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. In situ cloud observations at mountain-top research stations regularlymeasure ice crystal number concentrations (ICNCs) orders of magnitudes higherthan expected from measurements of ice nucleating particle (INP)concentrations. Thus, several studies suggest that mountain-top in situ cloudmicrophysical measurements are influenced by surface processes, e.g., blowingsnow, hoar frost or riming on snow-covered trees, rocks and the snow surface.This limits the relevance of such measurements for the study of microphysicalproperties and processes in free-floating clouds. This study assesses the impact of surface processes on in situ cloudobservations at the Sonnblick Observatory in the Hohen Tauern region,Austria. Vertical profiles of ICNCs above a snow-covered surface wereobserved up to a height of 10  m . The ICNC decreases at least by afactor of 2 at 10  m if the ICNC at the surface is larger than100  L−1 . This decrease can be up to 1 order of magnitude duringin-cloud conditions and reached its maximum of more than 2 orders ofmagnitudes when the station was not in cloud. For one case study, the ICNCfor regular and irregular ice crystals showed a similar relative decreasewith height. This suggests that either surface processes produce bothirregular and regular ice crystals or other effects modify the ICNCs near thesurface. Therefore, two near-surface processes are proposed to enrich ICNCsnear the surface. Either sedimenting ice crystals are captured in a turbulentlayer above the surface or the ICNC is enhanced in a convergence zonebecause the cloud is forced over a mountain. These two processes would alsohave an impact on ICNCs measured at mountain-top stations if the surroundingsurface is not snow covered. Conclusively, this study strongly suggests thatICNCs measured at mountain-top stations are not representative of theproperties of a cloud further away from the surface.

Impact of surface and near-surface processes on ice crystal concentrations measured at mountain-top research stations