A mixing scheme of ice particle models for global ice cloud measurements

The choice of ice particle model is critical for microwave remote sensing of ice clouds. Using a model-observation closure method, the global applicability of a series of single crystal and aggregate ice particle models is evaluated based on satellite-based microwave measurements from the CloudSat Cloud Profiling Radar (CPR) and the Global Precipitation Mission (GPM) Microwave Imager (GMI) high-frequency channels. The results indicate that the ice water path (IWP) retrievals from radar observations are highly dependent on the ice particle model, with the maximum difference between models exceeding a factor of 13. Although aggregate models generally perform better than single crystals, none of them can be well adapted to retrievals for all types of ice clouds on the global scale. A columnar single crystal works better for low IWP clouds, while a small block aggregate for high IWP. Consequently, a mixing scheme combining both with weighting by IWP is then proposed for global ice cloud measurements. By using this double-model mixing scheme, the simulations can well reproduce simultaneous active and passive microwave observations, regardless of the IWP. Specifically, the differences between CPR radar reflectivity simulations and observations do not exceed 0.1 dBZ, while the mean biases of brightness temperatures on all high-frequency channels of GMI are <1.5 K. This is expected to improve the retrieval accuracy of global ice clouds.