An airborne all-sky radiative closure test of microwave radiometers

Abstract. Microwave radiometers can observe the atmosphere through clouds and are widely used for all-sky atmospheric monitoring. New hyperspectral microwave sounders and cloud-sensitive W-band radiometers are expanding microwave observing capabilities, but their performance under cloudy conditions requires careful experimental evaluation before they can be fully used for all-sky retrievals. We present one of the first experimental all-sky radiative closure analyses of these advanced radiometers under liquid stratocumulus conditions, using the High Spectral Resolution Airborne Microwave Sounder (HiSRAMS) and the Radar Enhanced W-band Airborne Radiometer Detection System (REWARDS) aboard the National Research Council Canada (NRC) Convair-580 during a research flight in Canada. Observed brightness temperatures were compared with radiative transfer simulations constrained by collocated airborne radar, lidar, and in situ cloud microphysical measurements, together with modelled temperature and water vapour profiles. In situ microphysical measurements provided a reference cloud absorption profile, while radar and lidar observations constrained its instantaneous vertical location and extent. REWARDS observations were used to constrain the empirical cloud profile configuration, whereas HiSRAMS provided a more independent assessment across spectral regions sensitive to oxygen and water vapour. For both instruments, most observations were consistent with the simulations within the estimated radiative closure uncertainties, indicating broad agreement under all-sky conditions. The uncertainty analysis showed that the vertical distribution of cloud absorption was the principal limitation in cloud-sensitive channels, while water vapour variability contributed substantially at selected frequencies in the water vapour band. These complementary spectral sensitivities also suggest potential for joint retrievals of cloud and thermodynamic properties. Nevertheless, large residuals in some cloudy scenes reveal continuing limitations in representing rapidly varying cloud structure and microphysics. Overall, this study demonstrates the feasibility of all-sky radiative closure analysis for HiSRAMS and REWARDS under liquid stratocumulus conditions and highlights the importance of coordinated passive microwave, active cloud, and in situ microphysical observations for constraining cloud properties and diagnosing closure uncertainties. This multi-instrument approach provides a valuable pathway for future all-sky radiative closure studies and, ultimately, for the development of more reliable all-sky retrievals.