G-Band Differential Absorption Radar for Inside-Cloud Boundary Layer Humidity Retrieval

An airborne G-band (170 GHz) differential absorption radar is being developed to simultaneously measure inside-cloud water vapor in the boundary layer and total column water vapor over all surfaces. By using a frequency-tunable transmitter covering a portion of the lower flank of the broad 183 GHz water absorption line, relative attenuation of the radar echoes from both cloud water and the ground will be measured to retrieve range-resolved absolute humidity content. This proof-of-concept instrument will provide the first ever remote water vapor profiling capability within clouds with precise and high resolution height registration, overcoming limitations of more mature humidity remote sensing methods such as Differential Absorption Lidar (DIAL) and microwave or millimeter-wave passive radiometry. Numerical simulation with high fidelity cloud models suggest absolute humidity can be derived to within better than 25%. These humidity observations will help quantify the coupling between boundary layer humidity structure and cloud coverage, constrain surface fluxes of moisture and energy, and provide a potential new observation for assimilation in numerical weather models. The G-band radar design is based on a proven frequency-modulated continuous-wave system architecture that the Jet Propulsion Laboratory has developed for various security and science applications spanning 95-700 GHz. Notable enabling technology elements of the radar include a fast-sweeping, high-bandwidth, and low-noise signal source; state-of-the-art semiconductor frequency-multipliers and low-noise amplifiers maximize the radar sensitivity; very high-isolation transmit/receive duplexing; and novel humidity profile retrieval algorithms. Preliminary ground-based differential radar measurements of range-resolved humidity absorption will be presented that demonstrate the proposed technique.