Ice-nucleating particle depletion in the wintertime boundary layer in the pre-Alpine region during stratus cloud conditions

The remote-sensing equipment of LACROS (Leipzig Aerosol and Cloud Remote Observations System) was installed in Eriswil, Switzerland during the winter campaign of 2023 / 2024. We utilize a big dataset of in situ (especially ice-nucleating particle (INP) sampler; low volume sampler) and remote-sensing (especially cloud radar and Raman lidar) equipment. In addition, INP measurements were available as well in Hohenpeißenberg, Germany and Melpitz, Germany.

We evaluate the regional variability of the number concentration of INPs between the two pre-Alpine central-European sites of Eriswil and Hohenpeißenberg, supported by INP measurements from Melpitz during the winter months of 2024. The aim of the study is to spatially and temporally evaluate INP availability and removal within the planetary boundary layer (PBL) during Bise situations because reasons for the lack of ice and precipitation in the supercooled clouds observed over the Swiss Plateau remain unclear and may be caused by the lack of INPs. Target scenario of the study were situations when northeasterly winds (so-called Bise winds) prevailed and layers of stratus clouds formed at the top of the PBL at temperatures down to −10 °C. In these situations, it is expected that INPs are depleted along the transport path.

We will present our main insights from our measurements:

1) During the cold-Bise (cloud minimum temperatures as low as −10 °C) and warm-Bise (cloud minimum temperatures above 0 °C), almost no INP contrast was found between Hohenpeißenberg and Eriswil if both were within the PBL. Also, the INP concentration was overall found to be much lower during the cold-Bise than during the later warm-Bise situation.

2) When the Hohenpeißenberg site was located in the free troposphere during the cold-Bise situation, INP concentrations were much higher compared to Eriswil (still within the PBL) but similar to cloud-free Melpitz. These observations led to the conclusion that during cold-Bise situations the INP reservoir within the PBL is depleted, likely by the presence of supercooled stratus. The inversion-capped wintertime PBL, especially during periods of widespread snow cover, is apparently not capable to replenish the INP reservoir from the free troposphere.

3) INP observations of around 10^−3 L^−1 at Hohenpeißenberg, when this site was above the PBL were on a similar order as the ice crystal number concentrations (ICNC) observed during the same period at Eriswil. This supports the hypothesis that INPs are entrained from the free troposphere via turbulence and afterwards immediately removed as they interact with the Bise cloud layer, leading to reduced availability of INPs downwind. It must be noted that an ICNC concentration which is higher than the observed INP concentration can in principle also be a result of secondary ice formation processes. Nevertheless, secondary ice formation processes generally lead to orders of magnitudes of increase in ICNC, which was, besides occasional peaks in the ICNC, not observed in the average ICNC values during the investigated time periods.