Title: Boundary-Layer Humidity Sounding Using a G-Band Differential Absorption Radar
Presenting Author: Richard Roy
Organization: Jet Propulsion Laboratory, California Institute of Technology
Co-Author(s): Matthew Lebsock, Ken Cooper, Raquel Monje, Luis Millan, Robert Dengler, Jose Siles, Robert Lin, and Choonsup Lee, JPL

Abstract:
The vapor in-cloud profiling radar, or VIPR (IIP-16), is a proof-of-concept instrument for performing active measurements of humidity profiles using a differential absorption technique on the low-frequency flank of a strong water vapor absorption line. Leveraging significant technological development from a previous ACT-13 effort, the instrument features state-of-the-art GaAs Schottky diode frequency-doublers which yield record high powers of 350-550 mW across the 167-174.8 GHz transmit band. The radar transceiver is operated in a frequency-modulated continuous-wave (FMCW) configuration, and utilizes high-isolation transmit/receive duplexing optics and phase-noise cancellation to realize thermal-noise limited measurements. Successful ground-based tests of the differential absorption radar (DAR) in precipitating and non-precipitating cloudy scenarios will be presented, where differential absorption is observed beyond 2 km range with 100 mW transmit power and 39 dB antenna gain. An absorption-line-fitting humidity retrieval algorithm is implemented yielding range-resolved profiles with few-hundred meter resolution. Future plans for validation include DAR measurements coincident with airport radiosondes and eventual testing from an airborne platform, with instrument upgrades including the installation of a 60 cm primary aperture and corresponding 20 dB increase in antenna gain. Notably, the surface returns from airborne measurements will be investigated for the retrieval of total column water vapor within the atmospheric boundary layer.