Abstract
Abstract. In situ cloud observations at mountain-top research stations regularly measure ice crystal number concentrations (ICNCs) orders of magnitudes higher than expected from measurements of ice nucleating particle (INP) concentrations. Thus, several studies suggest that mountain-top in situ cloud microphysical measurements are influenced by surface processes, e.g., blowing snow, hoar frost or riming on snow-covered trees, rocks and the snow surface. This limits the relevance of such measurements for the study of microphysical properties and processes in free-floating clouds. This study assesses the impact of surface processes on in situ cloud observations at the Sonnblick Observatory in the Hohen Tauern region, Austria. Vertical profiles of ICNCs above a snow-covered surface were observed up to a height of 10 m. The ICNC decreases at least by a factor of 2 at 10 m if the ICNC at the surface is larger than 100 L−1. This decrease can be up to 1 order of magnitude during in-cloud conditions and reached its maximum of more than 2 orders of magnitudes when the station was not in cloud. For one case study, the ICNC for regular and irregular ice crystals showed a similar relative decrease with height. This suggests that either surface processes produce both irregular and regular ice crystals or other effects modify the ICNCs near the surface. Therefore, two near-surface processes are proposed to enrich ICNCs near the surface. Either sedimenting ice crystals are captured in a turbulent layer above the surface or the ICNC is enhanced in a convergence zone because the cloud is forced over a mountain. These two processes would also have an impact on ICNCs measured at mountain-top stations if the surrounding surface is not snow covered. Conclusively, this study strongly suggests that ICNCs measured at mountain-top stations are not representative of the properties of a cloud further away from the surface.
Highlights
Cloud microphysical properties next to dynamical processes are key parameters for the cloud’s lifetime, the cloud extent and the intensity of precipitation they produce (Rotunno and Houze, 2007)
Either sedimenting ice crystals are captured in a turbulent layer above the surface or the ice crystal number concentrations (ICNCs) is enhanced in a convergence zone because the cloud is forced over a mountain
This study assessed the impact of surface and near-surface processes on ICNCs measured at mountain-top stations and possible implications on the atmospheric relevance of such measurements
Summary
Cloud microphysical properties (e.g., phase composition, cloud particle number concentrations and size distributions) next to dynamical processes are key parameters for the cloud’s lifetime, the cloud extent and the intensity of precipitation they produce (Rotunno and Houze, 2007). Orographic precipitation plays a crucial role for the world’s water resources, as the headwaters of many rivers are located in alpine regions (Roe, 2015). In the midlatitudes, mixed-phase clouds (MPCs) consisting of a mixture of ice crystals and supercooled liquid droplets produce 30 to 50 % of liquid precipitation (Mülmenstädt et al, 2015) due to the rapid growth of ice crystals to precipitation size in the presence of supercooled liquid droplets. This is due to a higher saturation vapor pressure over liquid water than over ice, and ice crystals grow at the expense of evaporating cloud droplets. Correctly representing the fraction of ice in orographic MPCs is crucial for accurate weather and water resource predictions in alpine terrain
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