Early Online Release Transformation of the Csu-chill Radar Facility to a Dual-frequency, Dual-polarization, Doppler System 2

5 This paper describes the transformation of the CSU-CHILL National Radar Facility from 6 a single-frequency (S-band) dual-polarization Doppler weather radar system to a dual7 frequency (S and X band) dual-polarization Doppler system with coaxial beams. A brief 8 history regarding the development of dual-wavelength radars is first presented. In the past 9 dual-wavelength measurements were used to detect hail using the dual-wavelength ratio de10 fined as the ratio of intrinsic (or attenuation-corrected) X-band reflectivity to the S-band 11 reflectivity. Departures of this ratio from unity were taken to indicate the presence of hail, 12 produced by Mie scattering at the shorter wavelength by hail. Most dual-wavelength radars 13 were developed with attempts to match beams for S and X bands, which implies that the 14 sample volumes for the two frequencies were essentially the same. The X-band channel of 15 the CSU-CHILL radar takes a different approach making use of the already existing dual 16 offset-fed antenna designed to give a 1 degree beamwidth at S-band, resulting in an X-band 17 beamwidth of approximately 0.3 degrees, with very high gain. Thus the X-band provides 18 about a factor of three more resolution than the S-band component, while maintaining the 19 same sensitivity as the S-band component. Examples of cold season and warm season data 20 from the X-band and S-band radar components are presented, demonstrating the successful 21 transformation of the CSU-CHILL radar into a unique multi-frequency multi-polarization 22 system. The new CSU-CHILL dual-wavelength dual-polarization weather radar will serve 23 as an important asset for the scientific community. 24