Measurement report: Intercomparison of cloud spectrometers and a holographic probe during the ACTRIS sub-zero in-cloud Campaign at Sonnblick Observatory

Abstract. This study presents results from the Aerosol, Clouds and Trace Gases Research Infrastructure Cloud In Situ (ACTRIS-CIS) European Centre for Cloud Ambient Intercomparison (ECCINT-INT01) campaign, an intercomparison of cloud spectrometers conducted at the Sonnblick Observatory (3,106 m a.s.l.) from 21 November to 3 December 2022. Seven instruments measuring cloud microphysical properties were deployed, including three Fog Monitors (FM-120), three Cloud Droplet Analyzers (CDAs), and an open-path Single Particle Holographic Imaging probe (ICEMET), which served as the reference instrument. The campaign evaluated instrument performance in measuring cloud droplet number concentration, median volume diameter, effective diameter, liquid water content, and cloud droplet size distributions under sub-zero conditions. Significant sampling losses were observed in vertically oriented instruments compared with ICEMET, particularly for cloud droplets with median volume diameters of 10–15 µm. Cloud droplet number concentration and liquid water content were the most affected parameters, whereas median volume diameter and effective diameter remained comparatively robust. Horizontally aligned instruments showed improved agreement with ICEMET, highlighting the importance of wind-aligned sampling geometry. Despite these losses, all spectrometers showed strong consistency in derived sizing parameters, indicating that droplet size structure can be reliably retrieved under different deployment configurations. In contrast, liquid water content showed larger discrepancies, highlighting the need for correction methods accounting for sampling losses of larger droplets. Our results provide recommendations for instrument deployment, demonstrate the influence of inlet design on measurement quality, emphasize the value of multi-instrument intercomparisons, and support efforts to harmonize ground-based cloud microphysical observations within the ACTRIS framework.