ALBERT

Description: The formation and growth of ice in clouds are essential for precipitation formation. Despite advances in our understanding of ice phase processes arising from laboratory and modeling studies, a major gap exists in representative in-situ field observations. CLOUDLAB aims to fill this gap by performing targeted glaciogenic cloud seeding experiments (using silver iodide injections) in supercooled, predominantly liquid stratus clouds over the Swiss Plateau to induce ice crystal formation and subsequent growth processes. Downwind of the seeding location, the freshly generated ice crystals are observed using two scanning cloud radars and a tethered balloon system equipped with an optical particle counter and holographic imager. The holographic imager captures phase-resolved information about the number, size, and spatial distributions of hydrometeors with high spatio-temporal resolution (〉6 µm, 40 Hz). We present in-situ and remote sensing data observed during a series of cloud seeding experiments. The ice crystal and aerosol number concentrations increased by several orders of magnitudes during the passage of the seeding plume. Simultaneously at the same location, the radar reflectivity increased by 10 to 20 dB compared to the unseeded background cloud. The observed ice crystals formed three to fifteen minutes before the detection and grew to a diameter of around 100-200 µm. We will also assess how the seeding influences the liquid phase (e.g. liquid water content, cloud droplet size) and answer the question: Does the seeding produce fully glaciated patches? This dataset provides unique insights into early-stage ice processes and broadens our understanding of precipitation formation.

Description: Ice formation and growth play a critical role in the initiation of precipitation. However, fundamental knowledge gaps in microphysical processes exist, for example, in the efficiency of diffusional growth of ice crystals, which leads to uncertainties in weather forecasts and climate projections. The CLOUDLAB project aims to bridge this gap by using supercooled stratus clouds as a natural laboratory for glaciogenic cloud seeding experiments. Ice nucleating particles (particles containing mainly silver iodide) are dispersed into these clouds via an Uncrewed Aerial Vehicle (UAV), triggering ice crystal formation and growth. The use of a UAV for seeding in conjunction with the persistent nature of stratus clouds enables repeated seeding experiments under similar and well-constrained initial conditions. So far, 50 seeding experiments with seeding temperatures between -10°C and -3°C were conducted in clouds over the Swiss plateau. The seeding-induced microphysical changes were monitored using in-situ and ground-based remote sensing equipment positioned 3-15 minutes downstream of the seeding location. The seeding plume had an extent of multiple hundreds of metres and was detected by increased reflectivity in the vertically pointing and scanning cloud radars (additional instrumentation was provided by TROPOS in the frame of the accompanying PolarCAP project). Simultaneously, high concentrations of small ice crystals were detected with a holographic imager mounted on a tethered balloon. The findings are contextualized with simulations using the numerical weather model (ICON).

Description: From 27 October to 1 November 2020 we observed aged wildfire smoke layers from 10-11.5 km height with a multiwavelength Raman lidar at Limassol, Cyprus, continuously over 6 days. Aged smoke particles consist mainly of organic material. On all days, ice nucleation occurred just above the base of the smoke layer at temperatures around -50°C. The ice crystals grew fast and formed strong virga in clean air below the smoke layers. The observed smoke and cirrus features clearly indicates where the ice nucleation occurred. During the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition the German icebreaker Polarstern with a multiwavelength Raman lidar aboard drifted through the pack ice at latitudes from 85°-88.5°N from October 2019 to March 2020. During this period a wildfire smoke layer originating from record-breaking Siberian fires in the summer of 2019 and cirrus layers were continuously observed with the our lidar and a cloud radar of the ARM mobile facility in the upper troposphere and lower stratosphere (UTLS) from 7-17 km height. In this smoke layer, the evolution of more than 50 cirrus systems were monitored in the North Pole region within this 6-month period. We will discuss ice nucleation processes over Cyprus and the North Pole region. In the case of MOSAiC, the discussion is based on derived smoke ice nucleating particle concentrations and ice crystal number concentrations obtained from the combined lidar-radar studies.

Description: Low stratus clouds are a commonly occurring phenomenon over the Swiss Plateau during wintertime and can last up to several days. This almost stable characteristic is exploited in the CLOUDLAB project, which aims to gain a more profound understanding of ice crystal formation and growth by introducing seeding particles into a supercooled liquid cloud, thus initiating glaciation of the cloud. The seeding particles (silver iodide) are injected into the dynamically stable cloud from a drone enabling a highly precise and repetitive experimental setup. By observing the microphysical changes via an extensive observational setup of remote sensing and in-situ instrumentation, the low stratus clouds effectively serve as a natural laboratory for seeding experiments. To further our understanding of the occurring processes, we employ the ICON-NWP model in large-eddy mode with a horizontal resolution of up to 65 m to simulate the conducted experimental conditions. The existing two-moment microphysics scheme is supplemented by a seeding parameterization allowing us to mimic the ice nucleation behavior of the seeding particles. While it remains a challenge for numerical models to adequately represent low stratus, we successfully simulated several seeding experiments. Here, we present first comparisons between model simulations and observational data in the form of case studies and show the potential of ICON-NWP for interpreting our seeding signals.

Description: How cloud droplet activation and ice nucleation occur in clouds is still an open question that is important for predicting cloud occurrence, weather, and climate. More measurements in real clouds are vital to supplement laboratory studies to further our understanding of cloud microphysical processes. Here, we present results from CLOUDLAB, a field study deploying glaciogenic cloud seeding experiments, using a drone to release silver-iodide-containing particles into supercooled stratus clouds over the Swiss Plateau. We measure the downstream effects on aerosol and cloud particle populations in-situ with two portable optical particle spectrometers (POPS) mounted on a second drone and on a tethered balloon, as well as an in-situ holographic imager and ground-based remote-sensing instruments. With more than 50 in-cloud and 30 out-of-cloud experiments, covering a range of environmental conditions (temperature, wind speed, liquid water content), we address how the seeding material induces freezing in-cloud, specifically: Do the particles first activate into cloud droplets and then nucleate ice via immersion freezing, or do the particles collide with existing cloud droplets and cause contact freezing? By comparing POPS size distributions of the seeding material measured in- and out-of-cloud, at different distances from the seeding drone, we can identify how much hygroscopic growth occurs under different conditions to infer cloud droplet activation. Meanwhile, supporting in-situ cloud particle measurements and cloud radar reflectivity show the presence and extent of ice nucleation. These novel methods and data offer unique insights into cloud droplet activation and ice nucleation in real environmental conditions.

Description: The project Polarimetric Radar Signatures of Ice Formation Pathways from Controlled Aerosol Perturbations (PolarCAP) aims at tackling the complex problem of the evolution of the ice phase at slightly supercooled conditions by means of observations in a thermodynamically and aerosolcontrolled natural environment using radar polarimetry and spectral‐bin modelling. PolarCAP is implemented in close collaboration with the external ERC research project CLOUDLAB of ETH Zurich. The targets of the study are predominantly liquid supercooled stratiform cloud layers which frequently form during wintertime in the temperature range from ‐10 to 0°C over the Swiss Plateau. The observations will build on a solid foundation of previous achievements of TROPOS and previous collaborations. Doppler peak separation and multi‐wavelength techniques, retrievals of ice‐crystal size distributions, determination of particle habits techniques, all based on scanning polarimetric cloud radar observations, will be combined with fall‐streak tracking and liquid‐water retrievals to obtain a comprehensive picture of the cloud evolution. In collaboration with CLOUDLAB, evaluation data will be obtained in‐situ with UAV and holographic ice particle imagers, which will be used to challenge the remote‐sensing‐based retrievals. Within the PolarCAP project the remote sensing equipment of LACROS (Leipzig Aerosol and Cloud Remote Observations System) is installed in Eriswil, Switzerland during three winter campaigns 2022‐ 2025. This contribution will introduce PolarCAP. We will further present first results of our cloud microphysical measurements and their evaluation against CLOUDLAB in‐situ observations, as well as associated spectral‐bin model simulations, with the focus on hydrometeor characterization and retrieval evaluation.