How Representative are In-Situ Cloud Observations at Mountain-Top Research Stations?

Mountainous regions in the mid-latitudes frequently experience mixed-phase conditions. As such, mountain-top research stations provide an excellent platform for long term measurements with high temporal resolution at low operational costs. However, 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. Previous studies suggest that either secondary ice production or surface processes (e.g. blowing snow, hoar frost or riming on snow covered trees, rocks and the snow surface) contribute to the observed ICNCs. A strong impact on the observed ICNCs from the latter may limit the relevance of mountain-top ice crystal measurements.

To assess the impact of surface processes on the ICNCs measured at mountain-top research stations, vertical profiles of ICNCs were observed with an elevator up to a height of 10 m at the Sonnblick Observatory (SBO) in the Hohen Tauern Region, Austria (see figure, left). Independent of the presence of a cloud, the observed ICNC decreases with height (see figure, right). This suggests a strong impact of surface processes on the measured ICNCs. Further, we propose that the contribution of surface processes of several hundred per liter can partly explain the gap between the measured INP concentrations and the observed ICNCs at mountain-top research stations. Consequently, the high ICNCs measured near the surface at mountain-top stations are not representative for the clouds, which limits the relevance of ground-based measurements for atmospheric science.

Due to the limited vertical extent of the measurements, it is unclear if ice crystals re-suspended from the surface affect clouds. If only a few ice crystals re-suspended from the surface reach heights of several tens of meters, secondary ice processes may be triggered in the absence of primary ice nucleation. Indeed, at wind speeds higher than 20 ms^-1 the observed ICNC still exceeded 300 l^-1 even 10 m above the surface (see figure, upper right). Under these conditions, surface processes may significantly contribute to the in-cloud ICNC and affect the cloud's development and lifetime, potentially acting to glaciate orographically forced mixed-phase clouds on the leeward slope of mountain barriers.