizontal and vertical orientations. The additional information from vertical pulses will greatly improve many different types of forecasts and warnings for hazardous weather. These radars are expected to be scanning the skies over central Alabama by the end of 2011.

What variables are measured?

  • Differential Reflectivity - The differential reflectivity is a ratio of the reflected horizontal and vertical power returns. Among other things, it is a good indicator of drop shape. In turn the shape is a good estimate of average drop size.
  • Correlation Coefficient - A statistical correlation between the reflected horizontal and vertical power returns. It is a good indicator of regions where there is a mixture of precipitation types, such as rain and snow.
  • Linear Depolarization Ratio - This is a ratio of a vertical power return from a horizontal pulse or a horizontal power return from a vertical pulse. It, too, is a good indicator of regions where mixtures of precipitation types occur.
  • Specific Differential Phase - The specific differential phase is a comparison of the returned phase difference between the horizontal and vertical pulses. This phase difference is caused by the difference in the number of wave cycles (or wavelengths) along the propagation path for horizontal and vertically polarized waves. It should not be confused with the Doppler frequency shift, which is caused by the motion of the cloud and precipitation particles. Unlike the differential reflectivity, correlation coefficient and linear depolarization ratio, which are all dependent on reflected power, the specific differential phase is a "propagation effect." It is a very good estimator of rain rate.
  • Cross-polar terms - other variables yet to be explored!

Phased Array Radar

Current WSR-88D radars transmit one beam of energy at a time, listen for the returned energy, then mechanically tilts up a little higher, and samples another small section of the atmosphere. When it has sampled the entire volume of atmosphere, from bottom to top at a particular location, the radar goes back down, moves over a little, and starts the process over again. This continues until the radar has scanned the entire atmosphere, which takes around six or seven minutes. Phased arrays use multiple beams, sent out at one time, so the antennas never need to tilt. Scanning takes only 30 seconds, and it already has dual-polarization capabilities.

Phased Array radar has a unique antenna that collects the same information as a conventional radar in about one-sixth the time. Researchers believe phased array could extend warning lead times from 10 minutes to 18-22 minutes.



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