The supercooled cloud fraction in the mixed-phase clouds from Himawari-8 observations

The determination of supercooled cloud fraction (SCF) based on satellite remote sensing is important for research fields including estimation of global radiative energy balance, artificial weather modification, and prevention of aircraft ice accretion. However, nearly all retrieval algorithms for passive instruments provide binary phase information - ice, supercooled or liquid - making it difficult to retrieve mixed-phase cloud properties and understand the transition from supercooled water droplets to ice crystals. Motivated by these questions, we proposed a method to evaluate the ice partitioning of single-layer thermodynamic cloud top phase, under the assumption of the shape of ice crystals.

In order to retrieve optical properties of SCF, we use a droxtal habit model to investigate the scattering properties of frozen supercooled water particles. We compare the single scattering phase functions between droxtals and spherical particles at different wavelengths. Furthermore, the difference of satellite observed radiance reflected by supercooled water clouds and ice clouds are discussed. The difference between cloud ice water path of these two categories of clouds and observations from the same satellite channel can be used to evaluate the SCF in single-layer cloud top mixed phase clouds. Taking the ice-to-liquid ratio in the GCM (global climate model)-Oriented CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) Cloud Product (CALIPSO-GOCCP) as the criteria to validate our retrieved SCF, the average deviation, root mean square error and correlation coefficient are 6.98%, 9.62%, and 0.78, respectively. As a future work, we plan to adjust ice particle habits regarding ambient temperature to represent frozen supercooled water particles.

Our method could be applied to the to be launched EarthCARE (Cloud, Aerosol and Radiation Explorer) satellite in 2023 as it boards one multi-spectral imager and one atmospheric lidar simultaneously. This study is also of interest for related researches on assessing the climate impacts of supercooled and mixed-phase clouds and validating the associated global model simulations.