Status of the Vapor In-Cloud Profiling Radar

This presentation will be an update on the development of the Vapor In-cloud Profiling Radar (VIPR). VIPR is an airborne Differential Absorption Radar (DAR) designed to remotely profile water vapor within cloudy and precipitating volumes. The instrument uses multiple radar frequencies in the G-Band (167-174 GHz) on the flank of a water vapor absorption line. The differential reflectivity between the frequencies provides a direct measure of the water vapor profile in a manner analogous to DIAL (DIfferential Absorption Lidar). VIPR demonstrates the first ever DAR measurements of water vapor within clouds. These observations are highly complementary to other water vapor remotes sensing techniques which have either no capability within clouds or diminished performance as cloudiness increases.

The VIPR radar architecture employs a Frequency Modulated Continuous Wave (FMCW) approach with an all solid-state transceiver. A single antenna is used with quasi-optical duplexing to achieve > 80 dB of transmit/receive isolation. A compact, high-bandwidth device using GaAs Schottky diode frequency doubling and power combining is used to achieve 0.5 W of continuous wave power at G-band. A 25 cm diameter primary reflector provides sufficient gain for airborne measurements of typical liquid sized cloud droplets from a range of several km.

The instrument is tuned to frequencies that maximize sensitivity to water vapor within the Planetary Boundary Layer (PBL), a measurement area selected by the recent ESAS Decadal Survey for incubation over the coming decade. We will show our initial observations of PBL water vapor profiles and describe their resolution and sensitivity characteristics. Baseline resolution is 200 m oversampled in range by a factor of 8, which permits robust resolution enhancement. Precision errors are variable from scene-to-scene but typically are better than 20%. Both resolution and uncertainty are within the range recommended by the Decadal Survey. Finally, we will outline the necessary additional technology development required to make VIPR possible from a spaceborne platform.