Abstract

The project PolarCAP aims to uncover the complex entanglement of aerosol- and cloud-microphysical processes by exploring the evolution of the ice phase at slightly supercooled conditions of T > -10°C in a thermodynamically and aerosol-controlled natural environment using radar polarimetry and spectral-bin modelling.

PolarCAP collaborates with the ERC research project CLOUDLAB of ETH Zurich to investigate the evolution of the artificially triggered ice phase in supercooled stratus layers (see Fig. 1). Thereby, CLOUDLAB applies cloud seeding with silver iodide to initialize the freezing of cloud droplets, whose evolution is then monitored by means of in-situ measurements of drones and the unique holographic cloud-hydrometeor in-situ sensor HOLIMO, as well as by means of standard ground-based cloud remote sensing instrumentation.

In the framework of the collaboration between PolarCAP and CLOUDLAB, a unique data set will be produced and analyzed that includes polarimetric radar and lidar observations from the Leipzig Aerosol and Cloud Remote Observing System (LACROS) as well as data from the cloud-resolving spectral bin model COSMO-SPECS. PolarCAP will benefit strongly from the available cloud in-situ measurements. Progress will be achieved in the ability to constrain the efficiency of different ice nucleating substances, to link the time scales of microphysical processes and stratus dissipation, and to evaluate and develop remote-sensing-based retrievals for cloud properties.

Status Summer 2024

In winter 2022/23 and 2023/24, the mobile exploratory platform LACROS of TROPOS was part of a series of winter campaigns near Eriswil in the centre of Switzerland. LACROS joint the two 3-months campaigns, which were conducted under the umbrella of the ERC research project CLOUDLAB of ETH Zurich, in the framework of the PolarCAP (Polarimetric Radar Signatures of Ice Formation Pathways from Controlled Aerosol Perturbations) project.
The CLOUDLAB campaign jointly brought together a unique set of ground-based and airborne in-situ cloud and precipitation sensors and remote sensing instruments. During the campaigns, LACROS was on site with a large number of remote sensing equipment. In winter 2022/23, among other instruments, a scanning 35-GHz and vertical-pointing 94GHz cloud radar, as well as the 35-GHz scanning cloud radar of ETH Zurich were on site.
During the campaign 2023/24 two more cooperations took place. The PROM project CORSIPP of LIM (Leipzig Institute for Meteorology) joined the campaign in Eriswil with their scanning 94GHz polarimetric cloud radar. In addition, EPFL (École Polytechnique Fédérale de Lausanne) joined the campaign with a scanning polarimetric X-band radar. In the end, the campaign was one of the largest joint deployments of multi-wavelength radar and lidar systems. An overview of the campaign can be seen in Figure 1. An additional side project of TROPOS and the Hohenpeißenberg Meteorological Observatory of the German Weather Service (DWD) dealt with the characterization of the aerosol conditions during the supercooled stratus cloud events. Aerosol in-situ samplers were installed at Hohenpeißenberg observatory and Eirswil to characterize potential contrasts in the concentration of ice nucleating particles (INP) between the two sites. The analysis of these datasets (2 weeks of samples were taken) is ongoing in 2024.

With our radar measurements during the cloud seeding experiments conducted by the ETH, we were able to support the ETH colleagues with their studies on ice crystal growth mechanisms. In addition, we identified case studies of natural cloud seeding events. By exploring these case studies in a great detail, we hope to learn more about the involved processes that lead to enhanced precipitation during natural seeding events. Recently, we developed a fall streak tracking algorithm that helps to identify the evolution of the microphysical properties of the ice crystals on their pathway through the cloud system. The 35-GHz and 94-GHz cloud radar measurements gave us the chance to calculate a dual wavelength ratio which gives us more insight into the cloud microphysical processes.
Overall, the campaign serves as a unique chance to validate remote sensing measurements against surface in situ measurements qualitatively. A Master's thesis written at TROPOS focused on this topic (Gaudek, T., 2024: Co-located observations of liquid and ice precipitation hydrometeors with a two-dimensional video disdrometer, a holographic cloud in-situ sonde, and active remote sensing, Masterthesis, University of Leipzig.).

Moreover, the cloud-resolving spectral bin model SPECS driven by COSMO, a non-hydrostatic limited-area atmospheric model, was prepared for investigations in a special campaign-like mode. The COSMO-SPECS model was enhanced by developing a dedicated flare or cloud seeding extension in which ice nucleating particles (INP) and cloud condensation nuclei (CCN) are artificially introduced into a pre-defined grid cell in x-y-z coordinates for a certain period of time. This implementation enables a thorough investigation of the impact of the emitted AgI-AgCl plume on stratus cloud development. Initial model runs were conducted for 25 January 2023 from 9 - 10 UTC at two distinct spatio-temporal resolutions. Both simulations cover an area of 18km by 16km. Horizontal resolution of Run1 is 400m (50×40 grid cells) and 100m for Run2 (200×160 grid cells). The vertical resolution spans 100 height levels ranging from 900m to 21500m. Figure 2 illustrates the effect of the plume induced by seeding as it advances downwind. First comparisons of modeled liquid water path (LWP) to microwave radiometer (MWP) LWP shows that LWP biases are significantly reduced in the higher resolution Run2.

Figure 2: Temporal evolution of the ice crystal number concentration of COSMO-SPECS Run2 with artificial seeding at 10:52:00 UTC. The ruler shows the distance of the plume over time, with marks units of km. The x shows the location of the observational site.

Figure 3 illustrates a time-height cross-section, of the 35-GHz radar reflectivity factor and linear depolarization ratio from 10:30 to 11:40 UTC on the corresponding day as the simulations. The occurrence of the plume event above the radar at 11:02 UTC closely aligns with the simulations. Next steps involve the evaluation of this event through a comprehensive model-observation comparison. This step aims to investigate deeper into the characteristics and dynamics of the observed phenomenon, enhancing our understanding through analysis and validation against simulated data.