The Ice Cloud Imager: retrieval of frozen water mass profiles

Abstract. The Ice Cloud Imager (ICI) will be hosted on the second generation of the EUMETSAT Polar System (EPS-SG). By measuring at microwave and sub-millimetre wavelengths, ICI will provide unparalleled global observations of ice clouds. EUMETSAT's official ICI level-2 product will offer retrievals of ice mass column properties. This study explores whether the capabilities of ICI can be extended to retrieve vertical profiles of ice mass. Using a retrieval database of ICI simulations, we trained a quantile regression neural network (QRNN) to retrieve ice water content (IWC) and profiles of the mean mass diameter of ice hydrometeors. Our retrieval setup is fast and simpler to implement than previous ICI profile retrieval approaches, and the study is more comprehensive in scope than earlier efforts. Comparisons between our retrieved and database profiles demonstrate that ICI observations are sensitive to IWC within the range of 10−2 and 1 g m−3, and performance is strongest between altitudes of 3 and 14 km. Our results also show that ICI observations are sensitive to mean mass diameter values up to 600 µm, although successful retrievals of up to 800 µm are observed. To assess the vertical resolution of the retrievals, we computed approximations of averaging kernels on the model predictions. We estimate the resolution of IWC profiles to be ∼ 2.5 km. Retrievals of mean mass diameter achieve an estimated resolution of 2.5 km at an altitude of 5 km, with reduced resolution at higher altitudes. No operational product currently provides ice mass vertical information derived from passive microwave observations. However, this study demonstrates that ICI can fill this gap thanks to the presence of both microwave and sub-millimetre channels, with the sub-millimetre wavelengths providing particularly high sensitivity to cloud ice. Furthermore, the relatively broad swath of ICI observations lead to a higher spatial and temporal coverage than radar and lidar products can achieve. The global and long-term dataset that ICI will offer could therefore act as a valuable complement to CloudSat or EarthCARE-based retrievals. Future efforts could explore the inclusion of the Microwave Imager (MWI) observations to improve retrievals at low altitudes – a natural next step given that MWI is to be launched on the same platform as ICI.