Comparing ice-nucleating particles in extensive stratiform clouds

In the Southwest of Germany and the North of Switzerland, there is a wind known as Bise, roughly blowing from Northeast. Associated with this wind, an extensive and long-living stratiform cloud deck can form in the winter months.

For this study, measurements were done at two different locations, both protruding above the surrounding landscape: Hohenpeißenberg (47.801°N,11.009°E, elevation 950m, station of the German Weather Service DWD, ~ 50km southwest of Munich) and Eriswil (47.071°N,7.873°E, elevation 920m, ~ 60 km southwest of Zurich). During a Bise situation, there is typically a stratus cloud at and between both locations, with winds blowing roughly from Hohenpeißenberg to Eriswil. During two Bise periods in January and February 2024, we collected aerosol particles onto polycarbonate filters at both sites. Sampling was conducted with a Digitel low-volume sampler. Sampling time was 12 hours for each filter, with a flow rate of 25 l/min. Collected filters were examined for their INP (Ice Nucleating Particle) concentrations with well-established offline methods at TROPOS.

When the Bise cloud was present at both Hohenpeißenberg and Eriswil, INP spectra at both locations were very similar. In January (Bise-1), temperatures in the boundary layer were below 0°C, and INP spectra did not show a high fraction of INPs with biogenic origin. In February (Bise-2), temperatures in the boundary layer had already risen to be constantly above 0°C. Much higher INP concentrations were observed for the whole INP spectra during Bise-2. This increase was particularly strong for freezing temperatures above -12°C, caused by additional heat labile INPs of biogenic origin. The difference in INP concentrations between the two Bise situations may at least partially originate in INP removal through ice nucleation with subsequent precipitation formation during Bise-1 which did not occur during Bise-2.

During Bise-1, there was a phase when at Hohenpeißenberg the inversion decreased in altitude by a few hundred meters. With this, the measurement site protruded above the Bise cloud, while at the same time the temperature at the ground in Hohenpeißenberg increased from -10°C to just below 0°C. Meanwhile, the Bise cloud was still present at Eriswil. While INP spectra had been similar at Hohenpeißenberg and Eriswil when both sites were in the Bise cloud, during this phase of more than a day, INP spectra at Hohenpeißenberg showed much higher concentrations and additional heat labile INPs. The changed conditions reflect the situation in the free troposphere, and we suggest the free troposphere as a source for INPs for the Bise clouds.