Combined Remote-Sensing, In-Situ and Modelling of Cloud Microphysical Perturbations in Supercooled Stratus Clouds

Aerosol-cloud interactions in mixed-phase clouds still present major challenges for weather and climate models. The PolarCAP project (Polarimetric Radar Signatures of Ice Formation Pathways from Controlled Aerosol Perturbations) investigates how aerosols influence cloud-microphysical processes via cold cloud seeding experiments. Ice formation and evolution is studied under slightly supercooled conditions (T > -10°C) within a thermodynamically and aerosol-controlled environment, employing radar polarimetry, holographic imagery and spectral-bin modeling. In collaboration with the CLOUDLAB project at the ETH Zurich, PolarCAP investigates the development of an artificially initiated ice phase within supercooled stratus clouds. Utilizing cloud seeding with silver iodide, the freezing process of super-cooled cloud droplets is initiated. The subsequent evolution is monitored using in-situ measurements and ground-based cloud remote sensing tools. The collaboration has yielded a unique dataset, incorporating observations from the Leipzig Aerosol and Cloud Remote Observing System (LACROS) and in-situ data from CLOUDLAB in tandem with data from the cloud-resolving spectral-bin microphysics model COSMO-SPECS.

We present a comparative evaluation between observational and model data, complemented by a Lagrangian analysis that tracks ice formation and growth processes within the seeded cloud to provide detailed insights into the evolution of the ice phase. A multitude of ensemble model runs were performed on two different mesh sizes, with horizontal resolution of ~400m and ~100m, varying the flare INP injection rate and initial cloud condensation nuclei (CCN) number concentrations. First, we show the model's ability to replicate observed cloud responses, providing insights into primary ice growth processes, particularly the Wegener-Bergeron-Findeisen (WBF) process. During the seeding experiments, observations show simultaneous decreases in cloud droplet concentrations alongside increases in ice crystal concentrations, including periods where cloud droplets were entirely depleted. Second, the measured ice crystal sizes and growth rates, are compared to the model output. This comparison revealed discrepancies in ice crystal size distributions, highlighting potential model biases in parameterizations of ice nucleation and growth rates for columnar ice crystals.