Aviation Weather Hazard Detection with Polarimetric Radar

Plans are being developed to upgrade the national network of WSR-88Ds for polarimetric measurements. The radars will transmit electromagnetic energy at 45o (slant) polarization and receive returned signals at horizontal and vertical polarization. Because hydrometeors are not spherical, their radar backscattering cross sections are not the same for the two received polarizations. Signal properties change continuously as the radar waves propagate yielding information that can be used to determine hydrometeor type (rain, snow, or mixed-phase), size, shape, and orientation. The added measurements of differential reflectivity, differential propagation phase, and cross-correlation coefficient provide far more information regarding scatterers than is obtained from radar reflectivity alone. The measurements readily discriminate among ground targets, biological scatterers (insects and birds), and precipitation. The consistency among the measurements can be used to verify radar hardware calibration.

Demonstrated capabilities with polarimetric radar include improved rain-snow discrimination, hail detection, estimation of heavy rainfall rates, and freezing-level designation. Other benefits, likely to be developed long term, are improved quantification of winter (frozen) precipitation, detection of some mixed-phase icing conditions (especially situations involving embedded convection), the estimation of precipitation-impacted visibility, detection of lightning precursors, and improved microphysical parameterization in numerical forecast models. Importantly, polarimetric measurements should also be useful for eliminating regions of the atmosphere where particular hazards are not likely. The net result should be better detection and quantification of weather hazards in the terminal area and increased airport capacity.

The paper will describe the polarimetric measurements and illustrate their potential for detecting various weather hazards. Emphasis will be placed on prospects for rain-snow discrimination, visibility estimation, snowfall quantification, and designation of icing hazards.