TRANSFORMATION OF THE CSU-CHILL RADAR FACILITY TO A DUAL-FREQUENCY, DUAL-POLARIZATION DOPPLER SYSTEM

This paper describes the transformation of the Colorado State University-University of Chicago-Illinois State Water Survey (CSU-CHILL) National Radar Facility from a single-frequency (S band) dual-polarization Doppler weather radar system to a dual-frequency (S and X bands) dual-polarization Doppler system with coaxial beams. A brief history regarding the development of dual-wavelength radars is first presented. In the past, dual-wavelength measurements were used to detect hail using the dual-wavelength ratio defined as the ratio of intrinsic (or attenuation corrected) X-band reflectivity to the S-band reflectivity. Departures of this ratio from unity were taken to indicate the presence of hail, produced by Mie scattering at the shorter wavelength by hail. Most dual-wavelength radars were developed with attempts to match beams for S and X bands, which implies that the sample volumes for the two frequencies were essentially the same. The X-band channel of the CSU-CHILL radar takes a different approach, that of making use of the already existing dual-offset-fed antenna designed to give a 1 degrees beamwidth at S band, resulting in an X-band beamwidth of approximately 0.3 degrees, with very high gain. Thus, the X band provides about a factor of 3 more resolution than the S-band component while maintaining the same sensitivity as the S-band component. Examples of cold season and warm season data from the X-band and S-band radar components are presented, demonstrating the successful transformation of the CSU-CHILL radar into a unique multifrequency, multipolarization system. The new CSU-CHILL dual-wavelength, dual-polarization weather radar will serve as an important asset for the scientific community.