Evaluation of EarthCARE retrievals of ice microphysics and vertical wind using in-situ aircraft observations

Abstract. The novel observational capabilities of the EarthCARE satellite promise the most accurate retrievals of the vertical profile of ice clouds yet achieved from space, but independent evaluation is essential. In this paper we use in-situ sampling from five underflights of EarthCARE in ice clouds between -10 and -45 °C during the UK "VERIFY" and Canadian "ECALOT" campaigns to evaluate and improve its microphysical and vertical-wind retrievals, as well as testing prior assumptions such as the mass–size relationship and the radar ice scattering model. We find that, to a good approximation, the small-scale fluctuations in radar-measured Doppler velocity can be attributed to vertical wind and the larger-scale averages to ice terminal fall speed. When EarthCARE’s "C-CD" algorithm is updated to exploit this finding, its vertical wind retrieval is able to capture the amplitude and phase of gravity waves measured by the aircraft. Retrievals of IWC and extinction by the radar-only "C-CLD" algorithm, the lidar-only "A-EBD" algorithm and the synergistic "ACM-CAP" algorithm, are in good agreement with the aircraft but highlight the important challenge of retrieving ice particle density. Comparing the 94-GHz radar reflectivity observed by EarthCARE with values computed from the aircraft probes enables us to successfully validate the Self-Similar Rayleigh-Gans scattering model even when non-Rayleigh scattering reduces the reflectivity below the equivalent Rayleigh value by 15 dB. However, the calculations of reflectivity-weighted terminal fall speed from the aircraft are systematically 15–30 % higher than measured by EarthCARE’s Doppler radar, suggesting the need for further work on models of ice fall speed.