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Closure of the Cloud Phase

Periodic Reporting for period 2 - C2Phase (Closure of the Cloud Phase)

Reporting period: 2018-10-01 to 2020-03-31

Clouds strongly modulate the Earth’s radiative budget, are key elements in the hydrological cycle, and are major drivers of the uncertainty of the prediction of future climate. Clouds can consist of liquid water droplets, ice particles, or both at the same time. At temperatures below 0°C, liquid droplets remain in a metastable thermodynamic state until freezing is initiated heterogeneously, either by an ice particle with which it comes into contact or by an ice nucleating aerosol particle, or homogeneously at about -37°C. Whether and how many ice particles are present in an otherwise liquid cloud (i.e. the phase distribution) influences the dynamical development through latent heat release, the interaction with radiation, and the formation of precipitation. Despite its importance, the cloud phase distribution is poorly represented in many weather and climate models. In the ERC Starting Grant C2Phase (“Closure of the Cloud Phase”), we set out to improve the representation of the phase distribution in models by making use of recent progress in understanding of microphysical ice formation processes and in observational capabilities with space-based passive sensors.

More specifically, our objective is to test the following hypothesis: The relevant primary and secondary ice formation processes can be included in state-of-the art cloud models such that these agree in a statistical sense with space-based observations of the cloud thermodynamic phase for a wide range of conditions. The closure will be better (higher correlation between predicted and observed phase), the more physical details are included.
Within the first half of the project, we have demonstrated with idealized model studies that microphysical ice formation processes can exhibit specific fingerprints in the cloud-top phase distribution, which are expected to be observable from satellite. Furthermore, we have prepared our high resolution model for various setups which are suitable for the closure studies. Convective clouds, which transition the entire relevant temperature range, are particularly promising subjects. In a multiyear dataset obtained from the geostationary satellite instrument SEVIRI, several hundred convective clouds are tracked and their properties related to the cloud top phase transition from liquid to ice. The results point again at significant microphysical influence on the cloud top phase distribution, although variations of other factors complicate the analysis.
The analysis of global datasets from polar orbiting satellites has revealed large biases, but hemispheric differences and trends with cloud optical depth are consistent independent of the retrieval scheme.
We have constructed a unique and large database of the cloud top properties of tracked convective clouds, which will be useful for the rest of the project and other studies beyond C2Phase. The ICON-LEM model has been updated with state-of-the-art descriptions of primary and secondary ice formation. We expect to provide guidance on the correct parameterization of ice formation in detailed microphysics schemes by evaluation in the closure studies, and also recommendations for the parameterization of cloud phase in global climate models.