G-band Radar for Water vapor and Arctic Clouds (GRaWAC): novel insights on Arctic water vapor, clouds and precipitation

Abstract. Clouds are a central component in the complex interplay of feedback processes driving amplified warming in the Arctic. While state-of-the-art radar observations operating at X-, Ka-, or W-band provide detailed measurements of both hydrometeor distribution and cloud dynamics based on radar moments and spectra, current microphysical retrieval approaches are limited at small particle sizes ubiquitously present in Arctic clouds. Expanding observations to the G-band (110–300 GHz) can bridge this limitation and provide additional information on the vertical in-cloud water vapor distribution through the Differential Absorption Radar (DAR) method. Here, we introduce the dual-frequency, Dopplerized frequency-modulated continuous-wave (FMCW)  G-band Radar for Water vapor and Arctic Clouds (GRaWAC), which operates simultaneously at 167.3 GHz and 174.7 GHz. GRaWAC uniquely combines DAR and Doppler capabilities in a bi-static system, achieving a sensitivity of −43 dBZ at 1 km range and a 1 s integration time with a vertical resolution of 20 m. GRaWAC's flexible design enables operational usage from ground, ship, or aircraft. By applying the Differential Absorption Radar (DAR) approach, we retrieve in-cloud and in-precipitation water vapor profiles, as well as partial column water vapor in all-sky conditions, when deployed from aircraft. In order to highlight GRaWAC's potential, we present first measurements from Arctic deployments at (1) the German-French AWIPEV research base, Ny-Ålesund, Svalbard, with water vapor profiles derived at 60 s temporal and 200 m vertical resolution, and (2) aboard the AWI Polar 6 aircraft with profiles' vertical and horizontal resolution of 200 m and 1.6 km, respectively. 

We find that ground-based retrievals capture lower-tropospheric moistening well. We find a RMSD of 0.5 gm^-3 compared to coincident radiosonde profiles, which increases to up to 2 gm^-3 in the case of  differential scattering and liquid attenuation systematically affecting the retrieval approach. The temporal evolution of a mixed-phase cloud deck is well represented in Doppler spectra at 167.3 GHz, as characteristic bi-modal peaks form when supercooled liquid and ice coexist. During airborne operations, a case study shows that GRaWAC and a dropsonde water vapor profile agree remarkably well (within 0.5 gm^-3). Retrieved IWV is sensitive to instrument calibration, and shows an RMSD of 1.1 kgm^-2 compared to the statistics of dropsondes across different surface types, including sea-ice. While further investigation and retrieval development is needed to mitigate differential scattering and liquid attenuation effects on retrieved water vapor profiles, e.g., through a synergistic optimal estimation retrieval including microwave radiometer and W-band radar, our results highlight GRaWAC's potential to bridge the observational gap between Arctic cloud microphysics and thermodynamics.