Airborne differential absorption radar measurements with the 167-174.8 GHz Vapor In-cloud Profiling Radar (VIPR)

Abstract:

G-band (110 – 300 GHz) radars hold considerable promise to improve cloud remote sensing measurement capabilities, both for microphysics studies because of the large degree of non-Rayleigh hydrometeor scattering and for in-cloud humidity sounding using differential absorption radar near the 183 GHz water line. VIPR is a proof-of-concept airborne G-band radar developed at the Jet Propulsion Laboratory under NASA ESTO’s Instrument Incubator Program that primarily targets high-resolution humidity profiles inside of planetary boundary layer (PBL) clouds. By measuring differential absorption between 167 and 174.8 GHz within hydrometeor layers due to water vapor, VIPR allows for the retrieval of range-resolved humidity with a precision better than 1 g/m³ and resolution of 180 m. The ability to remotely measure water vapor profiles inside of clouds and precipitation with high spatial resolution, especially within the PBL, fills a critical observational gap and directly addresses the 2017 Decadal Survey PBL Targeted Observable. Here we discuss the results of VIPR’s first airborne campaign, which took place aboard a Twin Otter aircraft off the Southwest coast of the United States from October 2019 to January 2020. In addition to assessing the in-cloud profiling capabilities from an airborne platform by comparing with coincident radiosonde profiles, we demonstrate the ability to use G-band differential absorption radar to measure column-integrated water vapor between the aircraft and the Earth’s surface with a precision of 0.8 mm for 40 m along-track averaging. We also evaluate the angle dependence of the normalized ocean surface cross section from aircraft banking maneuvers for future applications in airborne and spaceborne G-band radar calibration, and find a normal incidence value of about 15 dB at 167 GHz for low surface wind speed conditions.